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1

FAO FISHERIES TECHNICAL PAPER 262

RIVER FISHERIES

.

by

R.L.Welcomme Senior Fishery Resources Officer

FAO Fishery Resources and Environment Division

The designations employed and the presentation of material in this publication do not imply the

expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the

United Nations concerning the legal status of any country, territory, city or area or of its authorities, or

concerning the delimitation of its frontiers or boundaries.

M-42

ISBN 92-5-102299-2

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or

transmitted in any form or by any means, electronic, mechanical, photocopying or otherwise, without

the prior permission of the copyright owner. Applications for such permission, with a statement of the

purpose and extent of the reproduction, should be addressed to the Director, Publications Division,

Food and Agriculture Organization of the United Nations, Via delle Terme di Caracalla, 00100 Rome,

Italy.

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 1985

© FAO

PREPARATION OF THIS DOCUMENT

This technical paper presents an updating and extension of material already published externally to

FAO in ‘Fisheries Ecology of Floodplain Rivers’ (Longman, 1979). It is intended as a general

summary of the current thinking based upon the literature on all aspects of river fisheries from the

2

physical and biological environment in which they are pursued to their management. As such it is of

interest not only to students, scientists and administrators working in the fisheries sector, but also to

biologists, ecologists and geographers working on aquatic organisms other than fish and more general

aspects of natural resources development and management.

The cover illustration shows a portion of the floodplain of the Magdalena River and illustrates the

complexity of the aquatic system in some floodplains.

Distribution:

FAO Fisheries Department

FAO Fisheries Regional Offices

Selector SI

CIFA

COPESCAL

EIFAC

For bibliographic purposes this document

should be cited as follows

Welcomme, R.L., River fisheries. FAO Fish.

1985 Tech.Pap., (262): 330 p.

SUMMARY

Rivers drain all but the most arid areas of the earth through channels that are regulated by physical laws that

impose on them certain forms. The ideal form is rarely encountered in practice and represents an end point to

which geographic process tend. In general a river may be divided into two principal zones, the steep and fast

flowing rhithron upstream and the sluggish and flat potamon downstream. While conditions in an individual

system are highly variable along its length, similar reaches of different rivers differ much less even between

continents and at different latitudes. All continents have a series of major river systems which consist not only

of the river channels but also the swamps, lakes and seasonally flooded lands associated with them.

Most rivers are highly conditioned by the patterns of precipitation in their basins. Differences in rainfall

intensity throughout the year generate a flood wave that progresses downstream in the majority of rivers (flood

rivers), although singular geographic circumstances may distribute discharge more evenly throughout the year in

some systems (reservoir rivers). The number of reservoir rivers is increasing through flow regulation and dam

building. Although the basic nature of the river is determined by the rocks over which it flows, the flood regime

seasonally modifies the physical and chemical conditions within the river particularly in the tropics. In higher

latitudes other features of climate, such as insolation or air temperature exert an increasing influence.

Seasonal changes in discharge, nutrient concentrations, pH, temperature and dissolved oxygen in their turn

influence the composition and abundance of the plant and animal communities inhabiting the river. These

changes are particularly marked in the floodplain area of the potamon where the rise in water during the floods

inundates extensive areas of land flanking the main channels. This increase in living space, together with the

release of nutrients associated with the submersion of the soil produces an annual surge of primary productivity

closely followed by an expansion in biomass of animal communities.

The number of fish species inhabiting rivers is a function of the size of the river, with larger basins such as

the Amazon having well over 1000 species. The individual species are highly adapted to the conditions in the

type of river reach in which they live. Such adaptations are not only morphological but also behavioural and

some species have developed extensive migrations to avoid adverse conditions or for breeding and feeding.

Alternatively elaborate breeding mechanisms have also evolved.

Other features of the biology of fishes are linked to the hydrological cycles within the river. Thus, the flood is

associated with spawning in the majority of species when the abundance of living space and food provides the

best conditions for the survival and growth of the young fish. Such is the influence of these factors that in years

of more intense flooding survival and growth are so improved that the total biomass of the fish community rises

and a strong year class is produced for transmission on to other years. In reservoir rivers and in the rhithron

seasonal and year-to-year differences of this type are not so marked.

3

Fish communities in rivers provide the basis for fisheries which are pursued with a great variety of gear.

Fishing intensity is also seasonal and is tied either to variations in temperature or to the flood. Because the fish

community can vary in abundance with the fluctuations in flood strength catch is similarly correlated with years

of high catch following after years of particularly good flooding. Fish communities respond to increases in

fishing pressure in a number of ways. In general catch in rivers having simple fish communities follow a typical

yield curve whereas those in which communities are complex show a plateau in catch which may persist over a

great range of effort. This plateau masks changes in the composition of the community with a drift from large,

slow growing to small, fast growing forms.

Rivers and their basins are used for many purposes other than fisheries. Many of these modify the quality or

quantity of water in the system and thus interact with the fish communities in the river to their detriment.

Management of the river for fisheries therefore becomes increasingly important as the intensity of use of the

river rises. Fisheries themselves also require management which may be accomplished either by direct

interventions on the fish stock or by legislative or economic activities on the fishermen themselves. As the river

becomes increasingly modified the capture fisheries originally pursued there become less viable. Although the

water courses may continue to provide food or recreation the major developmental emphasis is towards

replacement activities such as aquaculture on the former floodplain or the creation of new fisheries in the

reservoirs associated with the main channel.

Hyperlinks to non-FAO Internet sites do not imply any official endorsement of or responsibility for

the opinions, ideas, data or products presented at these locations, or guarantee the validity of the

information provided. The sole purpose of links to non-FAO sites is to indicate further information

available on related topics.

CONTENTS

INTRODUCTION

Chapter 1 - MORPHOLOGY OF RIVER SYSTEMS

FORM OF RIVER SYSTEM

TYPES OF RIVER

LONGITUDINAL PROFILE

TRIBUTARY DEVELOPMENT AND STREAM ORDER

Stream Order

Relation of River Length to Drainage Basin Area

MORPHOLOGY OF THE RHITHRON

MORPHOLOGY OF THE POTAMON

The Channels

The Floodplain

BRIEF REVIEW OF MAJOR RIVERS

AFRICA

AMERICA (NORTH)

4

AMERICA (SOUTH)

ASIA

EUROPE

AUSTRALIA AND NEW ZEALAND

Chapter 2 - PHYSICAL AND CHEMICAL PROCESSES

HYDROLOGY

FLOW

Variability of Flow Regimes

Velocity of Flow

ORIGINS OF FLOODING

WATER BALANCE ON FLOODPLAINS

CHEMISTRY

SEDIMENT LOAD AND TURBIDITY

NUTRIENT SPIRALLING

CARBON AND ORGANIC MATERIAL

IONIC COMPOSITION

HYDROGEN ION CONCENTRATION (pH)

TEMPERATURE

DISSOLVED OXYGEN

Chapter 3 - PRIMARY PRODUCTION IN RIVERS

THE RIVER CONTINUUM CONCEPT

MATERIAL OF ALLOCHTHONOUS ORIGIN

BACTERIA AND OTHER MICROORGANISMS

PHYTOPLANKTON

INFLUENCE OF ENVIRONMENTAL FACTORS

PRODUCTION

ATTACHED ALGAE

HIGHER VEGETATION

DISTRIBUTION AND ZONATION

SUBMERSED VEGETATION

5

FLOATING VEGETATION

FLOATING MEADOWS

FLOODPLAIN MEADOWS

THE ROLE OF HIGHER VEGETATION IN NUTRIENT BALANCE

Chapter 4 - SECONDARY PRODUCTION IN RIVERS

ZOOPLANKTON AND DRIFT

ANIMAL COMMUNITIES ASSOCIATED WITH FLOATING AND SUBMERSED

VEGETATION

BENTHOS

NEUSTON

VERTEBRATES OTHER THAN FISH

AMPHIBIA

REPTILES

BIRDS

MAMMALS

Chapter 5 - RIVER FISHES AND THE RIVERINE SYSTEM

NUMBERS OF SPECIES IN RIVER SYSTEMS

DIFFERENCES IN NUMBERS OF SPECIES BETWEEN SYSTEMS

RELATIVE ABUNDANCE OF SPECIES WITHIN ONE SYSTEM

SUB-POPULATIONS

SIZE OF SPECIES

DISTRIBUTION OF SPECIES IN RIVER SYSTEMS

DISTRIBUTION IN SPACE AND ZONATION

Zonation

Habitats of River Systems and Accompanying Floodplain

DISTRIBUTION IN TIME AND MIGRATION

Types of Migration and Movement

Migrations of Adult Fish

Movement of Juveniles

Distance and Speed of Movement

Timing of Migration

6

ADAPTATION TO EXTREME ENVIRONMENTAL CONDITIONS

LOW DISSOLVED OXYGEN CONCENTRATIONS

Adaptations to Air Breathing

Adaptations for Using the Surface Layer

Physiological Adaptations

ADAPTATIONS TO RESIST HIGH TEMPERATURE

ADAPTATIONS TO RESIST DESICCATION

ADAPTATIONS TO POOR LIGHT

ADAPTATIONS TO RESIST STRONG CURRENT

Chapter 6 - THE PRODUCTION BIOLOGY OF RIVER FISH

FEEDING

SOURCES OF FOOD

SPECIALIZATION AND RESOURCE PARTITIONING

Specialization

Resource Partitioning

SEASONALITY OF FEEDING

GROWTH

FACTORS AFFECTING GROWTH

MODELS OF GROWTH

YEAR-TO-YEAR VARIATIONS IN GROWTH

REPRODUCTION

SPAWNING SITES AND REPRODUCTIVE ADAPTATIONS

FECUNDITY AND SPAWNING PATTERNS

TIMING

THE INFLUENCE OF HYDROLOGICAL REGIME ON SPAWNING SUCCESS

MORTALITY

CAUSES OF MORTALITY

SEASONALITY OF MORTALITY

ESTIMATES OF MORTALITY RATES

MODELS OF MORTALITY

7

STANDING STOCK AND PRODUCTION

STANDING STOCK

Main River Channel

Backwaters

Standing Waters of Floodplains

Total System

PRODUCTION

Estimates of Production

Models of Fish Standing Stock and Production in Rivers

Chapter 7 - THE FISHERY

THE FISHERMEN

OCCASIONAL FISHERMEN

PART-TIME FISHERMEN

PROFESSIONAL FISHERMEN

FISHING GEAR

SEASONALITY

CAPTURE METHODS

Low Water

Fishing during Rising and Receding Floods

Fishing at Peak Flood

BOATS

PRESERVATION OF FISH

TYPES OF PRODUCT

Chilled Fish

Dried Fish

Salting

Smoking

Fish Meal

Fish Oil

Fermented Products

8

PROTECTION AGAINST INSECT INFESTATION

SPECIES CAUGHT

LATIN AMERICA

AFRICA

ASIA

EUROPE AND NORTH AMERICA

FISHERIES FOR JUVENILE FISH

CATCH

ANALYSIS OF CATCH IN DIFFERENT RIVERS

Catch as a Function of River Form

Catch as a Function of Fishing Intensity

Changes in Community Structure with Increasing Fishing Pressure

FLUCTUATION IN CATCH BETWEEN YEARS

Chapter 8 - MANAGEMENT OF RIVER FISHERIES

EFFECTS OF OTHER USES OF RIVERS AND THEIR BASINS ON FISHERIES

CHANGES IN FLOW

CHANGES IN SILT LOAD

CHANGES IN WATER QUALITY

INTERACTION WITH OTHER USES

Terrestrial Animals

Forestry

Agriculture

Urbanization

Hydraulic Engineering

DEVELOPMENT AND MANAGEMENT OF RIVER SYSTEMS FOR FISHERIES

OBJECTIVES AND STRATEGY

Objectives

Uses of Fish Resources

Developmental Stage

MANAGEMENT OF THE RIVERINE ENVIRONMENT

9

Preservation of the Natural System

Instream Improvement Structures

MANAGEMENT OF THE FISH STOCK

Introduction of New Species

Stocking

MANAGEMENT OF THE FISHERY

Regulation of Access

Increasing the Catch Capacity of Fishermen

Closed Seasons

Reserved Waters

Mesh Regulations

Banning of Certain Gears

Accessibility

DEVELOPMENT OF NEW OR ALTERNATIVE FISHERIES

Aquaculture

Reservoir Fisheries

ACKNOWLEDGEMENTS

BIBLIOGRAPHY

SPECIES INDEX

GEOGRAPHICAL INDEX

GENERAL INDEX

10

INTRODUCTION

Rivers have formed nuclei for human settlement from the origins of mankind. Many of the earliest

civilizations emerged upon the fertile floodplains and since about 5000 b.p., when the earliest

systematic colonization of the Nile, Mesopotamic, Indus and Chinese rivers occurred there has been a

concentrated effort aimed at the domination of the hydro-logical regimes for the benefit of agriculture.

The Roman culture and later that of Western Europe impounded many smaller rivers for water power.

In many of the more arid parts of the world streams and rivers were manipulated to provide water for

irrigation. These trends have increased until the present day efforts at impoundment, deviation and

canalization have left few rivers with undisturbed channels. To the environmental impacts of

hydraulic engineering must be added the hazards of the contamination of the waters with a variety of

agricultural, domestic and industrial chemicals. In addition bad, or non- existent basin management,

deforestation and farming of marginal hill slope lands has increased erosion and the silt loads of rivers

resulting in rapid modification of the lowland reaches of the river. These changes not only modify the

environment, depriving the fish of living space and access to parts of the river necessary for the

completion of their life cycles, but also changes in the quality and quantity of the water in which they

live.

The fish communities of rivers have provided the basis for fisheries presumably from the earliest

phases of human occupation of river valleys. The FAO Yearbook of Fishery Statistics (FAO 1984) the

shows the nominal catch of the worlds freshwaters to have grown from 7.1 million tons in 1977 to 8.9

million tons in 1983 representing 10.4 and 11.6 percent of total world catch respectively. The

relatively slow rate of increase of 3 percent per year would seem to indicate that catch levels are

reaching a maximum. Much of the present inland catch still derives from rivers or the seasonally

inundated ground associated with them especially in Latin America and South East Asia where large

lakes are rare. The increase in the number of uses competing with fisheries has brought about the

disappearance of some long established fisheries and others are on their way to extinction.

In general studies of fish communities have lagged behind those of lakes and reservoirs although there

has been an increase in interest in this topic in the last decade. Practical concern with the management

of rivers for fisheries began towards the end of the last century in North America and Europe and led

to research in such waters in support of stocking and physical improvement programmes principally

in support of sport fisheries for salmonids. These led to early classifications of rivers into zones in

Eastern Europe where commercial fisheries for coarse fish were also economically important. One of

the earliest systematic studies on large rivers were those of Antipa (1910) whose original work on the

Danube was continued by other workers until it has become one of the most extensively studied of the

world’s major rivers. Antipa's general conclusion that the fisheries production of the Danube was

directly proportional to the extent and duration of flooding in any particular year (Botnariuc, 1968)

has proved to be equally applicable to all other flood rivers investigated. The work on the Danube also

illustrated that the floodplain cannot be considered in isolation but must be treated as an integral part

of the larger system (Botnariuc, 1967; Balon, 1967). Somewhat later Russian workers commenced

studies on the Volga River although this work intensified only after the creation of the cascade of

reservoirs in that system and only part of the literature is available in translation. Detailed studies of

the Mississippi-Missouri system were further delayed and it is only in the last decade that

understanding of that system has been obtained. Modern work on these and other temperate rivers is

now highly complex and deals with many biological and ecological issues especially those connected

with the conservation of riverine habitats.

Systematic study of the fisheries ecology of tropical rivers began on the Niger when a laboratory was

set up in the Central Delta (Blanc et al., 1955) whose output through the numerous publications of

Daget clarified much of the taxonomy and biology of fish in that river several FAO projects have been

associated with the study of this river in Niger, Benin and Nigeria. The Nile Sudd in Sudan has been

studied by a series of missions including the Jonglei Investigation Team (1954) and Mefit Babtie

(1984). Intensive but short term duration studies on the Kafue River by the Universities of Idaho and

11

Michigan did much to shed light on the biology of the fishes of the Kafue flats. The ORSTOM team

studied the Yaeres floodplain of the Logone River during the 1970's and elsewhere workers have been

gathering information on the fisheries and general ecology of the Shire River, the Okavango delta

(Botswana Society, 1976) and components of the Congo River.

In South America most of the river systems have been examined to a certain extent. The numerous

works of Bonetto and his team have provided a great amount of information on the Parana River and

its tributaries while Godoy (1975) summarized an extensive amount of work on the Brazilian Mogi

Guassu tributary of the same system. In the Amazon staff of the Instituto Nacional de Pesquisas

Amazonicas has studied the area around Manaus and an FAO funded project enabled the Peruvian

authorities to collect information on the same river at the level of Iquitos. Work on the Orinoco was

begun by Mago-Leccia (1970) and has been continued by Novoa and his co-workers. Lowe-

McConnell carried out fundamental studies on the ecology of tropical river fish communities in the

Rupununi River. Surveys of the fisheries of the Magdalena River were started by INDERENA and

intensified through the activities of an FAO project.

Studies on Asiatic rivers have been somewhat more limited although work on the Mekong from

Chevey et Le Poulain (1940) onwards have contributed to the general understanding of large tropical

systems. Otherwise occasional studies have been carried out on rivers in Peninsular Malaysia, Borneo,

India, Sri Lanka and the Mesopotamic rivers. Doubtless there is abundant literature on the major

Chinese systems but this is not available in translation at this time although some Soviet studies on the

Amur River have been published.

All these works and many others combine to give a body of information on the fish and fisheries of

the world as presented in this Technical Paper. This information indicates that while there is a

considerable amount of diversity in rivers, some general conclusion can be drawn. Firstly, although

potamon and rhithron differ considerably the form and behaviour of these two major river zones

appears consistent irrespective of continent or latitude. In other words rhithron reaches resemble one

another wherever they are as do potamon reaches and each may be considered as forming a set

permitting information and data to be pooled irrespective of geographic origin. Secondly, the

dynamics and behaviour of fish communities in the potamon of flood rivers is different from those of

reservoir rivers (and those whose flows have been artificially modified by man). Thirdly, the biology

and ecology of many of the fishes in flood river is so finely attuned to the seasonal flood that

modifications to the hydrological regime produce changes in the composition and productivity of the

fish community consistent with the conversion from flood to reservoir conditions.

This technical paper summarizes the present state of knowledge on the fish and fisheries of river

systems although certain limitations are imposed by the need to contain the size of the document. It

emphasizes the role of rivers as food producers and, although other uses of the fish communities are

referred to, detailed analysis of the extensive literature on sport fishery management and practice is

omitted. The first section deals with the physical and chemical environment and briefly summarizes

those aspects of primary and secondary productivity of rivers in so far as they are significant for

fisheries. A complete treatise on the limnology of running waters is inappropriate here and would

require a volume this size for this topic alone. The analysis of the biology and ecology of the fish

concentrates on larger rivers as these are the locations of the major fisheries and, although mention is

made of the biology of rhithronic communities, the extensive literature on salmonid rivers is not

developed to the full as this has been published elsewhere. Consideration of the fisheries of rivers is

of necessity speculative as the quantity and quality of the statistical data available impose limits on

the depth of analysis. Nevertheless fishery scientists are increasingly called upon to make rapid

proposals for management strategies or evaluations of possible impacts of interventions by other users

within the river basins so some generalized models are needed. Clearly such models must be open to

criticism and eventual modification in the light of future experience. Similarly, the section on

management tends to examine the effects of management on the fish community and does not explore

fully the social and economic implications of the various approaches to the regulation and

12

improvement of fisheries in rivers, a more detailed treatment of these topics being found in more

specialized works.

CHAPTER 1

MORPHOLOGY OF RIVER SYSTEMS

FORM OF RIVER SYSTEMS

TYPES OF RIVER

Rivers are linear systems which serve to evacuate water falling on the continental masses towards the

oceans. This transfer involves the dissipation of the kinetic energy inherent in the water and the

morphology of river channels evolves so as to even out the loss of this energy along the length of the

river. The hydraulic processes arising from this loss act in a predictable manner within the river

channel, thus the forms taken by the various rivers of the world are closely comparable where

conditions of bed-rock, elevation and rainfall are similar (Leopold et al., 1964). In effect, greater

differences exist between the various zones of one river than between homologous zones of different

rivers. Thus biological studies on rivers tend to treat sub-sets of river systems, such as “trout streams”

or “potamon reaches”, rather than to consider the system as a whole from headwater to mouth.

However, such subdivisions are for convenience of study only and any river system should ultimately

be viewed as a continuum showing a succession of characters along its length.

Features of the geography of any particular river basin can impose certain characteristics on the river.

One useful distinction is between: (i) reservoir rivers, which have extensive lakes, swamps or

floodplains near their headwaters resulting in the gradual release of floodwaters and sustained flow

with only slight variations in rate; and (ii) flood rivers, where there are extremes of annual fluctuation

in water level from severe flood to sometimes complete desiccation in the dry season. The most

extreme form of flood rivers, i.e. those which frequently cease to flow or even dry out seasonally,

have been termed “sandbank” rivers by Jackson (1961a and 1963).

A second distinction originates from the type of landscape through which the river flows. Here (i)

tropical forest rivers have many of the characteristics of reservoir rivers in that variations of flow are

evened out by the retention of water in the flooded forest. Such rivers tend to have black waters with

low pH, low conductivity and ionic content, low silt load and high humic content; (ii) Savannah rivers

may be of either sandbank or flood type, depending on the form of their basins. The pH of their waters

is rarely extreme, varying from slightly acid to slightly alkaline, conductivities are often reasonably

high as are silt loads; (iii) Desert rivers, which receive no tributaries in their dry land course, tend to

conform to the flood type. They show greatly increased conductivity and alkalinity along their lengths

as the water becomes concentrated by evaporation, and in their more extreme form end up as salt

marsh or lake; (iv) Tundra rivers drain the arctic and sub-arctic regions northwards and tend to have

flow regimes that are dependent on winter freezing. Their ionic contents are frequently poor as the

lands over which they flow were denuded of topsoil during the glaciations. Mixed systems also occur,

and larger rivers, especially, may change their nature several times during their course. Equally,

developments within their basin may change what were once forest rivers into savannah rivers, and

eventually by erosion, siltation and water use into desert rivers.

Considerable modifications have been carried out in many river systems, particularly in the temperate

zone where there are few large water courses which now show all their original features, and the

13

numbers of reservoir rivers, in the form of regulated streams, are steadily increasing throughout the

world due to interventions in the aquatic system aimed at controlling river flow.

LONGITUDINAL PROFILE

Rivers tend to have longitudinal profiles which are concave open to the sky (Fig. 1.1). This means that

within any one river, there is typically a succession of types of water course with steep slopes near the

source to minimal slope near the mouth. This succession is by no means always adhered to, and many

major rivers, through accidents of terrain, alternate between fast-flowing, rocky and slow-flowing,

muddy stretches. Thus, after torrential upper courses, rivers such as the Niger (Fig. 1.1), the Congo or

the Danube show several successive reaches of floodplain and rapids along their length. The different

types of water course plainly support different communities of living organisms and this has formed

the basis for several systems of geographical and ecological zoning. Geographically, such zoning is

reflected in everyday speech, which distinguishes between a variety of types of water course,

including torrents, creeks, brooks, streams, rivers, etc. Ecologically, too, such distinctions have value

as they generally correspond to many differing conditions including flow, slope or bottom type, which

determine the types of plant and animal community living in them. Because of the variety of types of

flowing water that are recognized, it is not possible to produce a comprehensive classification of

rivers that is ecologically satisfying, although some authors such as Petts (1984) have recently

published such listings. However, a fundamental difference does occur between fast-flowing streams

and slow-flowing rivers. This corresponds to a classification proposed by Illies and Botosaneanu

(1963) after examination of the many schemes proposed for river zonation which divides the river

course into two main classes - the rhithron and the potamon.

14

Figure 1.1 Longitudinal profiles of river systems:

A. Theoretical ideal profile; B. The Niger River

The rhithron is defined as the region extending from the source to the point where mean monthly

temperatures rise to 20°C, where oxygen concentrations are always high, flow is fast and turbulent

and the bed is composed of rocks, stones or gravel with occasional sandy or silty patches. The

rhithron is subdivided into three zones - the epi-, meta- and hypo-rhithron covering a range of water

courses from strong streams to small rivers.

15

The potamon is the region where monthly mean temperatures rise to over 20°C, oxygen deficits may

occur, flow is slow and the bed is mainly sand or mud. Three sub-zones are distinguished - the

epipotamon, the metapotamon and finally, the hypopotamon, which is that brackish water zone

affected by marine waters.

Because under this system temperature is important in defining the various zones, the change-over

from rhithron to potamon tends to be at higher altitudes in the tropics, where at low altitudes a true

rhithron zone may be entirely absent. This accounts for the differences in emphasis in studies or river

biology in the temperate zone, which have almost completely addressed the rhithron and in the

tropics, which are much more preoccupied with the potamon. However, because of the fundamental

differences between animal and plant communities of torrential headwaters and slow-flowing lowland

rivers regardless of temperature, it is useful to conserve this distinction even for tropical systems.

Thus in this work I have adopted the term rhithron to cover the steeper, rocky, torrential upper reaches

of rivers and potamon to cover the slow, mature lowland reaches.

TRIBUTARY DEVELOPMENT AND STREAM ORDER

Stream order

The dendritic arrangement of the channels of a river throughout its drainage basin is well known.

Several suggestions for ranking streams forming this type of pattern have been proposed. The most

widely accepted is that whereby streams are categorized according to order in a hierarchy defined as

follows: first-order streams are those having no tributaries, second-order streams are formed by the

union of two first order streams, third order streams by the union of second order streams and so on.

In its original form the system provided for one stream, usually the longest, of each category to be

extended headwords in such a way that the main channel of the river extends continuously from

source to mouth (Horton, 1945) (see Fig. 1.2). Later modifications of the system suppressed this idea

in favour of the more simple classification of all streams of the same order into one class (Strahler,

1957).

For ecological studies of rivers, each system has its advantages. The former is of use when

considering the evolution of some characteristic, for example fish catch, along the whole length of the

river. The latter is a more natural grouping and is useful in generalized studies in that streams of any

particular order tend to form sets, members of which can be considered together. Sudden changes in

faunal abundance are not uncommon below the junction of streams, particularly those of similar

order, where abrupt differences in flow, sediment load and other hydrological factors produce

correspondingly gross changes in the channel of the river. These, in turn, lead to a shift in the

ecological factors favouring one species group over another.

Clear relationships emerge between the numbers and lengths of streams of each order, whichever

system of ordering is adopted. These show that the number of streams of different order in a

watershed increases with decreasing order, according to a logarithmic relationship of the form:

Ns = a.bs,

where Ns = Number of streams of any order and s = order.

The length of stream of any order (Ls) decreases with decreasing order (s) in a similar manner:

Ls = x.ys.

The factors for a, b, x and y will vary according to continent or climatic zone.

16

Figure 1.2 Different systems for ordering river systems:

A. Horton (1945); B. Strahler (1957); C. Strahler's system as applied to the Logone River at the

height of Moundou

These relationships show that there are a very large number of small tributaries whose combined

lengths make up a considerable percentage of the total in any system (Table 1.1). For example,

Leopold et al., 1964, estimated that, of the 5 million kilometres of United States' streams with a mean

length equal to or greater than 1.6 kilometres over 90% were located in streams of fourth-order or

less. The relevant formulae are:

Ns = 7 661 837 (-1.58s)

and

Ls + 0.697 (0.8320s)

17

Table 1.1

Number and length of river channels of various sizes in the USA (from

Leopold et al., 1964)

Order a Number Length km) Total length % Cu.

1 1 570 000 1.6 2 512 000 48.39 48.39

2 350 000 3.7 1 288 000 24.83 73.20

3 80 000 8.5 674 400 13.07 86.26

4 18 000 19.2 345 600 6.66 92.92

5 4 200 44.9 188 160 3.62 96.55

6 950 102.4 97 280 1.87 98.62

7 200 235.2 47 040 0.91 99.33

8 41 540.8 22 173 0.43 99.45

9 8 1 243.2 9 946 0.19 99.94

10 1 2 880.0 2 880 0.05 100.00

5 191 479

a According to Strahler

Low-order streams are often of a torrential, rhithronic nature, whereas potamon reaches tend to be

concentrated around higher-order rivers. Low-order tributaries in lowland areas tend to be potamonic

in character especially in the tropics.

Relationship between river length and drainage basin area

The area of drainage basins and the length of streams within them are also related. Thus for the fifty

largest rivers of the world, as ranked by mean annual discharge, a relationship:

L = 1.7084A0.5418 (r² = .70)

emerges, where L = the length of the main channel in kilometres and A = the area of the drainage

basin in km². When analysed by region the coefficient and the exponent of this relationship vary, as

shown in Table 1.2.

Table 1.2

Relationship of length of main river to basin area for several areas of the world

U.S.S.R. North flowing rivers L = 0.8747 A0.5901 r² = 0.97 n = 6

N. America, Canada L = 0.9618 A0.5789 r² = 0.81 n = 6

N. America, U.S.A. L = 1.2528 A0.5609 a

Europe L = 1.5421 A0.5420 r² = 0.56 n = 7

India L = 2.2475 A0.5138 r² = 0.61 n = 11

Asia, East flowing rivers L = 3.3608 A0.4849 r² = 0.37 n = 24

S. America L = 3.4641 A0.4843 r² = 0.93 n = 16

Africa L = 4.9500 A0.4521 r² = 0.76 n = 30

a Derived from Leopold et al., 1964

The coefficient (a) and exponent (b) are themselves related for this data set by the equation:

18

b = 0.5797a-0.1492

Similarly, Gregory and Walling (1973) summarized data on variations in drainage density for various

parts of the world where total channel lengths are related to area. These, together with the

relationships in Table 1.2 indicate that there are between 1.6 and 8 kilometres of channel per square

kilometre of land surface, depending on landform and rainfall.

MORPHOLOGY OF THE RHITHRON

The main morphological characteristic of rhithron reaches is the alternation of pools and riffles which

arise from changes in gradient (Fig. 1.3). The steeper epi-rhithron is dominated by rapids, waterfalls

and cascades, but as the river proceeds downstream, the proportion of pool-like reaches relative to the

riffles increases and eventually the hypo-rhithron merges into the potamon. Riffles are steep, shallow

zones having coarse bottoms of boulders, rocks or pebbles. Pools are flatter, deeper zones with

bottoms of finer material. They may be more complex in form, especially in the hypo-rhithron, with

diverticula and backwaters having muddy or detritus bottoms, giving greater ecological diversity.

According to Leopold et al., 1964, one pool-riffle sequence always occupies a length of channel

equivalent to between five and seven channel widths, irrespective of the form or geographic location

of the river. The ratios between the relative lengths of pool or riffle lying within this segment do,

however, differ considerably. The productivity of rhithron reaches is frequently expressed in terms of

pool-riffle ratios, in which a slight dominance of the riffle component is deemed favourable for

salmonids because of the increased invertebrate food to be found in these zones.

Historical evidence presented by Sedell and Luchessa, (1981), and the existing situation in streams in

some less settled parts of the world, indicate that the present appearance of upland streams throughout

the temperate and much of the tropical world is very different from that pertaining under regimes

unmodified by man. A considerable portion of the inhabited world was once forested and streams

running through such areas received large quantities of wood, vegetation, boulders and other flow

modifying structures such as beaver dams. Thus streams up to 7th order were, and still are in some

parts of the world, conditioned by such obstructions which produced much better regimes than are

common today, with extensive overbank flooding, greater retention of water outside the stream

channel, considerable instream floodplain storage of carbon and more diversified habitat structures,

(Swanson et al., 1982).

Few attempts have been made to classify the rhithron into morphological types other than pools and

riffles. The variability that exists within these two major habitat types is such that further division is

needed if the areas are to be adequately described. One attempt at such a division has been made by

Bisson and Sedell (1982), who, taking US salmonid streams as an example, produced a typology

which seems sufficiently flexible to encompass rhithronic streams in other areas of the world. This

classification subdivides the various habitats as follows:

Riffle: Low gradient riffles - shallow stream reaches with moderate current velocity and turbulence.

Substrate usually gravel, pebble and cobble sized particles.

Rapids - gradient greater than 4 percent with swiftly flowing water and considerable turbulence.

Substrate generally coarser with larger boulders.

Cascades - steep reaches with variations in gradient consist of alternating small waterfalls and

shallow pools. Substrate consists of boulders.

Pools: Secondary channel pools - often temporary pools remaining in areas flooded by freshets,

often associated with gravel bars; sand or silt substrate.

19

Backwater pools - found along channel margins caused by eddies from large obstructions. Often

shallow and dominated by fine-grained substrates.

Trench pools - long, often deep, slots arising downstream of major obstructions. Usually with

coarse-grained but stable bottoms.

20

Figure 1.3 The morphology of the rhithron showing the division of the channel into a sequence of

pools and riffles

Plunge pools - arise where a stream passes over a major obstacle and scours out the bottom.

Variable in depth and substrate.

Lateral scour pools - occur where flow is diverted by a large obstacle or obstruction.

Dammed pools - consist of water impounded upstream of a complete or nearly complete channel

blockage. Tends toward low current and finer substrates.

Glides: a third general habitat category consisting of moderately shallow reaches with even flow that

lack obstructions or pronounced turbulence. Bottoms usually gravel and small cobbles. Glides

are particularly common in large rivers passing through mountainous terrain where channels may

run relatively straight for many kilometres.

As rhithron reaches are for the most part situated in low-order streams, their flood regimes are apt to

be somewhat “flashy” with rapid changes in discharge. The water is usually confined within a well

defined bed, although peak spates may flood a narrow strip fringing the channel. During periods of

high discharge, the riffle pool complex tends to be completely submerged and from the surface the

two components may appear to lose their identity. At intermediate discharges the riffle has turbulent

flow while the pool has laminar flow, often with relatively calm areas at the banks. As discharge falls

the distinction between riffles and pools increases until eventually, when flow ceases completely, the

riffles are left dry and the pools retain water giving the channel the appearance of a string of beads.

At slightly lesser slopes for the same bankfull discharge braided channels arise. These consist of many

anastomosing anabranches, which may themselves meander, winding among rocky, sandy or

vegetated islands which are exposed at low water and flooded at high water (Fig. 1.4). Pool and riffle

systems may appear within the individual anabranches in steeper and stonier reaches.

21

Figure 1.4 An example of a braided channel; the Niger River at Ayourou

MORPHOLOGY OF THE POTAMON

The potamon of reservoir and regulated rivers consists only of the channel which may be meandrine

or braided in form. In flood rivers, however, there are two major components to the potamon: (i) the

channel, and (ii) the floodplain which correspond to the French terminology “lit mineur” and “lit

majeur” and represent the beds of the river in its two main phases - low water and flood.

The form of many of the world’s floodplains has undoubtedly been changed considerably by human

activities over the last few thousand years. Prior to human occupancy, it seems likely that most rivers

were associated with riparian gallery forests, and that, in more arid areas, the plains themselves were

covered with scrub of the bush savannah type. These conditions still persist in some Latin American

rivers, and give water regimes and channel forms which are more stable than those of the open

savannah rivers which have resulted from clearance of the trees for agriculture, grazing and firewood.

According to Leopold et al. (1964) floodplains will typically include the following features:

(a) the river channel;

(b) oxbows or oxbow lakes representing the cut-off portion of meander bends;

(c) point bars - loci of deposition on the convex side of curves in the river channel;

(d) meander scrolls - depressions and rises on the convex side of bends formed as the channel

migrates laterally down valley by the erosion of the concave bank;

22

(e) sloughs - areas of dead water formed both in meander-scroll depressions and along the valley

walls as flood flows move directly down valley scouring adjacent to the valley walls;

(f) natural levees - raised berms or crests above the floodplain surface adjacent to the channel,

usually containing coarser materials deposited as floods flow over the top of the channel banks.

They are most frequently found at the concave bank. Where most of the silt load in transit is fine-

grained, natural levees may be absent or nearly imperceptible;

(g) backswamp deposits - overbank deposits of finer sediments deposited in slack water ponded

between the natural levees and the wall or terrace riser;

(h) sand splay - deposits of flood debris usually of coarser and sand particles in the form of splays or

scattered debris.

These features, which are illustrated diagrammatically in Fig. 1.5, determine the quantity, distribution

and flow of water in the system throughout the year. Most of them are readily distinguishable in

moderate sized floodplains, but in smaller valleys are obscured by the rapidity with which changes

occur. Individual features of the floodplain are briefly described below (numbers in text refer to Fig.

1.5).

The channels - the lotic component

The main channel (1) or channels of the river and its anabranches (2) usually retain water, but not

necessarily flowing water, at all times of the year. As the river enters its alluvial plain it starts to

meander forming wide convoluted channels whose curves are proportional to river width (Leopold et

al., 1964). In some larger rivers, the Amazon and Congo, for example, braided channels occur in

which the islands form levees with depression lakes at their centre. Where such braided channels

occur the lateral floodplain is sometimes limited in width and its whole extent may come to be

contained within the main channel. In these instances the islands are analogous to the lateral

floodplain and fulfil a similar role in the biology of the fish (Svensson, 1933; Gosse, 1963).

In addition to the main channel or channels of the river, floodplains have a network of channels and

creeks which penetrate the levees to connect the main river with the back-swamps and meander scroll

lakes. Such channels may or may not retain water at all times of the year, but they represent the main

path of water and fish movement during the earlier periods of rising water and later phases of falling

water.

On many floodplains naturally occurring channels are supplemented by artificial canals constructed

for navigation, irrigation, drainage or even fisheries.

The floodplain

The floodplain is itself divided into two components: (i) the plain itself which is seasonally inundated,

but remains dry for at least part of the year, and (ii) the standing waters which remain in the plain

during the dry season.

The plain (seasonally inundated component)

The alluvial plain of a river can be divided into two main zones: Firstly, the levee regions (7), which

more or less follow the course of the river channel and its former beds, consist of raised areas that are

flooded for the shortest time annually; and secondly, the flats, which extend from the levee to the

terrace or plateau delimiting the plain. Exceptionally high levees sometimes occur immediately

adjacent to the channel which is only occasionally submerged by the highest of floods. Such areas,

23

together with the raised terraces bordering the plain are used for human habitation, or give island

refuges for cattle or wild game during the flood season. Levees may be much reduced or even

completely absent in rivers carrying very fine silt. Because coarser material is deposited early after the

slowing of the flow as a river enters the plain, there is a tendency for the raised areas bordering the

river channel to diminish in height downstream. In river basins with exceptionally high silt loads, for

example the Chao Phrya delta in Thailand prior to the installation of flood control structures (see Fig.

1.6), the process of levee building and of deposition within the main channel may raise the river

channel and levee high above the surrounding plain.

Figure 1.5 Diagram of the main geomorphologic features of a floodplain; numbers refer to

description in text

24

25

Figure 1.6 Plan and cross-sections of the Chao-Phrya delta, Thailand, before the installation of flood

control structures, showing channels raised within levees above the level of the surrounding

plain. (After Ohya, 1966)

The flats, which make up the greater proportion of the plain, show slight differences in relief due to

old depositional features. More depressed parts are interspersed with standing waters of various types.

The edges, adjacent to the terrace or valley wall delimiting the plain, may be more deeply excavated

where locally increased flow scours deposits to form lagoon or backswamp complexes (6).

Irregularities may also be formed where inflowing tributaries deposit alluvial fans. Both the levee and

the terraces may be covered with dense gallery forest (see Figs. 1.7a and 1.7b), but many floodplains

are occupied only with sparse scrub or are denuded of trees altogether (savannah plains).

Figure 1.7 Schematic profile of: A. The Amazon (after Sioli, 1964), and B. The Middle Parana (after

Bonetto, 1975), showing distribution of vegetation and main floodplain features

26

Standing waters (the lentic component)

Permanent or semi-permanent standing waters are left by receding floods in the form of sloughs in

oxbows (4), meander scroll depressions (5), backswamps (6), or the residual channels left by the

former course of the river (3). These water bodies expand and contract according to the annual flood

cycle (Fig. 1.8) and during the highest floods tend to merge into a continuous sheet of water covering

the whole plain.

Figure 1.8 Annual cycle of flooding of a typical floodplain depression of the Senegal River (Vindou

Edi). (After Reizer, 1974)

Distinction is often made between lakes, lagoons or pools on the one hand and swamps on the other.

Although the terms lake, lagoon and pool have been used interchangeably in the literature, and refer

more or less impartially to bodies of water of some depth and slight to moderate vegetation cover,

there is a useful distinction to be made between lakes, as large features of a floodplain system which

27

persist relatively unchanged over a number of years, and lagoons and pools as more transitory open

water feature. Lagoons remain connected to the river throughout the year, whereas pools are usually

smaller and more ephemeral bodies of water which become isolated and have a tendency to dry out in

the dry season. The term “swamp” is applied to those depression wetlands whose soil remains

saturated or more or less permanently covered with shallow waters and which support, as a result,

characteristic growths of vegetation which dominate the environment.

Permanent standing waters of the floodplain are generally shallow, rarely exceeding 4 m in depth and

may be in communication with the river. Alternatively, their deepest point may lie below the water

table of the plain enabling them to remain wet throughout the year.

Water bodies on the floodplain lose water by evaporation and to a lesser degree by filtration

throughout the dry season. This results in the contraction and eventual drying out of many water

bodies with a concomitant concentration of dissolved substances. In most river systems this does not

produce any appreciable result, as concentrated solutions are rapidly washed out by the next flood and

the average conductivity of the water remains low. However, in some regions of high salinity or one

way flow, temporary brackish pools or salt pans can result. The most extreme examples of such areas

are perhaps the one million hectare Plain of Reeds of the Mekong Delta, where there are

extraordinarily high concentrations of alum and the Ovambo floodplain which terminates in the saline

Etosha pan, (see Fig. 1.14) and the Helmand River in Afghanistan with its terminal Sistan Marshes.

Very large lakes or groups of lakes are associated with some floodplains. These are sometimes

distinct entities geologically, but ecologically they are usually completely integrated into the

river/floodplain system. The greatest of such lakes, the Grand Lac of the Mekong (Fig. 1.9) floods an

area of 11 000 km², most of which was forested, but reduces to about 2 500 km² of open water in the

dry season. It is connected to the main river by a broad channel with reversible flow, the Tonle Sap.

28

Figure 1.9 The delta of the Mekong River with the Grand Lac and Tonle Sap

A similar but smaller feature is found in the Senegal River valley where the Lac de Guiers is

connected to the main channel by the River Tawey. The lake has an extreme dry season area of 120

km² and expands to twice this when flood waters flow into it through the Tawey in August and

September. The lake district of the Niger Central Delta (Fig. 1.10) forms part of the general flood

system at high water, but breaks up into 18 major lakes with a combined area of 2 400 km² at low

water. The 11 000 km² Kamelondo depression floodplain of the Lualaba contains some 50 permanent

29

and semi-permanent lakes, having a total area of 1 545 km² and of which the largest, Upemba, has a

dry season area of 530 km². In the Magdalena River (Fig. 1.11) the waters are confined within some

800 lakes (cienagas) of varying size and permanence at low water. Their total area is about 3 400 km²,

although some individual water bodies, for example the Cienagas, Zapatosa (119 km²) and Ayapel

(123 km²) are of considerable size.

Figure 1.10 The internal delta of the Niger river and its tributary the Bani, in Mali, showing the

numerous lakes at the northern end of the plain

The varzea floodplain of the Amazon is similarly interspersed with lakes, some of which are very

large. There are 20 lakes of over 50 km² and the Lago Grande do Curuai covers 630 km². While these

resemble the floodplain lakes found in other systems a special type of water body is also found in the

Amazon. The clear water tributaries to the lower and middle reaches of the river have large widened

30

mouth bays formed originally from drowned valleys. Such “river-lakes” are the site of sedimentation

at their upstream ends where new varzea type floodplains are formed from the deposits.

The individual water bodies of the floodplain usually persist over long periods during which they age,

although particularly high floods can produce sudden changes through scour and deposition. The

ageing process consists of a natural succession from open water lake to dry land via the intermediate

stages of shallow lagoon and swamp ecosystems. Botnariuc (1967), in particular, has studied the

processes involved in the transformation of the lakes of the Danube floodplain. Here the transition

from lake (“Ghiol”) to reed-grown pond (“Japse”) is caused mainly by the growth of emergent

vegetation which slows water currents and accelerates siltation thus allowing a further extension of

the vegetated area, although the process is partially reversible when higher floods may scour deposits

and vegetation from the lake bottom. In the Amazonian floodplain too, the progression from water

bodies invaded by aquatic grasses, through shrub vegetation to different stages of floodplain forest,

noted by Junk (1983) is frequently prevented from reaching its climax through scour by exceptional

floods. Siltation can be detected in the deposition of new varzea in the mouth lakes of the Amazon or

in the change in form of lakes and channels viewed as a time series by remote sensing. Side looking

radar and satellite imagery show floodplains to be littered with the remains of old floodplain features

now silted over. The succession may also be reconstructed from series of lakes such as those

described by Green (1972) for the meander complex of the Suia Missu River in Brazil, or by Castella

et al. (1984) for the relic beds (“lones”) of the Rhone river in France. The disappearance of older

features of the floodplain in this way is compensated for by the generation of new bodies of water by

the constantly changing course of the main river channel. The speed with which this occurs is related

to the discharge of the river and its silt load, but it is to be supposed that, given relative constancy of

these parameters, the ratio of open water bodies on the plain to total area remains relatively

unchanged through time.

31

Figure 1.11 The internal delta of the Magdalena River at its confluence with the Cauca and San

Jorge rivers in Colombia showing the numerous floodplain lakes (cienagas)

Proportionality of floodplain features

The proportion of the floodplain which remains permanently under water is generally as difficult to

establish as is the total area submerged at the peak of the floods. There is for the most part a lack of

reliable maps showing floodplain features and even where they do exist the period of mapping rarely

coincides with the period of minimum water. However, the use of satellite imagery presents a possible

solution to this problem in areas where cloud cover remains within acceptable limits. Further

complications arise from the instability of floodplain features and the changes wrought by man

himself. In most populated areas the floodplain and its hydraulic regime have been considerably

modified by the digging of canals, the raising of artificial levees and the levelling of depressions.

Some information on relative areas is available from the best studied floodplains in Africa (Table 1.3).

As flood regimes vary in the intensity of both their flood and dry season components, values for high

water and low water areas fluctuate about a mean. The figures quoted in Tables 1.4 and 1.5 should be

treated as such averages, being only approximations derived from relatively small-scale maps.

In Africa the area of permanent water ranges from 5 to 59 percent of the total flooded area, with a

pronounced mode at between 10 and 20 percent. There is insufficient information to judge whether

significant differences exist between the flood ratios of the various categories of floodplain and these

may depend more on soil type or local climate. It is difficult to describe the simple ratio between

32

flooded and permanent water areas. Indeed the extent to which the various depositional features of the

plain can change from season to season makes such figures of only temporary value, except as

indicators. Table 1.5 shows, for some African plains, the proportion of the permanent waters that

remain on the floodplain as standing waters and those located in the divers river channels.

There are pronounced differences in this proportion and the smallness of the sample does not permit

definite conclusions to be drawn on the possible causes of this. However, it might be expected that on

floodplains where agriculture is intensively practised during the dry season, much of the standing

water and particularly the swamps will tend to disappear through drainage and fill. This, in fact,

appears to be the case for the cultivated Senegal, Ouémé and Pongolo plains as compared with the

Niger, Lualaba and Kafue plains, which are used primarily for cattle grazing. The main exception to

this is the Shire, in which a large amount of the standing water is classified as permanent swamp.

Unfortunately, the dividing line between swamp and lagoon is somewhat fine and vegetated areas of

lagoons are often placed in this category.

Table 1.3

Characteristics of some African floodplains

Floodplain Area at

peak flood

(km²) ‘A’

low water

(km²) ‘B’ B/A × 100 Authority

Senegal R. Mean for total system 5 490 800 15 OMVS

Niger R. Central delta 20 000 3 877 19 Raimondo, 1975

Fringing plain

Niger 907 270 30 FAO/UN, 1971

Fringing plain

Benin 274 32 12 FAO/UN, 1970

Fringing plain

Nigeria 4 800 1 800 38 FAO/UN, 1970

Benue R. Fringing plain

Nigeria 3100 1290 42 FAO/UN, 1970

Ouémé R. Coastal delta 1000 52 5 Pers. observations

Chari and Yaeres 7 000 NI - Ali Garam, pers.comm.

Logone R. Total system 63 000 6 300 10 Blache, 1964

Zambezi R. Barotse 10 752 537 7 FAO, 1969

Okavango Internal delta 17 000 3 120 20 Cross, pers. comm.

Pongolo R. 100 26 26 Coke and Pott, 1970

Kafue R. Kafue flats 4 340 1 456 27 Gay, pers. comm.

Shire R. Elephant and

Ndinde marshes 665 200 30 Hastings, pers. comm.

Total sytem 1 030 480 48

Luapula R. Kifakula (lagoon) 1984 195

depression (river) 1 500 75 13

Total 266

Lualaba R. Kamulondo

depression 11 840 7 040 59

Nile R. Sudd 31 800 16 300 51.2 Mefit-Babte, 1984

Volta R. Fringing plain

Ghana 8 532 1 022 12 Vanderpuye, pers.comm.

Ogun R. Fringing plain 43 25 59 Dada, pers. comm.

Oshun R. Fringing plain 37 20 73 Dada, pers. comm.

Masilli R. Fringing plain 15 2 13 Dada, pers. comm.

33

Table 1.4

Areas at peak flood of some Central and South American floodplains

Floodplain peak flood Area at high water (km²) Authority

Grijalva/

San Antonia R. Delta 8 000 USAF1

Atrato R. Delta 5 300 USAF

Magdalena R. Delta 20 000 Pardo, 1976

Catatumbo R. Delta 5 000 USAF

Orinoco R. Apure/Arauca

Internal delta 70 000 Matthes,

pers.comm.

Orinoco R. Coastal delta 20 000 USAF

Rupununi R. Internal flood zone 6 500 USAF

Amazon R. Central delta 50 000 Sioli, 1975

Amazon R. Coastal delta 25 000 Sioli, 1975

Paraguay R. Gran Pantanal 80–100 000 Bonetto, 1975

Parana R. Fringing plain 20 000 Bonetto et al.,

1 United States Operational Navigation Chart - Scale 1:1 000 000

Table 1.5

Composition by area of different types of permanent water on floodplains during the dry season

Standing waters

Floodplain

River and

channels (ha²) Lagoons (ha²) Swamps (ha²) Total (ha²) Total area

(ha²)

Niger R. Central delta 61 300(16) 300 400(77) 26 000 (7) 316 400(84) 389 700

Lualaba-Kamulondo 29 400 (4) 154 500(23) 480 000(72) 643 500(96) 663 900

Kafue-Kafue Flats 5 380 (4) 10 180 (7) 130 000(89) 140 180(96) 145 560

Shire 2 000 (4) 5 500(12) 40 599(84) 46 000(95) 48 000

Pongolo 392(14) N.D. N.D. 2 428(86) 2 820

Ouémé 1 402(27) 3 768(73) slight 3 768(73) 5 170

Senegal valley 28 100(57) 21 800(43) slight 21 800(43) 49 700

( ) Percentage of total

N.D. No distinction made

The various floodplain features are sufficiently important in the lives of the peoples inhabiting major

rivers for a particular terminology to have arisen in many languages. While by no means exhaustive,

Table 1.6 lists the main names that have been used in the literature by workers from various countries

to describe individual features.

Table 1.6

Glossary of terms for floodplain features in various regions of the world

Country

34

Levee Floodplain FEATURES Depression

lagoon or swamps Channel and side

arms

Benin Tikpa Ti

Brazil Varzea Lago de varzea Parana/Igarape

Cambodia Veal Beng Prek

Colombia Cienaga Cano

India Bheel Jheel

Romania Ghiol:Japse

Papua Roundwaters Barats

N. Guinea

Senegal Fonde Oualo Vindo Tiangol

Sri Lanka Villus

Sudan Toiche

Sumatra Lebaks (temporary)

Lebungs (permanent)

Congo Madzibe (large lagoon)

Edzibe (smaller Malala

oxbows)

Types of floodplain

It is perhaps hazardous to attempt any definitive classification of floodplains. However, three general

types can be discerned whose characteristics are sufficiently different that they may influence either

the behaviour of the fish populations inhabiting them, or the problems faced by a fishery.

Fringing floodplains: Nearly all tropical and sub-tropical rivers and many temperate ones have a

lateral flood zone. This takes the form of a relatively narrow strip of floodable land lying between the

river valley walls (Fig. 1.12). Fringing floodplains are normal developmental features of a river which

follow its course in all areas where the slope is favourable. They tend to increase in width the less the

slope of the river and this generally means a progressively greater elaboration of the floodplain along

the river's course to the sea.

35

Figure 1.12 Floodplain fringing the Niger river at the level of Gao in Mali

Internal deltas: Occasionally, river systems encounter geological features which cause them to spread

laterally over very large alluvial plains. Such features may be the site of a former lake filled with

alluvium, for instance the Yaeres of Lake Chad; the deltaic discharge of a small river into a larger

one, such as is found where the Apure flows into the Orinoco; or the backing up of the aquatic

36

systems by an obstruction downstream, the Kafue Flats for example. Such floodplains may occur at

any point along the course of the river. The main stream usually becomes divided into anabranches

which rejoin the main channel below the deltaic area, or several rivers flowing into the same plain

may interconnect with complex and shifting channels. In certain parts of the world, the Bahr Aouk

headwaters of the Chari system, the Gran Pantanal of the Paraguay River or the Apure-Arauca

tributaries of the Orinoco, for instance, the terrain is so flat that enormous areas of land are flooded to

a very shallow depth by rain-water as well as by overspill from the river. Such “sheet flooding” is

often dictated by land form processes other than those created by the rivers themselves, but the

network of channels and lagoons that arise by the erosion during drainage give such areas a deltaic

character.

Coastal deltaic floodplains: The terminal lateral expansion of the alluvial plain and the break-down of

the main river channel into distributaries produce the classic fan-shaped delta. Coastal deltaic

floodplains are influenced by the marine environment in that, in the dry season, sea water penetrates

the main channels as a saline tongue. Tidal effects are often transmitted far upstream even beyond the

limits of the saline tongue, but little invasion by salt water occurs during the floods, and only the

coastal fringe is submerged by tidal action.

Certain floodplains are intermediate between the internal delta and the coastal delta. These occur in

those rivers which discharge deltaically into either inland freshwater lakes, such as the Yaeres systems

located at the place of discharge of the Chari-Logone into Lake Chad, or into rivers much larger than

themselves. The principal ecological differences between fringing and internal deltaic floodplains on

the one hand, and coastal floodplains on the other are, firstly, the greater extent of the water body

available to fish in the dry season in the coastal floodplain and, secondly, in the penetration of sea

water and marine species into areas adjacent to the sea.

BRIEF REVIEW OF MAJOR RIVERS

This review is intended to highlight those features of the rivers of the world that are particularly

significant to fisheries. Thus less emphasis is placed on the numerucal3ly dominant small order

streams or upper torrential reaches, than on features such as the degree of modification or the

elaboration of the floodplains with their associated lakes and swamps.

It is difficult to define accurately the total number and lengths of the world's rivers, although by

simple extrapolation some estimate can be made. The relationship for river length as a function of

basin area for the world as a whole has a coefficient of 1.7, implying that for any one square kilometre

of land surface there are on average at least 1.7 km of channel. Taking the drained surface area of the

world as 1.1 × 108 km², a minimum length of channel of 1.8 × 108 km results. This is a minimum

estimate as the original equation relates only main channel length to basin area and thus omits any

secondary channels.

The drainage pattern of most continents is dominated by few very large river systems, the length,

basin area and mean annual discharge of the fifty largest of which are listed in Table 1.7.

AFRICA (Fig. 1.13)

Upland and torrential rivers are fairly common in Africa for, although there are few large mountain

chains, the elevated nature of the mass of the continent and the existence of individual massifs means

that most rivers take their source in highland regions. For example, many of the West African rivers,

including the Senegal and Niger, take their source in the Fouta Djallon mountains of Guinea; the

Benue and Logone rivers arise in the Mondara mountains of Cameroon; the north flowing tributaries

of the Congo in the highlands of Angola; the Blue Nile in the Ethiopian highlands, etc. However,

nearly all African rivers have extremely well developed fringing floodplains in their lowland courses

of which perhaps the best studied are the savannah plains of the Senegal and Niger rivers. The

37

Senegal flows through a broad valley which, during the dry season, retains about 500 km² of water

confined in “sloughs” of various kinds (66 km²), as well as in the Lac de Guiers (150 km²) and the

main river channel (281 km²). At peak floods the river covers 5 000 km² of valley. The Niger and its

tributary the Benue river, also cover an extensive lateral plain. In the Republics of Niger, Benin and

Nigeria, the Niger itself flooded 5 981 km² at high water, shrinking to about 35 percent of this area in

the dry season. Much of the Nigerian plain of this river has since been lost due to the flood control

effects of the Kainji dam and its reservoir. The Benue river, also in Nigeria, has a very impressive

fringing plain, broad for its length which has a flooded area of 3 100 km² and a dry season area of 1

290 km².

Table 1.7

50 large rivers of the world ranked by mean annual discharge at mouth

RIVER CONTINENT /

COUNTRY

MEAN

DISCHARGE

1000 m³ sec-1

DRAINAGE

AREA 1000

km²

LENGTH

km RANK BY

LENGTH

1. Amazon S.America: Brazil 212.5 5711 6437 2

2. Congo/Congo Africa: Congo 39.7 3968 4700 8

3. Yangtze Asia: China 21.8 1920 5980 4

4. Brahmaputra Asia: Bangladesh 19.8 924 2900 31

5. Ganges Asia: India 18.7 1047 2506 48

6. Yenisei Asia: USSR 17.4 2560 5540 5

7. Mississippi/

Missouri N.America: U.S.A. 17.3 3184 6020 3

8. Orinoco S.America: Venezuela 17.0 870 2151 60

9. Lena Asia: USSR 15.5 23.96 4400 10

10. Parana S.America: Argentina 14.9 2278 3998 18

11. St. Lawrence N.America: Canada 14.1 1274 4000 16

12. Irrawaddy Asia: Burma 13.5 424 2100 63

13. Ob Asia: USSR 12.5 2455 5410 6

14. Mekong Asia: Thailand 11.0 793 4000 16

15. Amur Asia: USSR 11.0 1822 4444 9

16. Tocantins S.America: Brazil 10.2 896 2700 38

17. Mackenzie N.America: Canada 7.9 1784 4241 13

18. Magdalena S.America: Colombia 7.5 238 1600 -

19. Columbia N.America: Canada 7.3 660 1954 76

20. Zambezi Africa: Mozambique 7.1 1280 3500 14

21. Danube Europe: Romania 6.2 806 2850 34

22. Niger Africa: Nigeria 6.1 1100 4200 14

23. Indus Asia: Pakistan 5.6 916 2900 31

24. Yukon N.America: Canada 5.1 921 2654 44

25. Pechora Europe: USSR 4.1 322 1809 88

26. Uruguay S.America: Uruguay 3.9 230 1612 -

27. Kolyma Asia: USSR 3.8 637 2513 47

28. Sankai Asia: China 3.6 117 1957 74

29. Godavari Asia: India 3.6 294 1440

30. Dvina Europe: USSR 3.5 355 726

31. Hwang-Ho Asia: China 3.3 665 4845 7

32. Frazer N.America: Canada 3.2 235 1360

33. Nile Africa: Egypt 2.8 2944 6650 1

34. Sao Francisco S.America: Brazil 2.8 665 2900 31

35. Neva Europe: USSR 2.6 279 -

38

36. Pyasina Asia: USSR 2.5 189 1056

37. Nelson N.America: Canada 2.3 1059 2570 43

38. Rhine Europe: Netherlands 212 143 1312

39. Krishna Asia: India 2.0 304 1120

40. Indigirka Asia: USSR 1.8 355 1725 95

41. Dnepr Europe: USSR 1.7 496 2200 58

42. Rhone Europe: France 1.7 94 816

43. Mobile/Tombigbee N.America: U.S.A. 1.6 107 598

44. Salween Asia: Burma 1.5 276 2400 52

45. Tigris/Euphrates Asia: Iraq 1.4 535 1900 81

46. Po Europe: Italy 1.4 69 648

47. Vistula Europe: Poland 1.1 194 1084

48. Susquehanna N.America: U.S.A. 1.1 71 710

49. Yana Asia: USSR 1.0 243 1067

50. Senegal Africa: Senegal 0.9 338 1633 100

39

Figure 1.13 Location of the major rivers and floodplains of Africa:

1. Senegal 7. Lualaba 13. Shire

2. Niger, Central delta 8. Barotse 14. Luapula

3. Ouémé 9. Okavango 15. Rufigi

4. Logone, Yaeres 10. Cunene/Ovambo 16. Ruaha

5. Chari 11. Pongolo 17. Nile, Sudd

6. Congo, Mbandaka 12. Kafue

The Zambezi has two plains on its upper course which flank the river for 240 and 96 km respectively.

They flood laterally for a greater distance than usual, penetrating up to 16 km² inland on either bank.

the combined area of these Barotse plains is over 10 000 km², but only 5 percent of this area remains

wet in the dry season.

40

The greatest of the African coastal deltas, that of the Nile whose floods were the basis of the wealth of

pharaonic Egypt, is no longer inundated seasonally. The fringing floodplains of the river have also

virtually disappeared due to the flood control measures of the series of dams barring the middle

course of the river. Savannah deltas occur in the Senegal river where nearly 8 000 km² are flooded

annually, and the Ouémé river, whose 1 000 km² delta terminates in the 180 km² brackish water Lac

Nokoue. The Niger delta covers 36 260 km² with a coastal fringe of saline mangrove swamps.

Internal deltas are common in Africa. The largest of these, that of the Niger (Figure 1.10), occurs

where sand blown from the Sahara has resulted in the deflection of the Niger river eastwards near

Timbuktu. A depositional plain has grown up behind this with lakes lying in the depressions between

rocky outcrops. It extends over 20 000–30 000 km² during the four to five month flood season, but its

area shrinks to 4 000 km² in the dry season, most of which is retained in the permanent lakes. The

Kafue river, backed up by a range of hills, which are now the site of the Kafue Gorge dam, forms an

alluvial plain of over 6 600 km² which is inundated almost completely during the rains. Only 1 456

km² of permanent waters remained throughout the year prior to the closing of the dam although the

duration and extent of inundation has been increased subsequently. The 7 000 km² Yaérés floodplain

is the site of the deltaic discharge of the Chari and Logone rivers into Lake Chad through the

Logomathia and El Beid rivers. The present lake and the floodplains of the lower Chari occupy the

site of a larger Paleo-Chad, now shrunk in size by the progressive desiccation of the Sahara Desert.

The Yaérés are only part of a much larger family of floodplains centred on the Chari and Logone

rivers. At least two other groups of plains subject mainly to sheet flooding by rainfall and local run-

off are found in the system. The largest of these extends from the Bahr Aouk and Bahr Salamat rivers

covering a considerable portion of southeast Chad and some 37 000 km² of northeastern Central

African Republic. The second group spreads between the Logone and Chari rivers and eastward along

the Bahr Erguig anabranch of the Chari. According to Blache (1964) the Chari/Logone basin

floodplains had a combined area of 90 000 km², of which about 70 percent were inundated at peak

floods (September-October) and only 7 percent remained wet in the dry months of April and May

during periods of normal rainfall.

Many African flood areas are associated with permanent swamp systems. The most famous of these

and the most extensive wetland in the continent is the Sudd of the river Nile. Located at the

confluence of the Nile and Bahr el Ghazal, the 16 300 km² permanent complex of papyrus swamp and

open water lagoon swells to twice this area when the floods of the Nile arrive from the south and

additional areas of variable extent are flooded by rainfall and local run-off. A large waterway, the

Jonglei canal, is currently being excavated to bypass the swamps and make more water available

downstream. The Elephant and Ndinde marshes of Malawi, which cover 673 km² when flooded,

shrink to 384 km² of swamps and lagoons in the dry season. They form part of the larger Shire river

system which covers a total flooded area of 1 400 km² or 480 km² in the dry season. A third large

swamp complex is centred on the internal delta of the Okavango river and the 800 km² Lake Ngami.

These swamps have a residual area of 3 120 km² and cover between 16 000 to 20 000 km² at high

water. The Kamulondo depression, one of the few true floodplain areas of the Congo, contains lakes

and swamps with a permanent water area of approximately 7 000 km², which nearly doubles during

the flood season. The Rufigi/Ruaha river system of Tanzania contains three large floodplains; the 1

451 km² fringing plain of the Rufigi river itself, the 4 400 km² Usungu plain of the Ruaha river which

includes the 518 km² Utungele swamp, and the 6 736 km² floodplain of the Kilombero tributary to the

upper Rufigi which contains the 89 km² Kibasira swamp. In common with most Africa wetlands the

extent of these large plains varies considerably according to the medium term climatic regime. The

sahelian drought of the 1970's and 1980's reduced the area of the plains of the Niger, Senegal and

Chari/Logone systems whereas the increased pluviosity of the 1960's in the Central Africa plateau

resulted in an enlarged Sudd. Indeed it is now suspected that the environment effects of such natural

variation exceed the results of such human interventions as the Jonglei canal.

41

Two smaller African plains resemble in some ways the cienaga floodplains of Latin America. The

Kifakula plain, formed by the deposition of alluvium by the Luapula river between the Johnson Falls

and Lake Mweru, covers 1 500 km² and is scattered with permanent lagoons. The Pongolo floodplain

is even smaller (130 km²), but is of interest in that its upstream end is blocked by the Pongolapoort

dam, which has permitted experiments on the discharge requirements for the maintenance of fisheries

in small floodplains.

True forested floodplains similar to the Amazonian Igapó and várzea forests are more or less confined

to the Congo basin and a few smaller river basins in Cameroon and Gabon. The course of the Congo

below Kisangani broadens to include an increasing floodable area. which culminates in the Bangala

swamps and the vast complex of flood-lands at the confluence of the Congo, Ubangui and Sangha

rivers. These areas are subject to bi-modal flooding and tend to retain their water to a great extent and

therefore differ very considerably from the highly seasonal type of plain. The true extent of this area

is difficult to assess and it is almost completely unpopulated.

An unusual type of flood wetland is formed by the overspill from the Cunene river in South Angola.

In the wet season this river discharges a considerable part of its flow southwards where it is contained

within a graben. As it flows to the south through numerous channels and pools the evaporation raises

the salt content until finally the system becomes dry at the Etosha pan. Some 10 000 km² of this

Ovambo floodplain consist of highly saline soils and water but conditions are less saline towards the

North where some 20 000 km² of waterways and floodable land are retained in a humid condition by

ground water seepage.

AMERICA (NORTH) (Fig. 1.14)

Many of the northern rivers of the continent consist of relatively short channels connecting the large

numbers of lakes of the Northern forest and tundra. The land is mainly flat with heavy flooding of

swampy areas during the spring when melting snow forms the major part of the run-off. Because of

the severe winters, streams and swamps freeze for up to six months of the year and the floodplain of

these are of the tundra type. Several large rivers drain the area, including the Mackenzie and the

Yukon. The Peace and Athabasca rivers together formed a 2 560 km² complex of flood lakes and

swamps at their entry to Lake Athabasca (Blench, 1972) but this environment has now been

superseded by a series of isolated mud flats following the closure of a dam on the Peace river. Within

the United States most rivers are highly modified by various flood control works. For instance, the

Colorado is now a cascade of reservoirs and abstraction of water from the system for irrigation is on

such scale that there is little outflow to the sea. Similarly the rivers of the Mississippi - Missouri

basin, which drain most of the United States, have also been extensively altered, the Missouri and

Mississippi mainstems having been converted into a series of pooled reaches. While many of the

rivers draining the Rockies and the Coastal ranges are turbulent and encased in steep gorges, most

streams are associated with floodplains and the 100 year flood area of the US is estimated by Sabol

(1974) at 543 000 km² or 6 percent of the total land area of the Nation. Most of Mexico is arid and

few large rivers drain the area. However the Grijalva, Usumacina, San Antonio and San Pablo rivers

of Tabasco state combine near their mouths into a series of anastomosing channels running through a

broad flood zone which covers an area of about 8 000 km² at maximum floods. The plain extends up

to 150 km inland and abundant floodplain water bodies are located along the main water courses. The

other rivers of Central America are short, often steep and swift flowing streams associated with the

mountainous terrain.

AMERICA (SOUTH) (Fig. 1.15)

The Chilean, Peruvian, Equadorian and Colombian rivers draining the Andes westwards to the Pacific

are all short and torrential. So too are the upper reaches of the North flowing Atrato, Magdalena and

Sinu rivers of Colombia, and the headwaters of those tributaries which drain the Andes eastwards to

the Orinoco, Amazon and Parana systems. The Marañon and Ucayali tributaries to the Amazon

42

particularly have long courses in the mountains. Other upland rivers are found in the various plateaux

highlands of the continent but on the whole Latin America is relatively flat and most rivers are of the

lowland type with extensive plains which are flooded by local rainfall as well as by river overspill.

Figure 1.14 Location of the major rivers and floodplains of North America:

1. McKenzie 5. Saskatchawan 9. Grande

2. Yukon 6. Fraser 10. Mississippi

3. Peace 7. Colorado 11. Missouri

4. Athbasca 8. Usumacinta/Grijalva 12. St. Lawrence

Several morphologically similar floodplains clustered around the Carribean consist of internal or

coastal deltas which contain enormous numbers of permanent or semi-permanent lakes, locally termed

‘cienagas’. The most important of these plains is that of the Magdalena river (Fig. 1.11), whose

internal delta with the San Jorge and Cauca rivers extends over 20 000 km² of savannah. Most of the

plain is well inland but extends seaward along the Canal del Dique on one hand and down the main

river channel to the coastal Cienaga Grande on the other. The whole area of 20 000 km² can flood for

up to a month, 16 000 km² for between 1 to 3 months, 13 000 km² for 3 to 6 months, and some 4 000

km² for periods varying between 6 to 8 months. When the floods recede completely about 800

cienagas, with an area of 3 260 km² remain. The Atrato river, also in Colombia, extends over a

forested alluvial plain for the last 100 km of its course. The plain, which covers 5 300 km², is

interspersed with numerous cienagas. Of similar size is the 5 000 km² of savannah flooded by the

Catatumbo river as it flows into Lake Maracaibo.

Several vast areas subject to sheet flooding are found on the continent. The greatest of these is

probably the Gran Pantanal of the Paraguay river whose shallow interconnecting complex of lakes

43

extends over 80–100 000 km² at peak floods. The savannah “llanos” of Colombia and Venezuela are

also subjected annually to shallow sheet flooding. This area is drained by the Meta, Arauca,

Capanaparo and Apure rivers which combine into a more deeply inundated deltaic floodplain of over

70 000 km² at their confluence with the Orinoco. The Rupununi river annually floods a very variable

area of savannah to a depth of 1 to 2 m. In exceptionally wet years the flooded area extends from the

headwaters of the Essequibo river to the Tokutu and Ireng rivers, which flow via the Rio Branco into

the Amazon. These sheet flooded plains have in common the shallowness and extensiveness of their

inundated area, their numerous anastomosing channels and small temporary lagoons.

44

Figure 1.15 Location of the major rivers and floodplains of South America:

1. Usumacinta/Grijalva 5. Orinoco 9. Paraguay

2. Atrato 6. Rupununi 10. Parana

3. Magdalena 7. Amazon 10. Parana

45

4. Catatumbo 8. Sao Francisco

The Amazon river and its major tributaries have densely forested floodplains for much of their length.

However, the main river channels are fringed by, and enclose, vast alluvial plains up to 50 km wide in

the upper reaches and 100 km wide further downstream. The Amazonian flood zone reaches its

greatest extent in the central delta located between the confluence of the Amazon and Tapajos. The

heart of this area, the Ilha Tupinamborana covers about 50 000 km². A similar combination of fringing

floodplain and internal delta is found along the Paraguay river below the Gran Pantanal and along the

Paraná river after its confluence with the Paraguay.

Coastal deltaic floodplains are uncommon in South America, although both the Orinoco and the

Amazon have densely forested terminal flood areas of 20 000 and 25 000 km² respectively.

ASIA (Fig. 1.16)

Asia is by far the largest and most geographically diverse land mass and its rivers cover a

correspondingly large number of types. The North flowing rivers of the Soviet Union, the Lena, the

Ob and the Yenisei, form a set of very large rivers which, despite having rapids in their upper reaches,

flow for most of their courses through plains which at their Northern extent lie within the permafrost

zone. These lower reaches remain frozen for 6–7 months of the year, and this produces a damming

effect where the still frozen lower reaches block waters released by the earlier break-up of the ice and

the snow melt in the upper reaches to cause extensive flooding. These floodplains, which may be

termed “tundra Floodplains”, differ from the temperate and tropical floodplains in that their water

bodies freeze completely for a considerable period of the year, and during the summer they retain a

marshy character.

The Tigris-Euphrates river system is isolated from the rest of Asia by the arid tracts of Iran and

Afghanistan. The upper courses of both rivers are torrential arising in the mountainous regions of

Turkey and Syria, but an extensive floodplain is present in the Mesopotamic region between them.

Here despite several thousand years of irrigation, associated with some of the most antique

civilizations, which have left many traces, flooding still continues over the 20,000 km² at high water

and the river channels are then confluent with the marshes. At low water, the lake area is reduced to

about 5 000 km². The largest lake in the system, the 5,200 km² L. Hammar, serves as a terminal drain

(Al Hamed, 1966).

46

Figure 1.16 Location of the major rivers and floodplains of Asia:

1. Amur 8. Indus 15. Irawaddy

2. Lena 9. Narmada 16. Chao Phrya

3. Yenisei 10. Ganges 17. Mekong

4. Ob 11. Krishna 18. Sui Chiang

5. Syr Darya 12. Godavari 19. Yangtze

6. Amu Darya 13. Brahmaputra 20. Hwang Ho

7. Tigris/Euphrates 14. Gangetic Delta

The hydrography of most of Eastern and South Eastern Asia is dominated by the central mountain

massif of the Himalayas and the Tibetan plateau. Most of the great rivers of China, India, and the

South East Asian Peninsula take their source in these highlands to flow out through a series of coastal

plains from China to Pakistan. In China, the Hwang Ho, Yangtze and Si rivers, drain the plateau

eastwards and flow across vast plains which were once liable to widespread and repetitive flooding.

The Yangtze in particular, is associated with an extensive complex of flood lakes. These systems are

now highly controlled by dams and other flood control devices so inundations are less common. The

Red, Mekong, Salween, Brahmaputra, Ganges and Indus rivers all rise on the plateau and have very

long torrential upper courses before debouching onto extensive lowland floodplains. The Indus river,

the site of very early civilization, has a wide braided channel for much of its course. This periodically

shifts its bed due to excessive silt deposition raising the river channel and its levees high above the

level of the surrounding plain. When it overspills its banks the course of the Indus may be displaced

laterally for up to 40 km, losing water by seepage and evaporation in the otherwise desertic region.

The river flows into the sea through a large but infertile deltaic plain. In the virtually rainless areas of

Pakistan, the Indus and its floods are essential for irrigated agriculture. Both the Ganges and the

47

Brahmaputra rivers also have unstable beds flowing through braided channels. The main fringing

floodplain of the Ganges is over 600 km long and between 16 and 80 km wide. The Brahmaputra has

an especially broad channel, up to 12 km in places. The two rivers combine to form an immense delta

liable to sheet flooding from rainfall as well as river discharge. The types of floodplain, classified by

the duration and depth of inundation of the portion of this plain lying in Bangladesh, are shown in Fig.

1.17. In fact most of the active part of this delta lies in this country as the western region, centred on

the Hooghly river in India, is silted and rarely floods through the now dead distributaries. In

Bangladesh alone, there are 93 000 km² of floodable land including 28 340 km² of paddy fields which

are inundated for 3–4 months of the year. In addition, there are an estimated 14,000 km² of permanent

open inland waters. The seaward parts of the plain have dense brackish water mangrove forests know

as “Sunderbuns” some 5 000 km² in area.

The 31 000 km² deltaic alluvial plain of the Irawaddy river is flooded both by rainfall and upper river

discharge. Submergence by local precipitation precedes the arrival of the river floods by at least a

month. Artificial levees constructed for flood control will eventually change the whole pattern of

flooding. A similar but smaller deltaic plain flanks the Chao Phrya river and its distributaries in

Thailand (Fig. 1.6). Here the main river channels are raised high above the surrounding backswamps

which are deeply flooded each year, although recently the construction of an upstream dam seeks to

control the timing and extent of the inundation. Another plain that is fast being altered by flood

control measures, irrigation and an ambitious programme of upstream dam construction is that of the

Mekong (Fig. 1.9) which is over 74 000 km² in total area. About 21 000 km² of its area are now no

longer flooded although 8 850 km of river, canals and irrigation channels take water to every part of

the plain. The area of permanent water is only 4 000 km², of which the majority is in the 2 500 km²

Grand Lac. In fact this body of water is one of the region’s most deeply affected by the modification

of the hydraulic regime of the river as its floods have been drastically curtailed.

The peninsular rivers of the Indian sub-continent, the Godavari, Krishna and Cauvery, once had

extensive floodplains as did the Mahaweli system in Sri Lanka. These systems have now been

modified by dams for power generation and irrigation.

Marshy flooded areas are common as the Indonesia island of Sumatra, Borneo and West Irian, and as

some of the Philippine islands. Fundamentally similar in nature these are located on the flat coastal

alluvial plains and are submerged and drained by numerous small rivers. In Sumatra, there are the

eastward flowing, Hari, Kampari, Rokan and Musi rivers; in Borneo the Southward flowing Kahajan,

Barito and Mendawa rivers and in New Guinea, the Fly, Diguil and Pulan rivers. The most complete

description of the fish and fisheries of such coastal flood rivers is that of Vaas et al. (1953) who make

it clear that, on the island of Sumatra at least, such areas are classic floodplains. In the Oga and

Komering rivers, which cover over 500 000 km at peak flood, the lateral plains of “Lebaks” are used

for both rice culture and fisheries.

48

Figure 1.17 The delta of the Ganges and Brahmaputra rivers in Bangladesh showing the major land

types classified by depth and duration of flooding

More typical seasonal floodplains, such as the 6 555 km² Kapuas lake district of West Borneo, the 7

000 km² lake district of the Mahakan River in East Borneo (Dunn and Otte, 1982) or the 7 500 km²

lateral plains of the Sepik River in Papua New Guinea, are also common on the larger islands.

EUROPE (Fig. 1.18)

Western Europe is drained by a number of relatively short streams although the Danube, Rhine,

Rhone and Vistula rivers have discharges sufficient to place them among the first 50 in the world. In

eastern Europe a number of larger rivers drain into the Black, Azov and Caspian seas including the

Dniester, Dnieper, Don and Ural. Torrential streams are present in the Alps, Pyrenees, Carpathian and

Caucasus Mountains, but on the whole European rivers are of the lowland type, particularly in the

East where the drainage is dominated by relatively few large rivers. On the whole European rivers

49

have been highly modified by canalization and dam building so that few retain much trace of their

original floodplains. For example, the largest river in Europe, the Volga, has been converted into a

cascade of reservoirs and only a very short terminal reach retains its original form, although even here

the flow from upstream is modified. The Danube and Ural are perhaps the only major rivers in which

some original characters survive (Liepolt 1967). The total area of the Danube floodplains was given

as 264 500 km² by Liepolt (1972), but recent damming, flood control and land reclamation schemes

have only left some 5 000 km² as liable to inundation in Romania. In the Czechoslovak-Hungarian

reach of the river 230 km² are regularly flooded from May to August and 30 km² remain as permanent

water in side arms and 86 km² in the main river channel. However, even these will disappear when

present plans for the hydraulic management of the system are completed.

Figure 1.18 Location of the major rivers and floodplains of Europe:

1. Vistula 6. Po 10. Dneister

2. Elbe 7. Rhone 11. Dneiper

3. Thames 8. Garonne 12. Don

4. Rhine 9. Danube 13. Volga

5. Loire

AUSTRALIA AND NEW ZEALAND (Fig. 1.19)

The hydrology of Australia is dominated by the aridity of its Central regions. Thus the Murray

Darling system which drains most of the inland South-Eastern Australia with a basin area of 1 029

000 km² has a remarkably small total annual discharge of 22 km² or 6% of the total discharge for the

continent. Its regime is very variable with long periods of low flow and occasional massive floods.

During its wetter phases the system is associated with a lowland marsh system which takes the form

50

of an internal delta with some permanent lakes, e.g., Lakes Meninolee and Tandou, although much of

the area may reduce considerably in size at times of drought. In contrast to this south flowing river

system there are several north flowing rivers which drain the more humid tropical areas of Northern

Australia and Queensland. These carry about 40% of the total discharge by Australian rivers and have

regular monsoonal regimes with annual floods which inundate the lowland plains associated with the

various rivers. New Zealand's rivers are mostly short and torrential particularly in the mountainous

South Island where they are often associated with deep narrow glacial lakes.

Figure 1.19 Location of the major rivers and floodplains of the East Indies and Australia:

1. Mahakan 4. Fly

2. Barito 5. Sepik

3. Murray/Darling 6. Kapuas

51

CHAPTER 2

PHYSICAL AND CHEMICAL PROCESSES

HYDROLOGY

FLOW

Variability of Flow Regimes

Flow regimes have their origin mainly in run-off from precipitation over the river basin. This may be

in the form of rainfall in warm temperate and tropical areas, in which case the response of the low

order streams to changes in precipitation is related directly to seasonality of pluviosity. In cold

temperate or high altitude regimes the precipitation is usually in the form of snow in which case

release of water into the stream channel is more closely related to seasonal patterns of snow-melt.

Retention and release of ground water, important factors in maintaining flow in the river channel

during the dry season, contribute less to spates and tend to smooth out flows, reducing the amplitude

but increasing the duration of a flood. The proportion of rain water that appears as run-off or as

ground water depends on the type of terrain and its vegetation cover. Local difference in these factors

can cause very different flood patterns even in two adjacent streams. Because of this, and because of

the rapidity with which run-off from local rain storms can affect low order streams, graphs of water

level or flow in the smaller water courses are apt to appear “spiky” with numerous rapid changes in

flood condition. As streams amalgamate into larger and higher order rivers their individual regimes

merge so the flow is smoothed (Fig. 2.1) and reflects the average of the precipitations over a

progressively larger area. The fate of the flow thereafter depends on the form of the river basin and

here reservoir rivers have internal features which reduce the variability of the flow, distributing it

more evenly throughout the year and suppressing pronounced flood peaks. Such features as extensive

swamps, forests, lakes or even floodplains themselves, may store water for later release. Alternatively

the cumulative discharge of many tributaries arising in different latitudes and having correspondingly

diverse flood peaks may also act in this way. The lower reaches of rivers, therefore, may tend to take

on “reservoir” characteristics, a good example being perhaps the Congo River below Kisangani. Even

here there are two pronounced flood peaks (Marlier, 1973). The existence of swamp, forest and lakes

in the river basin do not by themselves ensure the complete smoothing of flow and many flood rivers

have all these features.

Velocity of flow

Although Leopold et al. (1964) maintain that the rate of flow remains more or less constant as one

progresses downstream in rivers in the United States, it is the more general observation that flow rate

decreases with decreasing slope. Thus lower order rivers tend to have faster flows consistent with the

greater slope of upland streams, whereas the higher order streams have slow flows in the meandering

lowland course. Some higher order rivers however, such as the Magdalena of Colombia or the Sepik

of Papua New Guinea, do have considerable flow velocities in their potamon reaches with main

channel rates exceeding 2–3 m/sec at low water. By contrast flow may cease completely in low order

streams or even main channels during the dry season, and in many modified rivers abstraction of

water for irrigation has reached such a level that there is rarely any net outflow to the sea. Obviously

it is difficult to generalize as to the speed of current in various streams as it depends on many physical

and climatic variables, but the range of velocities quoted by Zhadin and Gerd (1961) for Middle Asian

rivers of the Soviet Union serves to illustrate the general slowing of the flow as the river proceeds

downstream. Here flow velocity were 5–6 m/sec in the upper reaches, 3–3.5 m/sec in the foothills and

2–2.5 m/sec in the flatland reaches. Furthermore, velocities change over relatively small stretches of

stream especially in the rhithron. For example, the rate of flow in Caucasian streams at low water was

1.25 m/sec over the riffles and only 0.6 m/sec through the succeeding pools.

52

Figure 2.1 Flood regime of rivers with different basin areas within the Chari-Logone river system

showing the increasing smoothing of the flood curve with increasing size of basin

It is very important for living aquatic organisms within the river channel that the velocity of the water

does not increase proportionally with discharge. Leopold and Maddock (1953) and Drury (1969) both

present graphs which show that the mean velocity at any point on a river only doubles for a tenfold

increase in discharge. Furthermore, since these figures apply to peak velocities at mid-channel the

increases in velocity at the bottom or sides of the channel would be even less.

Same idea of velocity of flow in any reach can be obtained from the size of particles in the stream

bed. Fig. 2.7 sets out the mean current velocity required to just move particles of various sizes along a

53

stream, thus the existing bed deposits should indicate that flows are just below the equivalent rate

listed. This serves only as a rough guide to the mean velocity which might be expected, as results

from various authors do not agree all that closely. Furthermore, resistance to movement is not entirely

a function of particle size for hard packed clay is far less easily entrained than is sand, even though its

particle size is far smaller.

Origins of flooding

Flooding originates from three sources: (i) overspill from the river channel; (ii) local rainfall; and (iii)

tides.

Overspill from the river channel

As flow increases a point is reached where the channel is no longer able to drain the volume of water

passing down the river. Further rises in discharge above this point (bankfull level) result in overspill

onto the floodplain. Here, because of the flatness of the terrain, increases in volume are achieved by

lateral expansion rather than by increases in depth, and the water spreads slowly and diffusely

outwards, hampered in its progress by the floodplain vegetation. Such flow is termed “creeping flow”

and is depositional (i.e., silt comes out of suspension onto the plain), as opposed to the erosive flow

where a strong directional current is confined within the main channel or along the terrace walls.

54

55

Figure 2.2 Flood regimes of typical river systems for various continents

Plots of the variation of water level or flow of a flood against time (flood curves) for large rivers have

the appearance of waves (Fig. 2.2), which usually are unimodal, although downstream of the

confluence of two systems having differing flood regimes, polymodal floods may occur. The flood

wave has amplitude (the total difference in level between maximum and minimum discharge) and

duration, (the time taken to pass from one low water stage to the next). It moves down valley at a

speed which may depend on many factors, including the slope and degree of enclosure of the river

valley, the type of vegetation and the intensity of the flood. In the Niger River, for instance (Fig.

2.3A), the flood crest takes over 100 days to move from Koulikoro to Malanville. As this is over 1

160 km, the average rate of travel is about 17 km/day. Similar speeds of transmission are found in the

Senegal, where the 620 km from Galougo to Dagana take over a month (17 km/day) (Fig. 2.3B) and

the Chari, where the 745 km between Mossala and N'Djamena are traversed in 42 days (18 km/day)

(Fig. 2.3C). The flood moves down-valley at about 13 km/day in the Brahmaputra, 29 km/day in the

middle Mekong and on the lower Mekong flood-plain 13 km/day. On the Kafue plain the speed of

transmission is 11 km/day and on the Central Delta of the Niger 13 km/day. These figures are

averaged over considerable distances and speed differs within various reaches of the same river. In

general, the more expansive the plain, the less its slope and the greater its vegetation cover, the slower

will be the velocity of the flood crest. The progression of a flood wave across a flood-plain is now

especially easy to illustrate using satellite imagery. For example, four phases in the passage of the

flood over the Yaeres floodplain are shown in Fig. 2.4 (after Quensiere, 1981).

Figure 2.3 Time taken for the flood wave to traverse the distance between selected points in three

African river systems. The time taken for the flood wave to cover the distance between fixed

56

points is indicated by the vertical lines showing the time of maximum flood and the line

transposed from the preceding point upstream as shown in the graph; figures refer to the

distance between one point and the next

In long rivers, where the timing of the rains is fairly uniform along their courses, the arrival of the

main flood crest in downstream reaches may be delayed until the dry season. Such a delay is typical

of the Niger River, where two floods occur at Malanville (Benin) (Fig. 2.3), one in August-November

caused by rainfall and drainage of local rivers and a second in December-March due to the arrival of

the flood from the headwaters of the river.

57

Figure 2.4 Successive stages in the flooding of the Yaeres by overspill from the Logone river (after

Quensiere, 1981). ---- : Limits of total area flooded

As the flood wave passes along the floodplain there is a tendency for it to decrease in amplitude and

increase in duration, provided no large amounts of water are added to it by other tributaries. This

process is best seen on a river such as the Niger where the channel runs for a considerable distance

through arid and semi-arid landscapes.

Local rainfall

Precipitation on the floodplain itself, or on the basins of the lower order streams immediately

surrounding it, saturates the soil and causes local flooding which often precedes the main flood by

some considerable period of time. Early work by Svensson (1933) showed that in the Gambia River

certain depressions are flooded only by local precipitation and are connected to the river and other

low lying swamps by a single channel or creek. Such swamps may be unaffected by variations in river

height, but fish may migrate to them by ascending the connecting channel. Early flooding of

floodplain depressions by rainwater has been noted from several systems. Elsewhere in Africa, Carey

(1971) described flooding of this type in the Kafue. The inundation of the Upper Chari (Bahr Salamat

and Bahr Azoum) appears to be almost entirely by local rainfall (Durand, pers.com.), and in the Nile

Sudd the swamps are filled one to two months before overspill from the river occurs. Extensive areas

of the Venezuelan Ilanos (Matthes, pers.comm.) and the Gran Pantanal or Paraguayan Chaco of the

Paraguay River flood in this manner, (Carter and Beadle, 1930). A similar type of flooding has been

recorded from the Irrawaddy River in Burma by Ohya, (1966), where submersion by rainwater

precedes the main floods by about one month. Flooding by local rainfall differs from overspill in that

the net flow of the water is from the floodplain depression to the river rather than the other way round.

Flooding with rainwater prepares the floodplain by saturating the soil and filling flood-plain

depressions and replenishing the ground water so that relatively little of the subsequent overspill flood

is absorbed by the ground.

Tides

Flooding caused by tidal action obviously affects only those parts of floodplains adjacent to the coast.

However, secondary effects are produced by the backing up of freshwater floods by high tidal levels.

Volker (1966) divided river estuaries into four reaches (Fig. 2.5); Zone A where vertical tides produce

a reversal of the direction of flow and the penetration of saline water into the river channel, or onto

the floodplain; Zone B where the river water remains fresh but tidal effects and current reversals still

occur; Zone C where the water levels are still affected by the tides giving rise to differences in current

velocity, although the flow always proceeds downstream and Zone D where water level and flow are

affected by upstream discharge only; this zone merges into the main river upstream.

58

Figure 2.5 Zones of interaction between marine and freshwater in estuaries and coastal deltaic

floodplains (after Volker, 1966)

Water balance on floodplains

The flood curve at any point in the river, results from discharge transmitted from upstream, input from

local tributaries and precipitation in the immediate area. This can be represented diagrammatically as

in Fig. 2.6 where the x axis is time. The y axis may represent any of the indicators of water quantity,

the most common of which are water level, discharge rate or storage volume. Although there is a

continuous fluctuation in water level the flood can be broken into two parts. In the first, the water is

confined in the river channel and alterations in discharge produce changes in depth and velocity.

Changes in volume are caused mainly by increase in depth so long as the flow remains confined in the

channel are caused mainly by increases in depth. This phase is delimited by the bankfull stage beyond

which the water overflows the channel limits and floods the plain. Thereafter changes in inflow

produce relatively little change in outflow, the excess water being temporarily stored on the floodplain

where the volume increases mainly from the extension of the water area. The bankfull level is usually

difficult to define precisely, as it varies from place to place in the river and also from year to year. The

detailed hydraulics of floodplains are extremely complex and beyond the scope of this discussion.

They have been the subject of many studies and attempts at mathematical modelling such as those

elaborated by Weiss and Midgley (1975 and 1976).

59

Figure 2.6 Diagram of (A) a generalized annual flood regime, and (B) probability of occurrence of

maximum and minimum water stages

The intensity of flooding varies from year to year in response to fluctuations in rainfall. Because of

this variability, a distribution of the frequency with which any flood height occurs can be established

over a number of years. Such distributions (Fig. 2.6) may be normal but are more often skewed. The

number of years which must elapse on average between a flood of a particular height and the next

flood of the same height is known as the return period. The 100 year flood, for example, is that level

of flooding which can be expected once in 100 years. The probability of occurrence of any flood is the

reciprocal of its return period; the 100 year flood having a probability of occurrence of 0.01 in any

year.

Not only does the intensity of flooding vary from year to year, but there is increasing evidence that the

variability is of a cyclic nature which is traceable to long-term global climatic patterns (Winstanly,

1975; Bryson, 1974). According to such cycles, years of exceptionally high flooding or of drought

tend to be grouped together so there is a good chance of two or more years of intense flooding to

appear in sequence or for one year of low flood intensity to be followed immediately by another poor

year. When the latter happens drought conditions can ensue such as was shown in the Sahelian rivers

between 1970 and 74 and again in 1980–85. The grouping of good and poor floods in this manner can

have severe consequences on the ecology of the fish as we shall see in the section on the dynamics of

floodplain communities. The variation in flood level is a normal phenomenon and knowledge of the

range as well as the mean value is essential to an understanding of the ecology of these systems.

Differences in the amplitude of the flood curves as measured by maximum and minimum discharge

rate are shown in Table 2.1. These are very variable and especially high values tending to infinity are

obtained in those rivers where flow virtually stops during the dry season. An example of the effects of

such variations on the actual areas flooded is that of the Senegal River as shown in Table 2.2.

60

Table 2.3 shows the mean rainfall over certain floodplains. In the tropics and sub-tropics, rainfall

makes only a moderate contribution to the water balance of the plain and is largely exceeded by the

evapotranspiration rates shown in Table 2.4. This excess of evaporation over precipitation causes the

net outflow from some floodplains to be very much lower than the inflow. Thus, of the 7.11 × 1010 m³

of water entering the Central Delta of the Niger through the Niger and Bani rivers in years of normal

flow, only 3.82 × 1010 m³ emerges at Dire, a net loss of 46 percent. Similarly, 2.7 × 1010 m³ of water

enter the Sudd at Bor and only 1.4 × 1010 m³ leave it at Lake No, a loss of 48 percent of the water. In a

few systems, the Okavango swamps for instance, evaporation almost balances inflow over the year so

that there is no appreciable outflow from the system.

Table 2.1

Mean annual maximum and minimum discharge rates of various rivers (from

figures compiled by Van der Leeden, 1975)

Discharge m³ sec.-1

Basin area Mean Mean Max/

(km²) max. min. min.

Amazon(Obidos) 4 688 000 207 400 89 300 2.3

Brahmaputra (whole basin) 580 000 72 460 2 680 27.0

Mekong (whole basin) 795 000 67 000 1 250 53.6

Ganges (whole basin) 977 500 61 200 1 170 52.3

Orinoco (Ciudad Bolivar) 850 000 46 200 7 520 6.1

Indus (whole basin) 970 000 31 200 490 63.7

Mississippi(Tarbet landings) 3 928 000 30 800 7 930 3.9

Paraná (whole basin) 2 039 000 25 130 6 300 4.0

Magdalena (Calamar) 238 000 10 000 3 900 2.6

Oubangui (Bangui) 500 000 9 640 1 070 9.0

Blue Nile (Khartoum) 325 000 5 950 138 43.1

Senegal (Bakel) 218 000 3 430 26 131.9

Chari (N'Djamena) 600 000 3 390 199 17.0

Chao Phrya (Nakhon Sawan) 110 371 3 363 32 105.1

Niger (Mopti) 281 600 2 820 67 42.1

Danube (Bratislava) 131 338 2 746 1 402 2.0

Tigris (Baghdad) 134 000 2 720 286 99.5

Benue (Garoua) 64 000 1 920 32 60.0

Ouémé(Pont du Save) 23 600 758 0.63 1203.0

Table 2.2

Areas flooded by the Senegal River for different flood regimes (after

OMVS)

Year Statistical significance Maximum area flooded (ha)

1964 Five-year strong flood 766 000

1969 Mean flood 549 000

1926 Five-year weak flood 215 000

1972 Weakest recorded flood 104 000

61

The high evaporation rates also mean that, to be permanent, pools or lagoons must either be filled

annually to a depth greater than the loss by evapotranspiration (between 1.2 and 2.2 m from the data

in Table 2.3), or must receive inflow from ground water.

Table 2.3

Precipitation at sample locations on various rivers (from Van der Leeden, 1975)

Annual precipitation (mm)

J F M A M J J A S O N D Mean

AFRICA

Mali (Bamako) - - 0.4 15.2 37.7 137.2 348.0 205.7 43.2 15.2 - - 1120.1

Sudan (Wau) - 5.1 22.9 66.0 134.6 165.1 190.5 208.3 167.6 124.5 15.2 - 1099.8

Botswana (Maun) 106.7 8.7 71.1 17.8 5.1 2.5 - - - 22.9 5.84 8.64 434.3

Zambia (Lusaka) 235.1 190.5 142.2 17.8 - - - - - 10.2 91.4 149.9 835.7

ASIA

Dem. Kampuchea 7.6 10.2 25.4 78.7 144.8 147.3 152.4 154.9 226.1 251.5 139.7 43.2 1391.9

(Phnom Penh)

Thailand 5.1 27.9 27.9 58.4 132.1 152.4 175.3 233.7 233.6 355.6 45.7 2.5 1470.7

(Bangkok)

Bangladesh 7.6 30.5 61.0 137.2 243.8 315.0 330.2 337.8 248.9 134.6 25.4 5.1 1877.1

(Dacca)

Pakistan 63.5 63.5 68.6 48.3 33.0 58.4 205.7 233.7 99.1 15.2 7.6 30.5 927.1

(Rawalpindi)

SOUTH AMERICA

Venezuela 35.6 20.3 17.8 25.6 96.5 139.7 160.0 180.3 91.4 101.6 71.1 33.0 972.8

(Ciudad Bolivar)

Colombia 10.2 - 10.2 22.9 86.4 86.4 76.2 18.2 12.9 274.3 226.1 114.3 934.7

(Cartagena)

Brazil 248.9 231.1 261.6 221.0 170.2 83.8 58.4 38.1 45.7 106.7 142.2 203.2 1811.0

(Manaus)

Argentina 78.7 78.7 99.1 124.5 66.0 30.5 30.5 40.6 61.0 71.1 94.0 114.3 889.0

(Parana)

Table 2.4

Annual evapotranspiration rates from various floodplain regions

Annual ETP mm

Mean Range

AFRICA

Niger delta (Mali) 2 150 1 900–2 450

Sudd (Sudan) 1 900 1 800–2 000

Okavango swamp (Botswana) 1 800 1 700–1 900

Kafue flats (Zambia) 1 650 1 550–1 750

ASIA

62

Mekong delta (Viet Nam) 1 600 1500–1 700

Chao Phrya (Thailand) 1 600 1 500–1 700

Ganges delta (Bangladesh) 1 500 1 400–1 600

Indus valley ± 30°N (Pakistan) 1 750 1 600–1 900

Indus valley ± 25°N 2 000 1 900–2 300

SOUTH AMERICA

Llanos (Colombia) 1 400 1 300–1 600

Magdalena ± 5°N (Colombia) 1 500 1 400–1 600

± 8°N (Colombia) 1 300 1 100–1 400

Amazon river - Manaus (Brazil) 1 300 1 200–1 400

Parana river (Argentina) 1 250 1 150–1 350

CHEMISTRY

SEDIMENT LOAD AND TURBIDITY

There is a clear relationship between the velocity of the current in a river and the amount of matter

carried suspended in its waters - the greater the current, the larger the load and particle size (see Fig.

2.7) that can be carried by the stream. This has many repercussions both for the morphology of the

river and in the biology of many of the organisms living therein. Morphologically, because any river

will tend to transport the maximum amount of material in suspension for any given current, sediment

will be picked up or deposited as the current and the existing load vary. Slowing of flow results in

aggradation, the deposition of part of the load leading to the development of many of the

morphological features of the river, including levees, point bars and floodplains. Acceleration of the

flow leads to the river picking up sediment causing erosion or degradation. During the flood cycle

these two processes tend to alternate. Thus during the rising flood rivers are heavily charged with silt,

but as the water spills over the flood-plain the general slackening of flow leads to a deposition of

suspended matter producing decanted waters of some clarity. At falling flood acceleration in water

velocity causes localized scour but as flow slackens progressively through the low water period the

standing waters of the floodplain, and frequently of the main channel itself, deposit most of their load.

The amount of suspended matter in the water affects the penetration of light into the water, for

example Bonetto (1980) found a relationship S = a e bh for the Parana river, where S is the light

penetration as measured by Secchi disk, h is the velocity of the current as measured by the height of

the river and a and b are constants.

Sediment yields of representative rivers of the world are shown in Table 2.5. The suspended matter

carried by a river relative to its discharge is indicative of the stability of the system. Rivers which

carry a large amount of sediment relative to their discharge, for example the Indus, tend to be unstable

with shifting beds and rapidly evolving successions of floodplain feature, whereas rivers with low

sediment yields relative to their discharge tend to change much more slowly.

63

Figure 2.7 The speeds of flow required to move mineral particles of various sizes (line plotted from

data in Nielson, 1950); terminology of particle sizes from Gregory and Walling (1973)

NUTRIENT SPIRALING

The idea of nutrient spiralling posits that a nutrient atom cycles through three generalized components

of the stream ecosystem (Newbold et al., 1981). Each component is transported downstream with

current so the path of the atom describes an imaginary spiral. The three components are: (a) the water

in which the nutrient atom is dissolved; (b) the particulate phase in which the atom is sorbed onto

particulate matter and is thus located in the bottom although downstream transport continues through

saltation, suspension, erosion, etc.; (c) the consumer phase in which the atom is incorporated into

living matter. During this phase it may pass directly through a series of organisms before its release

once more into the water phase. The total distance travelled downstream between one release into the

water and the next, is termed the spiralling length (S). This length may be taken as a measure of the

efficiency with which nutrients are used within any particular system, the shorter the spiralling length

64

the more efficient the use in that any nutrient atom is utilized more frequently for any given length of

stream.

During the consumer phase, most of the nutrients are held in micro-organisms, but invertebrates

grazing in these and on higher plants can influence the spiralling length through accelerated release of

nutrients. Collector and shredder organisms tend to shorten spiralling length whereas grazers may

increase it.

Table 2.5

Representative rivers of the world and their sediment yields

Drainage area Average annual suspended load

River km² Total t × 106 t/km² Load/discharge

Amazon 6 062 080 406.4 67 1.92

Negro 93 875 1.5 16

Congo 3 968 000 72.4 18 1.82

Mississippi 3 184 640 349.5 109 20.0

Missouri 1 352 192 243.4 180

Ohio 196 044 15.2 77

Nile 2 944 000 123.8 42 44.2

Parana 2 278 400 91.4 40 6.13

Uruguay 384 000 15.2 39

Volga 1 335 014 21.1 16

Indus 1 231 360 488.7 396 87.3

Niger 1 100 800 5.1 5 0.8

Murray-Darling 1 059 840 35.7 33

Ganges 1 047 552 1625.6 1551 86.9

Brahmaputra 552 960 812.9 1469 41.0

Yangtze 1 012 864 560.8 553 25.7

Orinoco 938 752 96.9 103 5.7

Danube 806 400 21.8 27 3.5

Mekong 386 560 190.0 491 17.3

Irrawaddy 362 752 336.3 927 24.9

Vistula 190 873 1.7 9 1.55

Euphrates 119 219 4.8 40 3.4

Chao Phrya 105 216 12.7 120

CARBON AND ORGANIC MATERIAL

Rivers transport large, but usually not accurately quantified, amounts of dissolved and particulate

organic material from the terrestrial component of the drainage basin to the oceans (See Fig. 2.8).

Direct inputs usually consist of coarse particulate matter (CPOM) in the form of leaves, fruit, flowers,

pollen, branches, etc. whereas indirect inputs consist of leachates and other dissolved material

including inorganic carbonates from rocks and soils. Biological processes downstream tend to degrade

the particulate organic matter from CPOM (particle size full size trees to 1 mm) through fine

particulate organic matter (FPOM - particle size 50 mm - 1 mm) to ultrafine particulate organic matter

(UPOM - particle size 0.5 - 50 mm). Dissolved compounds increase in concentration down-stream

65

and a wide range of amino acids, fatty acids, sugars, hydrocarbons and pigments have been recorded

by Visser (1970) and Balogun (1970), either free in the water or absorbed onto sediment particles. At

the same time living organism contribute to the CO2 concentration through respiration.

Particulate carbon and compounds absorbed onto sediment particles follow similar processes to

sediment and other nutrients in that they may be stored within the plain through deposition, may be

recycled by biological agents and may be reincorporated into the flow towards the oceans through

erosion, (Fig. 2.9). Storage rates may be quite high. For example, in the floodplains of small rivers of

the forested zones of North America storage can amount to between 48 and 96 g C/m/yr (Sedell et al.,

1974) and in mature floodplain swamps can be as high as the 113 g C/m/yr recorded from the

Atchafalaya river (Gagliano and Van Beek, 1975).

66

Figure 2.8 A general model of the biogeochemistry of carbon in rivers (from Richey, 1981)

IONIC COMPOSITION

The chemical composition of river water depends on a wide variety of physical, chemical and

biological features, but three basic mechanisms control surface-water chemistry: precipitation, the

nature of the bedrock and the evaporation-crystallization process (Gibbs, 1970) (Fig. 2.10). In most of

the large tropical rivers the ionic composition of the water derives primarily from the rain and the rock

or sediments over which the river flows. In rivers flowing through hot arid regions, the rate of

evaporation and crystallization of the concentrated salts exerts the main influence and flood areas

located in such zones eventually transform into salt marsh habitats. Secondary influences on the ionic

composition are exerted by macrophytes and phytoplankton and to an increasing extent by the various

industrial, agricultural and domestic human activities.

Figure 2.9 Organic carbon flow between a floodplain wetland and its bordering river (Mulholland,

1981)

67

Figure 2.10 Diagram of the basic mechanism controlling surface-water chemistry (Adapted from

Gibbs, 1970)

Total dissolved solids (TDS) may be taken as one convenient measure of the total ionic concentration

in water. Naturally, the concentration and relative abundance of ions in river water is highly variable,

although there is a tendency for larger rivers to average out the variability of the tributaries. Fig. 2.11,

taken from Holland (1978), shows clearly that, while there is considerable spread in TDS for any

particular runoff the increase in concentration with decreasing runoff is unmistakeable.

68

Figure 2.11 Total dissolved solids in some major rivers as a function of the mean annual runoff (after

Holland, 1978)

The mean composition of the river waters of the various continents of the world are shown in Table

2.6. Holland (1978) discusses in more detail the concentration and origin of most of the major ions

present in these rivers.

Table 2.6

Mean ionic composition of river water of the continent of the world (after Holland, 1978)

Continent HCO SO Cl NO Ca Mg Na K Fe SiO Total

America N. 68.00 20.00 8.00 1.00 21.00 5.00 9.00 1.40 0.16 9.00 142.00

America S. 31.00 4.80 4.90 0.70 7.20 1.50 4.00 2.00 1.40 11.90 69.00

Europe 95.00 24.00 6.90 3.70 31.10 5.60 5.40 1.70 0.80 7.50 182.00

Asia 79.00 8.40 8.70 0.70 18.40 5.60 9.30 - 0.01 11.70 142.00

Africa 43.00 13.50 12.10 0.80 12.50 3.80 11.00 - 1.30 23.20 121.00

Australia 31.60 2.60 10.00 0.05 3.90 2.70 2.90 1.40 0.30 3.90 59.00

World 58.40 11.20 7.80 1.00 15.00 4.10 6.30 2.30 0.67 13.10 120.00

Anions 0.96 0.23 0.22 0.02 1.43

Cations 0.75 0.34 0.27 0.06 1.43

69

Detailed lists of ionic content of individual tropical rivers are outside the scope of this book.

Compilations of such data based on the work of numerous authors exist for Africa (Welcomme, 1972)

and Latin America (Ardizzone and Ziesler, 1977). Observations from Asia have not been compiled

systematically, but Jhingran (1975) cited data from Indian rivers and Johnson (1967) gave details of

the chemistry of some Malaysian rivers. Studies on the significance of ionic transport on a world scale

have been made by Meybeck (1976 and 1979) and Martin and Meybeck (1976 and 1979).

In those regions where the rocks are impoverished and soils leached, the main supply of nutrients

comes from precipitation, such rivers being grouped in the lower right hand area of Fig. 2.10. In the

Amazon basin for instance, the chemical composition of the blackwater streams of the Hylaea, or of

the larger blackwater rivers, can resemble that of the rainwater. The frequent similarity of the

composition of rain and river waters indicates that such ions as are leached from the soil may be

balanced by those carried by the rain. Similarly the chemistry of the headwaters of the Paraná and

Paraguay rivers are dominated by atmospheric precipitation (Bonetto, 1975) and in the same general

area the conductivity of rainwater falling over the Mato Grosso, Brazil, is often higher than that of the

streams (Green, 1970). In Africa, Visser (1974) noted that the composition of river and lake waters

within a radius of 100 miles of Kampala indicated that rain contributed on average all of the sodium,

ammonium and nitrate, over 75 percent of the chloride and over 70 percent of the potassium. In the

Gombak River, Malaysia, Bishop (1973) considered precipitation as a very important factor in

determining ionic concentrations.

The rivers of the three main tropical rainforest regions arise on very poor, leached, podsolitic soils.

Sioli (1968) described such waters in the Amazon basin as more or less transparent, devoid of

significant amounts of inorganic particles, but with very poor light penetration because of the brown

colour imparted by dissolved humic substances. PH is usually very low and dissolved nutrient

concentration poor. More detailed analyses of the composition of blackwaters of L. Tupe of the Rio

Negro (Rai and Hill, 1981) show such waters to be biased to sodium and sulphate dominance. Similar

waters have been reported from the headwaters of the Orinoco (Edwards and Thorne, 1970) and have

also been described from both Africa (Matthes, 1964) and Malaysia (Johnson, 1968). Descriptions

and analyses from the three continents are shown in Table 2.7.

Table 2.7

Description of chemical composition of blackwaters from Asia, Africa and South Americaa

Congo (Tshuapa

R.) (Matthes, 1964) Amazon (Rio Negro)

(Ungemach, 1972) Malaysia

(Johnson, 1967) Amazonian rainwater

(Ungemach, 1971)

pH 4.0–6.5 4.6–5.2 3.6–5.9 4.0–5.4

Conductivity 24–32 6.8–10.4 1.5–20

TDS 30–65 mg 1-1

Fe fairly abundant 268–535 μg 1-1 10–100 μg 1-1

+Ca very low 232–450 μg 1-1 0–1 904 μg 1-1 20–500 μg l-1

+Mg very low 108–254 μg 1-1 182–17 867 μg 1-1 10–30 μg 1-1

+Na 435–1 358 μg l-1 2 092–22 480 μg1-

1

+K 235–601 μg 1-1 586–7 429 μg 1-1

NNO3 trace 15–53 μg 1-1 5–300 μg 1-1

NNO2 trace 0.035–1.27 ug 1-1

PPO4 trace 3.2–8.6 μg 1-1 1–3 μg 1-1

N organic 70–80 μg 1-1 209–478 μg 1-1 10–150 μg l-1

Cl 1 800–2 600 μg 1-1 283–30 631 ug 1-1 80–320 μg1-1

Si 2 000–2 740 μg 1-1 2 000–2 740 μg1-1

SO4 absent absent 4 803–216 903 μg

1-1

70

A These figures are often based on only a few observations and do not therefore necessarily reflect

the full range of variations of the parameters measured

Blackwaters from the three continental areas are similar in general composition, except that sulphate

is even more common in the Malaysian waters than in those of South America and Africa. In South

America it is virtually absent in many blackwaters and in Africa is present only in those waters

flowing from volcanic regions. Johnson (1968) described an aberrant type of blackwater from

Malaysia which, by flowing over limestone, acquires higher total ionic concentrations of between

150–700 mg/l, accounting for the relatively high maximum values in Table 2.7. The “Gelam” type

waters also had very low PH values of 3.5–4.9 showing considerable excess sulphate. A third series of

blackwaters have become enriched by organic pollution and their PH was raised to 6 and above. This

series may indicate how blackwaters on other continents will react to pollution from urban sources as

human settlement of these areas becomes more intense.

In addition to the blackwaters, the Amazon and Orinoco are fed by clear water rivers. They originate

in the case of the Amazon from the massifs of central Brazil and in the Orinoco from the Guianian

shield. These waters are as poor in nutrients as the black-waters and have similarly low PH values of

between 4.0 and 6.6. They do not, however, have the dark staining by humic matter. A third category

of Amazonian rivers, the white water rivers, more closely resemble the general pattern of other

tropical rivers as described in the rest of this section and are the main source of nutrients in the basin

where, by the deposition of their heavy silt loads, they maintain the productivity of the várzea flood-

plains.

The lowland equatorial environments of the Amazon and the Congo give these two rivers many

common features. However, there are considerable differences between the two systems which arise

from the altitude and relief of the basins as well as from the geological formation and the vegetation

(Marlier, 1973). The forest is less extensive in the Congo and the main channel of the river is divided

into quiet and rapid reaches. Flooding is also less severe than in the Amazon.

CONDUCTIVITY

Conductivity is another measure of the total amount of ions present in a body of water and is a useful

approximation to chemical richness. As a measure, however, it does not give an indication of the

actual ionic composition of the water and may, therefore, fail to convey information on limiting

factors such as the lack of essential nutrients.

River waters are less variable than those of lakes and the conductivities of most major whitewater

rivers tend to resemble one another, with a few exceptions. In fact variations between tributaries

within any one river system are apt to be greater than those shown between different main river

channels. This is because the lower order streams are more sensitive to local geological and

vegetational patterns; whereas higher order channels average the conditions from a range of lower

order ones (Table 2.8).

Conductivity changes throughout the year within any one system. The broad, ionic composition of the

water is largely determined by the three processes mentioned in the introductory passage to this

section. The concentration of dissolved substances which include organic compounds not measured

by conductivity is influenced by five factors: (i) dilution effects, whereby flood or rainwater with

weak ionic concentration reduces conductivity; (ii) solution effects, the “shore factor” of Braun

(1952), whereby salts locked on previously dry land by decaying vegetation, animal dung, ash from

burnt vegetation etc., enter solution as the flood waters extend over larger areas; (iii) wash-out effects

whereby nutrient rich ground water is displaced into the river channel by increased infiltration; (iv)

concentration by evaporation and (v) absorption by living components of the system. Normally, these

effects combined tend to produce higher conductivities in the dry season than in the wet, both in

lagoons and in the river channels, giving an inverse relationship between water depth and

71

conductivity. The same trends have been recorded from the Oshun river (Egborge, 1971), the Ogun

river (Adebisi, 1981), the Kafue river (Tait, 1967; Carey, 1971; and University of Idaho et al., 1971),

the Senegal (Reizer, 1974) and the Zambezi (FAO/UN, 1969) in Africa. However, wash out of

nutrient rich ground waters may cause local rises in conductivity in small streams during the rains.

Several of these authors have also noted a secondary maximum of conductivity (due to solution

effects) as water invades the floodplain but this is of short duration.

Table 2.8

Conductivity and pH of some tropical river systemsa

Conductivity

(k20 μmhos cm-1)

ASIA

Gombak 29–41 6.6–7.0 Bishop, 1973

Mekong c. 280–630 6.9

Comm. for Coordination of

Investigations of the Lower Mekong

Basin

Meklong 8.2 Sidthimunka, 1970

SOUTH AMERICA

Magdalena system

Magdalena: mouth 490–630 6.2–7.2 Ducharme, 1975

above confluence 131 6.7–7.4 Ducharme, 1975

Cauca 250–500 6.5–7.6 Ducharme, 1975

San Jorge 320–350 6.5–6.6 Ducharme, 1975

Amazon system

Amazon/Solimoes 28.1–83.8 6.8–7.5 Schmidt, 1972

Maranon 129 Gessner, 1960

Ucayali 150 Gessner, 1960

Negrob 8.4–8.6 4.4–4.7 Schmidt, 1972

Tapajos 11.9–14.9 4.5–5.3 Schmidt, 1972

La Plata system

Paraná/Paraguay

alto 31.1–72.1 7.0–7.9 Bonetto, 1975/76

Paraná medio 112.0–184.0 7.2–7.7 Bonetto, 1975/76

Paraguay 69.7–335.0 6.9–8.2 Bonetto, 1975/76

AFRICA

Konkoure 22.1 5.9–6.2 Livingston, 1963

Bandama 90–200 6.7–7.6 Welcomme, 1972

Chari 22–73 6.9–7.7 Welcomme, 1972c

Logone 41–82 Welcomme, 1972c

Congo/Congo system

Congo main stem 37.1–76.7 5.5–6.5 Gosse, 1963

Ubangi 19.4–56 6.2–6.7 Micha, 1973

Luapula 150–180 6.2 Soulsby, 1959

Ruzizi 628 Marlier, 1951

Lualaba 145–255

Lt. Scarcies 35–55 7.1–7.4 Welcomme, 1972c

Gt. Scarcies 60 7.1 Welcomme, 1972c

Moa 36 6.6 Welcomme, 1972c

Oshun 57–96 Egborge, 1971

Niger 31–70 6.7–7.2 Daget, 1957

Sokoto 6.9–8.1 Holden & Green, 1960

72

Nile system

White Nile 220–500 8.0–8.9 Hammerton, 1972

Blue Nile 140–390 8.2–9.1 Hammerton, 1972

Kagera 93–99 Talling & Talling, 1965

Sobat 112 702 Talling, 1957

Bugungu Stream 245–395 7.1–7.8 Welcomme, 1969

Bahr-el/Ghazal 550 7.8 Talling, 1957

(L.No)

Semliki 400–910 Beauchamp, 1956

Orange 159 7.7 Keulder, 1970

Ouémé 60 Welcomme, pers.cos.

Ruaha 32–136 6.3–7.9 Petr, 1974/76

Senegal 72 6.8–7.1 Reizer, 1971

Volta system

Black Volta 41–124 6.5 Welcomme, 1972c

Red Volta 62 6.5 Welcomme, 1972c

White Volta 119 7.2 Welcomme, 1972c

Zambezi system

Kariba Lake 50–96 7.4 Coche, 1968

Upper course 57–126 FAO/UN, 1969

(Barotse plain)

Shire 220–450 7.5–8.8 Hastings,1972

Kafue 130–320 7.5–8.8 FAO/UN, 1968

Lower course 108–153 7.7–8.0 Hall et al., 1977

(Mozambique)

a These figures are often based on only a few observations and do not, therefore, necessarily reflect

the full range of the parameters measured

B Blackwater River

c Welcomme, 1972 compiled data received in response to a circular FAO questionnaire

The situation for Latin American rivers is less clear. Bonetto et al., 1978, show that, as may be

expected, conductivity in the Laguna La Brava rose during the low water period. Also, when floods

invade isolated floodplain lakes, they dilute the dissolved solids that have been concentrated during

the dry season, (Bonetto, Dioni and Pignalberi, 1969). Schmidt (1972) noted consistently higher ionic

concentrations during the dry season in the Amazon river as compared to the rains, and Junk (1973)

remarked on a similar fluctuation between high conductivities in the dry season and lower

conductivities in the flood in some floodplain lakes. However, both he and Schmidt (1972a) have also

described lakes in which the conductivity is minimal during the dry season and maximal during the

first phase of river inflow. The explanation advanced for this was that initial flooding by nutrient -rich

whitewaters, and solution of salts from inundated lands, gives high ionic concentration during periods

of increasing level. During the non-flood season water is diluted by drainage of nutrient-poor

blackwater from the forest streams and by rain-water.

In the littoral zone of the floodplain, represented by the vanguard of the advancing flood water,

nutrient-rich soil is freshly inundated and the solution of the salts produces a local increase in

conductivity. This is undoubtedly of great significance in the productivity patterns of floodplains

during the earlier part of the flood phase, when such shallow littoral zones must represent a

considerable proportion of the total area of the aquatic system.

73

HYDROGEN ION CONCENTRATION (PH)

Differences in water chemistry including pH, lead to variations in the type of aquatic organisms which

inhabit various waters. For instance distinctive communities of fish are particularly found associated

with highly acidic waters; these often have only low resistance to more elevated pHs.

Forest rivers, with their characteristics blackwaters and abundant humic materials, are slightly to very

acid with pHs ranging from 4 to neutrality. Savannah rivers are usually neutral or slightly alkaline

(Table 2.8). Because there is decaying vegetation on the bottom of most standing waters of the

floodplain, slight gradients often exist with higher pH values at the surface (Bonetto, 1975; Carey,

1971; Schmidt, 1973). Gradients in pH are also common at the water’s edge where the newly flooded

soil may produce either a local drop in pH, as in some African swamps, or a brusque rise in others.

The sudden rise in the pH of floodwaters on the Central Delta of the Niger, to as high as 8, has been

attributed by Blanc et al. (1955) to the effects of dung entering solution. Experimental evidence

confirmed that cow droppings raise the pH of water rapidly. The effects of this are, however,

temporary and more acid conditions are quickly restored. Laterite, which is a common soil in tropical

regions, and the peaty types of soil found on the surface of floodplains are both capable of reducing

pH (Mizuno and Mori, 1970). There is a tendency for swamps to become more acid as the dry season

progresses; Carey (1971) and Holden and Green (1960) particularly having noted that pH is lower in

swamps than in the river at this time. Drops in pH have also been noted by D'Aubenton (1963) under

the floating vegetation mat of the flooded forest of the Tonle Sap. By contrast conditions in some

open water lagoons can become increasingly alkaline as calcium salts are concentrated by evaporation

(Holden and Green, 1960). The lower pH values in swamps usually cause a general drop in pH

throughout the system when the acid waters are flushed out by rain or flood water with poor buffering

capacity early in the flood season.

Diurnal fluctuations in pH are primarily associated with the respiratory and photosynthetic activities

of the phytoplankton. A decrease in pH in the euphotic zone during the night, with a minimum early

in the morning, is a common feature of Amazonian várzea lakes (Schmidt, 1973). Similar trends can

reasonably be expected to occur throughout the day in the majority of floodplain lakes.

TEMPERATURE

Latitude, altitude degree of insolation, substrate composition, turbidity, ground or rainwater inflows,

wind, and vegetation cover, can all influence the temperature of water in rivers and floodplain lakes.

Generally, surface water temperatures follow the ambient air temperature fairly closely although

under dry hot conditions this is more likely to be correlated with air temperature minima due to the

cooling effect of evaporation. In the normal annual cycle in the tropics dry season water temperatures

are higher than those of the wet season. In the temperate zone winter temperatures are lower than

those in the summer and rivers may freeze for a considerable part of the year at higher latitudes and

altitudes. At relatively low latitudes, or at high altitudes in the tropics, temperatures may still fall

sufficiently to give conditions which are cold enough to cause growth checks and even fish

mortalities. In the Paraná River, for example, a combination of sub-zero temperatures and low water

caused massive mortalities in 1962 (Vidal, 1964).

Water in the main channel of a river rarely stratifies, as good mixing is maintained by the turbulence

associated with river flow. On the other hand stratified conditions are common in even very shallow

floodplain water bodies. Hastings (1972), for instance, recorded diurnal stratification in lagoons of the

South Elephant marsh (Fig. 2.12) which were only 1.6 m deep. Similarly, Sanchez (1961) and Junk

(1973) found 1–2 m deep lagoons from the Peruvian and Brazilian Amazonian várzea to be stratified

at least temporarily, and Ducharme (1975) (Fig. 2.12), Mikkola and Arias (1976), Arias (1975) and

Zarate and Cubides (1977) have all recorded stratified conditions from various cienagas of the

Magdalena system ranging from between 1.5–6 m depth. In the dry season the Amazonian Lago do

Castanho is only about 1.5 m deep, but clear stratification develops on windless days (Schmidt, 1973).

74

In flooded condition the lake is up to 11 m deep and stratification is more persistent, although the

temperature profiles are highly variable from one day to the next (Fig. 2.13) due to local changes in

conditions. The main day-to-day fluctuations occur in waters of less than 3.5 m depth, indicating that

only water down to this depth is normally affected by daytime heating. In the Paraná River, which is

subject to lower temperatures especially in winter and during the night, inverse stratification has been

noted by Bonetto, Dioni and Pignalberi (1969).

Figure 1.12 Diurnal cycle of temperature and dissolved oxygen in the Cienaga de Ayapel

(Magdalena River) 24 October 1974 (After Ducharme, 1975)

75

Figure 1.13 Daytime temperature fluctuations and changes in the stratification of Lago do Castanho

within short-time intervals during May 1969 (high-water). (After Schmidt, 1973)

It may be concluded that floodplain lakes not covered by vegetation normally stratify thermally

during at least part of the year. In shallower lakes stratification due to surface heating reaches a

maximum between 14.00 and 18.00 hrs, but tends to disappear at night or under the influence of

strong or persistent winds. The difference in density per degree at high temperatures is greater than

that at low temperature, for instance the drop in density between 29 and 30°C is about the same as that

between 4 and 10°C. For this reason at high temperature density differences are quite large for even

slight differences in temperature and the resistance to mixing is correspondingly high. In deep lagoons

stratification may last for several weeks or even months.

Water shaded by flooded or fringing forest tend to be cooler and more thermally uniform than that

exposed to direct sunlight, although thermal stratification has been described both from the flooded

forest fringing the Tonle Sap (D'Aubenton, 1963) and from Igapó forests of the lower Rio Negro

(Schmidt, 1976 and Geisler, 1969). Floating vegetation on the other hand gives rise to more varied

temperature conditions. Because of the restricted evaporation and high absorption of sunlight in calm

areas very high temperatures can be attained within masses of floating vegetation. Bonetto, Dioni and

Pignalberi (1969) found exceptional temperatures of up to 40°C in the surface waters of an oxbow

lake covered with Salvinia (Fig. 2.14). Similar temperatures have been found on the surface of Lago

76

Calado in stands of Paspalum repens by Junk (1973). Temperatures fall off more rapidly than they do

in open water due to shading by the vegetation. Floating mats of vegetation also encourage the

maintenance of stratified conditions by damping wind and wave action.

Figure 2.14 An extreme case of temperature accumulation in the waters under a bed of Salvinia at

midday. Solid line = open water; dotted line = water under vegetation. The arrow marks the

air temperature. (After Bonetto et al., 1969)

An important but little investigated habitat is the shallow open water littoral at the fringe of the flood

zone. At the time of rising water this represents the first contact between the advancing water and the

nutrient-rich dry soil. Because of its shallowness, daytime water temperatures may be very high and

gradients are set up between the very edge of the zone and the cooler water offshore (Fig. 2.15).

DISSOLVED OXYGEN

The distribution of dissolved oxygen within the aquatic system is one of the main factors influencing

the distribution of fish. In the rhithron waters are usually well oxygenated due to the turbulent flow

across the riffles. However, in the tropics these zones are apt to become separated into a series of

pools during the dry season when flow is minimal. At this time the pools may become deoxygenated

or even anoxic due to the decomposition of organic material, the oxygen demand of the fish and to

increased temperatures. In the Potamon the situation is more complex. As floodwaters invade the

floodplain of the Parana River there is an initial rise in dissolved oxygen concentrations in open water

and under floating vegetation. Similar effects have been noted from many other systems, particularly

the Sokoto (Holden and Green, 1960) and the Kafue (University of Idaho et al., 1971) in Africa, the

Paraguayan Chaco (Carter and Beadle, 1930) and the Magdalena (Arias, 1975; Zarate and Cubides,

77

1977) in South America. During the earlier part of the floods, flood or rainwater may flush

deoxygenated water out of depression lakes and swamps remaining from the preceding dry season.

There is, therefore, a tendency for dissolved oxygen levels to be low during the earlier period when

water is rising, although they remain fairly high throughout the major part of the floods. Higher

dissolved oxygen levels during the flood season are ascribed mostly to the aerating action of wind and

the mixing brought about by turbulence. In some deeper lakes, however, stratified conditions with low

oxygen tensions near the bottom may persist throughout the year. Vertical oxygen gradients have also

been observed in flooded Igapó forests in the Rio Negro basin by Geisler (1969) and Schmidt (1976).

In deoxygenated water, H2S concentrations may build up through the decomposition of bottom mud

and detritus.

Figure 2.15 Changes in the physical and chemical conditions with distance from the water’s edge in

a floodplain lagoon: (x--x--x) at 0800 hrs •--•--•) at 1200 hrs. (After Welcomme, 1970)

In the dry season dissolved oxygen conditions are linked to a number of factors including the size of

the water body, biological oxygen demand (BOD) of organic detritus or pollutants, degree of thermal

78

stratification, vegetation cover, phytoplankton development and wind action. In general there is a

tendency for even comparatively shallow bodies of water to stratify with higher dissolved oxygen

concentrations near the surface. Wind action is generally insufficient to ensure complete mixing in

deeper lakes where strong thermal stratification also exists. In shallower water bodies which lack

vegetation cover, wind action may ensure mixing through the whole water column (Schmidt, 1973).

However, resistance to mixing is comparatively high for the reason given in the preceding section.

Ducharme (1975) considered that, whereas wind is the major oxygenator in large lakes, dissolved

oxygen concentrations in smaller pools are largely controlled by the photosynthetic activity of the

phytoplankton. This means that smaller water bodies show greater diurnal changes in dissolved

oxygen than do larger ones. Photosynthetic activity raises the oxygen content of the water during the

day, particularly near the surface, but dissolved oxygen is withdrawn from water by plants and the

biochemical oxygen demand of silt and suspended organic solids and levels drop during the night

(Fig. 2.16).

Mass fish mortalities have been noted from Brazil (Brinkmann and Santos, 1973) and the Kafue flats

(Tait, 1967). In both cases the mortalities were attributed to sudden reductions in temperature cooling

the surface water of floodplain lakes which, coupled with strong winds, caused a rapid breakdown of

stratification, abrupt deoxygenation and contamination with H2S to lethal levels. Mortalities on the

Apure river floodplain have also been attributed to deoxygenated conditions (Matthes, pers.com.).

These arise when depressions containing unburnt grasses are inundated with rainwater. Because of the

persistence of thermal stratification the upper layers of the water (epilimnion) retain oxygen and the

fish survive there. However, the oxygen rapidly becomes depleted in the lower layers and H2S

accumulates. Later the turbulence associated with the entry of river flood waters mixes the layers

rapidly killing the fish. Other massive fish kills caused by the abrupt purging of oxygen depleted

waters from swamps have been recorded by Bryan and Sabins (1979) for the Atchafalaya river, a

tributary of the Mississippi. Such mortalities appear widespread in tropical floodplains as they have

been reported from many systems. Similar mortalities have been reported from cold water floodplains

where high BOD originating from pollution reduce dissolved oxygen levels below ice cover in winter

to levels below those tolerated by fish.

Floating mats of vegetation are an extremely common feature of tropical rivers and floodplain lakes.

The effects of these are similar whatever species of plant forms the mat. Fig. 2.16 for instance shows

the effect of stands of Paspalum repens on oxygen concentrations in Lago Calado of the Amazonian

várzea, but similar graphs have been drawn for the water column below Eichhornia crassipes (Arias,

1975 and Zarate and Cubides, 1977), Salvinia herzogii (Bonetto, Dioni and Pignalberi, 1969), Leersia

hexandra (Junk, 1973), Cyperus papyrus (Carter, 1953) and, Vossia (Tait, 1967). The general effect

of plant cover is to reduce dissolved oxygen concentrations, frequently to zero (Beadle, 1974) with a

coincident development and release of H2S. Because of the large amount of decaying organic matter

trapped in the root masses of floating plants, the deoxygenation often reaches a maximum in this zone

and sometimes rises slightly with increasing depth thereafter. The degree of deoxygenation below the

floating mat depends on water movements, for in strong currents the oxygen gradient beneath the mat

may be eliminated. It is, therefore, to be suspected that conditions under vegetation mats are less

extreme during the floods, when the constant movement of water may maintain at least some level of

dissolved oxygen. The density of the plant cover also influences the extent to which water becomes

deoxygenated; the more dense the cover, the less oxygen below it. Plant cover contributes to low

dissolved oxygen conditions in two ways. Firstly, a considerable biochemical oxygen demand is

created by both trapping organic matter and by the decay of their own vegetative parts, thus removing

oxygen from the water. Secondly, re-aeration of the water is prevented by reducing wind and wave

action on the surface of the water, and by producing shaded conditions unsuitable for phytoplankton

growth.

79

Figure 2.16 Diurnal changes in dissolved oxygen content and temperature in open waters and under

floating vegetation (Paspalum repens) in Lago Calado, 8 September 1968. (After Junk, 1973)

Oxygen gradients also seem to exist in the shallow littoral zone (Fig. 2.15). Wave and turbulence at

the water’s edge may here assure a restricted belt of oxygenated water, especially at night, even

though the water a little further offshore becomes completely deoxygenated. This effect, if

widespread, may be of considerable importance in the ecology of the fish, as many of the juvenile

forms are to be found in this zone.

80

CHAPTER 3

PRIMARY PRODUCTION IN RIVERS

THE RIVER CONTINUUM CONCEPT

Many workers have noted the apparent succession in ecological states along a river associated with

changes in morphology. Thus a river was early classified as young in its headwaters, where erosion

processes are dominant, mature in its mid-section where erosion and deposition are balanced and old

in its lower reaches where deposition is the major process. Rejuvenation zones may also occur in

mature or old reaches where increased gradient may temporarily reverse the more normal succession.

These descriptions are based on sedimentology but corresponding biological changes are also very

marked and have been summarized variously by Illies and Botasaneanu (1963), Hynes (1970) and

Hawkes (1975) who demonstrate the existence of different associations of flora and fauna along the

river which usually take the name of the dominant faunistic element, often fish. The nature of the flora

and fauna depend on the slope, current and the type of bed material in the river. As a river ideally

follows an orderly parabolic profile, the succession of the various elements appears correspondingly

orderly. Recently the river continuum concept, introduced by Vannote et al. (1980) has attempted to

assemble the various morphological and biological changes along a river into a coherent description

of this progression.

This concept assumes that the geo-physical variables within a river system present a continuous

gradient from source to mouth. Communities of living organisms succeed each other along the length

of the river in such a way as to minimize energy loss. This requires striking a balance between the

most efficient use of available energy through specialization and a contrasting tendency towards a

uniform rate of energy processing throughout the year. The structure of such communities is

summarized in Fig. 3.1. In the temperate rivers examined by the river continuum group, the following

shifts in community structure can be identified. Riverine communities can be separated into three

main groups, headwaters (orders 1–3), medium-sized streams (orders 4–6) and large rivers (orders

>6). Headwater streams are heavily influenced by riparian vegetation, which is responsible for large-

scale inputs of allochthonous nutrients while at the same time hindering autotrophic production by

shading. In some types of river, particularly blackwater rivers of the equatorial rainforests

allochthonous inputs in headwater streams or on floodplains may represent a major source of new

nutrients to the system. As stream size increases allochthonous inputs become less important and the

aquatic communities tend to concentrate more on autochthonous processing of nutrient transported

from upstream. This transport, with the exception of new allochthonous material arising from

floodplains and feeder tributaries, is the basis for all subsequent living processes. The first effect of

this process is in the nature of the allochthonous material which degrades from coarse particulate

organic matter (CPOM) in the low order streams to progressively finer particulate organic matter

(FPOM), ultrafine particulate organic matter and eventually to molecular components, amino acids,

sugars, etc. as one proceeds downstream. The composition of the living aquatic communities reflects

these changes in the nature of the nutrient substrates and in the physical form of the river ecosystems.

Plants progress from anchored submerged macrophytes in upstream reaches through periphyton to

phytoplanktonic communities within the main channel. Infrasubstrate communities shift from

shredder-dominated areas in the low-order headwater streams through grazer-dominated areas in the

medium-sized streams, to collector-dominated communities in the higher order streams. Fish

communities also tend to undergo a similar transition from invertivorous predator dominance in low-

order streams, through grazer-dominated communities in medium-sized rivers to iliophagous

dominance in the potamon.

Since its proposal many workers have used the concept as a framework for the analysis of small river

systems. Hawkins and Sedell (1981) for example, confirmed the predictions of the concept in four

Oregon streams, and Culp and Davis (1982) similarly successfully applied it to the Oldman and South

Saskatchewan river system. The ecological changes envisaged by the river continuum concept are

81

mostly accomplished in the progression through smaller order streams and little change is predicted in

rivers from order 6 onwards. This means that it may be used as a descriptor for the first 200 km or so

of a river course during the transition from rhithron to potamon but once the stable potamonic phase is

reached little further change can be anticipated for several thousand kilometres. Furthermore, within

the potamon, the floodplain is somewhat of a special case, for here the recycling of nutrients and

organic matter through the growth and decay of plants parallels, to a certain extent, the situation

nearer the headwaters, with the renewed input of CPOM. However, the nutrient base for this

productivity may be regarded as autochthonous in that it arises from the nutrients deposited in river

borne silt.

82

Figure 3.1 conceptual relationships between stream size and the progressive shift in structural and

functional attributes of lotic communities. (After Vannote et al., 1980)

Work by Bruns et al. (1984) indicates that, while this theoretical continuum may be applied to the

spatial evolution of conditions within the main channel, it is strongly influenced by presence of

tributaries. The ways in which these inflowing rivers affect the progression depends on the size of the

tributary relative to the main channel. This explains the sometimes abrupt transitions that may occur

at the confluence of two large water courses. Similarly the continuum may be interrupted or even

reversed by geomorphological irregularities in the normal catenary shape of the profile.

MATERIAL OF ALLOCHTHONOUS ORIGIN

An important input into the aquatic system is the rain of small non-aquatic creatures and organic

matter from terrestrial sources. As the floods advance, invertebrates, especially ants and termites, are

caught by the rising flood and incorporated into the aquatic system. There is also a continuous input

of insects, seeds, leaves, pollen and other material from flooded forests and grasslands which either

enter the drift in flowing waters, or settles to the bottom where it is decayed by bacterial and fungal

activity.

Workers on inundated forest regions all consider nutrients of allochthonous origin to be the single

most important, if not the only, input into the system. Geisler, for example, has in preliminary

experiments found that up to 56 individual pieces of organic material fell onto a quarter m² glue board

in the course of one day (Geisler, et al., 1973). The extent of leaf fall is indicated by data from many

parts of the world. For example, Blackburn and Petr (1979) summarizing data from low order

temperate streams showed that averages of between 3.5 and 8 t of plant litter (leaf, bark and

branches)/ha fall into the water every year. Comparable figures from the tropics indicate leaf falls of 6

t or more/ha/yr. Standing crops may be equally high, for instance in one small stream from the new

territories Hong Kong, Dudgeon (1982) found the detrital standing crop to be one hundred times

greater than that of the periphyton and more consistent at 80–120 gm/m² (= 1 t/ha). About 60% of this

was organic matter and thus accessible to the food chain.

Studies on the flow of nutrient through the Volga system indicate that the nutrient cycle in the river

and its reservoirs is greatly influenced by external inputs. The main trophic flows are thought to

proceed from allochthonous organic matter via bacteria direct to invertebrates, primary production by

phytoplankton and higher vegetation playing a very secondary role.

Annual litter fall on floodplain wetlands is very high, although the precise classification of material

generated on the floodplain as allochthonous or autochthonous is arguable. Records from the United

States range from 350 g (dry wt.)/m²/yr for an Illinois swamp to over 700 g (dry wt.)/m²/yr in North

Carolina (Mulholland, 1981). In Amazonian rain forests litter fall may be as high as 1 000 g (dry

wt.)/m²/yr although Adis et al. (1979) calculated that in the poorer Rio Negro, floodplain forest values

are closer to 580–790 g (dry wt.)/m²/yr. Even in savannah floodplains the presence of gallery forest

and floodable scrub vegetation provides a substrate from which materials fall into the water.

BACTERIA AND OTHER MICROORGANISMS

Much of the initial breakdown of CPOM in headwater streams is carried out by fungi and bacteria

which are abundant in the leaf letter of these zones. Sesile bacteria also form an important part of the

biomass of the riffle zones where they form epilithic slimes. Bacteria may became detached during

period of high flow and between 0.4 an 9.2 × 106 cells have been found suspended in flowing waters

depending on discharge (Marxsen, 1980).

Little information is available on the abundance of microorganisms in unpolluted rivers and

floodplain lakes, although they are obviously of immense importance both in the breakdown of

83

vegetation debris, dung and other organic remains, as well as in the diet of many species of detritivore

fish. The distribution of such microorganisms as fungi, actinomycetes, or starch, pectine and hemi-

cellulose decomposers, which obtain their energy from organic substrates, as well as aerobic and

anaerobic nitrogen fixing bacteria, were substantially higher in the swamps now submerged by the

Kainii reservoir, than in the river (Imevbore and Bakare, 1974). For example, a total of 6.3 × 107

organisms/ml was estimated to be present in the surface water of the swamps and 5.1 × 1011/ml were

estimated in swamp mud. This contrasts with the 3.5 × 104 and 1.3 × 1011/ml found in river water and

mud respectively. Fairly constant amounts of bacteria, between 2.3 × 105 organisms/ml for the Rio

Negro and 5 × 105/ml for the Solimoes were found in the rivers of the Amazon basin (Schmidt, 1970).

Greater numbers were found in the Lago do Castanho, a representative várzea lake. Here total

estimates of up to 7.3 × 106/ml were made although there is considerable seasonal and spatial

variation in density (Schmidt, 1969). In the lake, bacterial activity closely follows that of algae and

seasonal maxima of algae are always associated with maxima in bacterial number. Similarly there is a

distinct vertical stratification in both algal and bacterial numbers, with differences of between 0.5–4 ×

106 organisms/ml. Maximum densities occurred at about 1 m depth and at the bottom. Rai (1979) also

found a strong correlation between bacterial counts and water level in four Amazonian lakes. The

numbers of saprobic bacteria in a blackwater lake, ranged from 4 × 10³ - 2.2 × 105/ml during high

water to 1.1 × 105 - 9 × 105/ml during low water, and total bacterial counts in varzea lakes ranged from

2.1 to 11.6 × 108/ml during high water and 4.2 to 15.6 × 109/ml during low water. Rai further

confirmed Schmidt's findings that bacterial maxima were strongly correlated with algal maxima.

These examples from tropical systems indicate the richness of the bacterial flora in the more sheltered

tropical waters. The values do not in fact differ greatly from those quoted for the Danube by Mucha

(1967), where 1.5–2.6 × 106 organisms/ml were present in the bacterial plankton in Czechoslovakia

and Yugoslavia. In the Soviet portion of the delta 3.2–23.6 × 106 organisms/ml have been noted from

the river and 3.6–12 × 106 organisms/ml from the standing water Kilia arm. The Danube, however, is

highly polluted, particularly with organic matter. Other Ponto-Caspian rivers show similar ranges of

bacterial plankton numbers: 9-23.7 × 106 organisms/ml from the Volga and 1–3 × 106 organisms/ml

from the Dniester (Gavrishova et al., 1982).

PHYTOPLANKTON

The contribution made by phytoplankton to primary production within rivers is generally regarded to

be low when compared to other types of aquatic systems. However, phytoplankton is present in rivers

and contributes to the nutrient balance and to the trophic requirements of some of the fish species.

INFLUENCE OF ENVIRONMENTAL FACTORS

The major factors determining the presence and abundance of phytoplankton are temperature, velocity

of the current, availability of nutrients and availability of light.

TEMPERATURE

In high latitudes temperature is one of the principal regulators of planktonic abundance with a well

defined seasonal cycle based on the alternation of winter and summer. Thus in temperate rivers there

is a minimum in phytoplankton production and biomass during the winter. Even in the lower Parana,

where large seasonal temperature differences are normal, the summer high water figures for

phytoplankton numbers are frequently higher than the low water figures suggesting that temperature is

the major factor influencing phytoplankton abundance in this portion of the river (CECOAL, 1977).

In tropical rivers temperature plays a much diminished role and the greatest densities of

phytoplankton coincide with low water.

84

CURRENT

Phytoplanktonic organisms are sensitive to velocity and turbulence of flow in rivers as the rapid

currents and mechanical stresses of rapids and waterfalls inhibit the development of new plankton and

rapidly suppress any existing organisms discharged from associated lentic waters. Thus the agitated

waters of the rhithron generally support little plankton, although some does develop in the occasional

quiet backwater and pool. In addition a drift is present in the main channel made up of primarily

bottom living algal forms which are dislodged by the rapidity of the flow.

In the potamon, studies confirm the strong influence of flow and, consistent with this, show that

phytoplankton is more common in the lentic components of the system than in the lotic.

Prowse and Talling (1958) demonstrated the strong correlation between phytoplankton growth and

current velocity in the Nile at the Gebel Aulia dam. The dam slowed the Nile current and produced a

rapid increase in planktonic concentration. When the dam was open the flow was faster and plankton

concentration dropped. The build up of phytoplankton from this source has been thought to account

for the majority of potamonic plankton; a conclusion justified by the progressive disappearance of

such phytoplankton downstream from the point of discharge. Storage and discharge of water from

mainstream or major tributary reservoirs can also alter the indigenous patterns of phytoplankton

abundance. This has occurred in the Volga River where the whole algal fauna has been considerably

modified following the conversion of the river into a cascade of reservoirs (Kuzmin in Mordukhai -

Boltovskoi, 1979). Before impoundment the spring flood peak of phytoplankton biomass was less

than the summer low water peak. After reservoir construction, species composition changed, with a

reduction in the number of taxa present in the unmodified lower floodplain and delta. Here the

seasonal cycle of abundance has remained relatively unmodified although now the spring peak in

biomass (6.3 g/m³), when diatoms predominate, is greater than the prolonged period of high biomass

through the summer low water (2.4 – 3.8 g/m³) when green and blue algae are most common. The

phytoplankton of the Dniester River likewise underwent qualitative and quantitative changes after the

closure of the Dubassery reservoir with increases in Cyanophyta downstream of the dam. The

phytoplankton biomass varied between 0.66–0.85 g/m³ before the dam was built but rose to about 4

g/m³ in the river after its construction.

Discharge is assigned the major role in regulating phytoplankton abundance in the Mississippi river

(Baker and Kromerbaker, 1979), although in the river seasonal variations of temperature are high and

influence the succession of the various components of the phytoplankton. Phytoplankton abundance,

as represented by chlorophyll ‘A’ concentrations, was less in the main channels of the river than in the

backwaters or in a river lake where the current was slowed. In the Missouri river, Berner (1951)

associated the low planktonic densities (0.067 cells/ml) with high current and turbidity and a lack of

subsidiary floodplain waterbodies to feed into the mainstream. The Illinois River, a tributary of the

Missouri/Mississippi system had much higher phytoplankton densities (about 400 cells/ml) when it

was still connected to its floodplain lake and backwaters (Kofoid, 1908). The literature of the

Mississippi system between St. Louis and Cairo has been summarized by Schrammn et al., 1974 who

confirm these findings and commented on the difference in species composition and density between

main channel, riverine backwaters and floodplain lakes. Diversity is far higher in the lentic

environments where chlorophycea and Cyanophycea are dominant than in the river where

Chrysophycea are the major element. Similar findings are recorded by Bryan et al. (1975 and 1976)

who commented that phytoplankton has a very characteristic distribution within the Atchafalaya

system. When the flood is in progress habitats are inundated and flushed out producing homogenous

community structures. After the floods recede communities differentiate and representative forms

characteristic of the different types of water body again exert themselves. Clearly if much of the main

stream phytoplankton originates from flushing and discharge from the lentic components of the

system, build up in numbers must occur where the flow of a river is slowed or halted in backwaters or

in the standing waters of the floodplain. Rzoska and Talling (1966) found phytoplankton to be much

more abundant in backwaters of the Nile Sudd than in the main channels and thus Rzoska (1974)

85

quoted values of between 40 to 140 cells/ml for the river, whereas densities in lagoons reached from 1

720–2 330 cells/ml at river post 12. Differences in the specific composition of the phytoplankton were

also common. Blue-green algae such as Anabeana and Lyngbya dominated in the standing waters,

whereas in the river the sparse flora consisted mainly of diatoms especially Melosira. Samples from a

small West Africa river, the Oshun, showed similar trends to occur there. The main river is inhabited

mainly by desmids and diatoms, and colonial chlorophycea were the first to colonize the backwaters.

Phytoplankton abundance is also associated with seasonal differences in flow. Densities generally

reach a peak in the dry season and diminish in the floods in both types of water unless otherwise

influenced by temperature. Thus Egborge (1974) found a good negative correlation in the Oshun

between phytoplankton abundance and both water level and current velocity, with maximum

abundance at times of low water, although even then the total numbers of organisms were very low.

Iltis (1982) also found that algal populations were maximal during low waters and that the floods

were characterized by very poor phytoplankton in the main channels of six rivers of the Ivory Coast.

Carey (1971) had earlier found phytoplankton densities to be less during the floods in the Kafue River

with dense blooms occurring in the river at Nampongwe and in Namatenga lagoon between August

and November when the floods had receded. Phytoplankton was generally scarce in the Sokoto River

at most times of year, but was maximal in the dry season between March and June, especially in a

floodplain lagoon. Holden and Green (1960) suggested that, although the relative abundance of

organisms in terms of numbers per unit volume is lower during the floods, the absolute abundance

may well remain the same due to the dilution of the number of organisms by the enormously

increased volume of water in the system.

A similar argument was proposed by Bonetto, Dioni and Pignalberi (1969) who remarked at the same

time on the generally low contribution made by phytoplankton to the primary production of the

Parana River. This is particularly small during the floods or in water bodies with dense vegetation due

to high turbidity and shading effects, but may rise in some lagoons during the dry season. For

instance, CECOAL (1977) showed cycles of phytoplankton abundance in the upper Parana which

fluctuated between about 25 cells/ml in the floods to over 250 cells/ml during low water. In the

Middle Parana values of 2 331 cells/ml (low water) and 44 cells/ml (flood) represent values midway

between those of the two confluent systems (Bonetto, 1982). The algal population of the main channel

of the Amazon was also found to be higher at low water (15 000 cells/ml) than at high water (3 000

cells/ml) (Schmidt, 1970), and the same held true in a varzea lake with 500 000 algal cells/ml at the

period of minimum water level. Much of the rise in the number of phytoplankton in the river was

attributed to discharge of algal rich waters from lagoons. This cyclic pattern of activity is by no means

universal in the Amazon system as the Rio Negro showed a remarkably constant regime of about 10

000 algal cells/ml, and in lake Redondo the peak of algal production was reached during the rising

waters when the nutrient-rich whitewaters were invading the lagoon (Marlier, 1967). Because of the

interaction between the various water types in the Amazon system the nutrient regimes are likely to

differ considerably from the more normal regimes of rivers with only one dominant water type, and in

certain circumstances generally nutrient concentrations may be greater in the floods when lagoons are

invaded by nutrient-rich waters than during the dry season when nutrients have been diluted by

rainfall and inflow of poorer ground waters.

AVAILABILITY OF NUTRIENTS

There are several pointers to the important role nutrient availability plays in determining the

abundance of the phytoplankton and in particular in limiting it development beyond a certain level. In

the Gebel Aulia Dam for example, Prowse and Talling (1958) attributed the failure of phytoplankton

to develop beyond a certain level at slack water to nutrient depletion, particularly of nitrates. Talling

(1957) had earlier traced the high negative correlation of phytoplankton abundance and nitrate

concentration in the Nile. On the other end of the scale, extraordinary high concentration of nutrient

associated3 with eutrophication may result in blooms in excess of the usual.

86

The normal cycle of abundance associated with water velocity is shown in the Danube, where in

Romania 0.8 cells/ml were found during the June floods and 4.0 cells/ml were found during the

October low water (Szemes, 1967). Similarly in the slow reaches of the Russian part of the delta, cell

counts ranged from 192 cells/ml in the floods to 2 621 cells/ml at low water. This pattern was

disturbed by heavy pollution in the upper reaches of the river, where in the fast flowing Austrian

stretch 300 cells/ml were already present. Densities of 10–15 000 cells/ml were attained in the highly

polluted Czechoslovakian and Hungarian reaches, where water blooms are common during the

autumn giving a foul taste to drinking water. Juris (1975) even records numbers as high as 20 000

cells/ml in periods of maximum development, which may rise in high as 50 000 cells/ml in years with

especially low water. Further downstream, in Yugoslavia, cell counts dropped as low as 320–1 060

cells/ml and continued to fall to the figures shown for the delta. The Danube pattern indicates that

when abundant nutrients are available, flow becomes a secondary consideration in limiting

phytoplankton number.

Elevated standing stocks of phytoplankton in the Laguna Gonzales (54 642 cells/ml - 252 906

cells/ml) as compared to other lagoons of the Riachuelo (Parana river) - Laguna Totoras 129–1 330

cells/ml Laguna La Brava 335–9 235 cells/ml, Laguna Sirena 194–434 cells/ml and Laguna Meritta

72–654 i/ml are traceable to the highly eutrophicated state of Laguna Gonzalez (Bonetto et al., 1978).

The actual families of algae comprising the plankton vary much with water quality and in the Danube

Blue-green algae predominated under eutrophicated conditions. In the tropics too the major forms

present differ, for instance desmids tended to dominate in the flora of black-water streams both in the

Kapuas R., Borneo (Vaas, 1953) and in the Amazon.

In clear white waters of neutral pH diatoms and green algae are more abundant and in eutrophicated

waters or those with high pH, blue-green algae are the more common and indeed are often the only

element of the flora.

AVAILABILITY OF LIGHT

The amount of suspended matter in the water affects the penetration of light into the water, for

example Bonetto (1980) found a relationship S = a e bh for the Parana river, where S is the light

penetration as measured by Secchi disk, h is the velocity of the current as measured by the height of

the river and a and b are constants. It would thus appear that in many cases the limitation of plankton

development in the main channel of swift flowing rivers stems not from the current per se but from

the low penetration of light in such waters. In the main channel of the Parana just below the

confluence with the Paraguay river, for instance, the primary production varied between 0–285 mg

C/m²/day and the number of organisms varied between 80–2 000 cells/ml. The maxima and minima

were very strongly correlated with flow (Bonetto et al., 1979), abundance and production during the

flood being reduced not only by the strong current but by the poor light penetration. In many Latin

American floodplain waters, and probably in African areas too the productive zone is limited to a

relatively thin layer near the surface. This rarely exceeds 3 m in the Amazonian Lago de Castanho

(Figure 3.2) or in the flood lakes of the Riachuelo river (Bonetto et al., 1978a and b), or 2 m in the

Cienagas of the Magdalena (Mikkola and Arias, 1976). Limitation of photosynthetic activity during

the rainy season occurs when rising waters bring silt into the lagoon. It may be restricted in a similar

manner during the period of low waters when wind induced turbulence resuspends bottom mud.

87

Figure 3.2 Vertical patterns of primary production by phytoplankton and secchi disc transparency

(vertical bar) in Lago do Castanho from August 1967 to October 1968. (After Schmidt,

1973b)

OTHER FACTORS

Higher vegetation may also influence plankton abundance. In Bangula lagoon in Malawi, the waters

within patches of Nymphaea supported some 16 731 = 5 512 algal units (cells, filaments or

colonies)/ml. Here shading effects possibly reduce plankton densities. Over submersed vegetation,

higher but very variable figures were obtained of 38 107 – 52 188 units/ml (Shepherd, 1976).

Concentrations of planktonic organisms were some 13 times greater in patches of open water within

floating vegetation in the Laguna la Brava of the Riachuelo River than they were in the open water of

the lagoons on the same date (Bonetto et al., 1978a). Reduced phytoplankton densities near emergent

and floating vegetation and higher values over submersed vegetation have also been remarked upon

from the Danube.

PRODUCTION

Bonetto (1982) has proposed a model linking transparency with solar radiation, temperature and the

existing plankton density to predict the productivity of phytoplankton in the Parana River:

P = [10 + 0.28 (AmaxF)]e-0.019S

where: A max = -0.018 + 0.002Q + 0.021T

88

and

P = Productivity per unit area (mgC m³/day)

T = Mean temperature (°C)

F = Density of phytoplankton

A = Photosynthetic activity as defined by (2)

Q = Daily radiant solar energy (Cal/m²)

S = Secchi disc reading (in cm)

Considerable work on primary productivity of floodplain lagoons has also been carried out by

Schmidt (1973a) on the Lago de Castanho of the Amazonian várzea. His estimates of biological

production ranged from 2.15 gC/m/day at the lowest water level to 0.32 gC/m³/day during the inflow

of new river water. The net annual production was 297 gC m equivalent to a gross productivity of 358

gC/m²/year. The algal biomass was 1.9 gC/m² or 17 kg/ha with a gross productivity of 1.1 gC/m²/day.

Production from the Rio Negro was found to be considerably lower, ranging from 0.030 – 0.434

gC/m³/day giving a net productivity of 0.063 gC/m³/day or a gross annual production of 23

gC/m²/year (Schmidt, 1976). Other workers have found values between these two extremes. Bonetto,

Dioni and Pignalberi (1969) obtained values between 0.050 gC/m²/day and 1.0 gC/m²/day from two

different lagoons in the Paraná floodplain and two other lagoons, L. Totoras and L. Gonzalez of the

Riachuelo tributary to the Parana give somewhat higher readings of 0.2 – 2.5 gC/m²/day and 0.45 –

2.46 gC/m²/day respectively (Caro et al., 1979). Marlier (1967) found productions of between 0.14

and 0.7 gC/m²/day on the Lago Redondo. Values for various Magdalena cienagas range from 0.16 to

1.77 gC/m²/day (Mikkola and Arias, 1967) (Table 3.1), with mean values of 0.09 gC/m³/h from 18

lagoons of the Magdalena system (Arias, 1977). Values for gross primary productivity in the main

stream of the Godavari River (India) ranged from 0.30 to 1.06 gC/m³/day (Rajalakshmi and

Premswarup, 1975). Maximum values were recorded during the post flood period as transparency

rises and flow rate falls. At this time there was a good phytoplankton bloom. A second bloom

appeared after the summer rains. These figures are influenced by the discharge of organic polluting

effluents at one of the sites where mean values of primary production were the highest and also by the

anicut weirs which slow the flow and even out water level fluctuations.

These figures from tropical systems compare with temperate zone production rates of between 0 – 15

g 02/m²/day (equivalent to 0–4.7 gC/m²/day tabulated by FAO/UN (1973), and show the productivity

of phytoplankton in rivers to be extremely low, although peak production at low water from isolated

floodplain pools may temporarily reach the order of magnitude of production from lakes.

Table 3.1

Primary production estimates for various rivers and floodplain lakesa

System Production

gC/m²/day Authority

Amazon: Lago do Castanho 0.82 Schmidt, 1973

Lago Redondo 0.29 Marlier, 1967

Rio Negro 0.063 Schmidt, 1976

Rio Tapajos (0.44–2.41) 1.366 Schmidt, 1982

Paraná: Lago Los Espejos 0.05 Bonetto, Dioni,

Pignalberi, 1969

Lago El Aleman 1.00 Bonetto, Dioni,

Pignalberi, 1969

89

Laguna Totoras 1.2 –1.6 Bonetto et al., 1978

0.2 –2.5 Cara et al., 1979

Laguna Gonzalez 0.8 –2.5 Bonetto et al., 1978

0.45–2.45 Cara et al., 1979

Magdalena: Cienaga Guajaro 0.67 Calculated from data

Cienaga Maria la Baja 0.34 Mikkola & Arias, 1975

Cienaga Carabali 1.77

Cienaga Palotal 0.16

Godavari R.: 0.30–1.06 Rajalakshmi-Premswarup, 1975

Paraguay. Laguna Herradura 0.14–4.50 Zalocar et al., 1981

a N.B. These figures are often based on only a few observations and do not therefore necessarily

reflect the full range of variations of the parameters measured

ATTACHED ALGAE

Production by algae, especially diatoms growing on rocks, submerged wood and floating or

submersed vegetation may well be more important than the phytoplankton. This is certainly true in the

rhithronic headwaters where phytoplankton is virtually absent but where the rocks of the riffles

support. Mats of epiliths which in turn, provide the substrate for the complex of organisms comprising

the “aufwuchs”. In the pools too, the floating leafed vegetation of the slacks are colonized by dense

aggregations of epiphytes. The ecology of such communities in the running waters of the temperate

zone has been surveyed by Hynes (1970).

Few quantitative data appear to be available on this community in the potamon, although several

authors have remarked upon the abundance of such organisms. Rzoska, (1974) described the stems

and surfaces of emergent, submerged and floating vegetation in the Nile Sudd as being covered with

epiphytes including the red algae Compsogon. Indeed, Mefit Babtie (1982) record epiphytes as

comprising 16.6% of the total dry weight of Naias pectinata where 0.17 g of epiphytes were found per

gram of Naias. In the flood the proportion dropped to only 2.6%. Epiphytic algae were also recorded

as being very abundant on the Kafue flats by Carey (1971). The littoral of the Lake Chilwa swamps,

which closely resemble these of riverine floodplains, also support considerable populations of

epiphytes on Typha stems and on floating dead plant material wherever there is sufficient light

(Howard-Williams and Lenton, 1975). In Latin American waters Ducharme (1975), and Mikkola and

Arias (1976) considered the production by periphyton to be considerably superior to that of the

phytoplankton in the cienagas of the Magdalena river floodplain. The epiphytic community has also

been considered very important in the Middle Parana because of the abundance and density of support

in the form of floating and emergent vegetation, (Bonetto, Dioni and Pignalberi, 1969).

Although few figures are available it does appear that biological production by epiphytes is very high

in most flood zones, especially at the periphery of vegetation masses where light is adequate for

growth. In Bangula lagoon, Malawi, for example, the number of cells loosely attached to

Ceratopyllum was estimated by Shepherd (1976) at 3.35 ± 2.10 ×106/g fresh wt. of Ceratophyllum.

This gives an extrapolated number of 45.21 ± 17.70 ×109 cells/m² of lagoon surface, which was

between 0.8 and 20 (mean 9) times the number of cells of the same organisms found free in the water.

Investigations using artificial substrates in the Danube (Ertl et al., 1972) showed that periphyton

abundance was limited by frequent fluctuations in water level and by scouring. These effects may

have been magnified by the fixed nature of the artificial substrates used, which more closely

simulated rocks or emergent vegetation, than floating vegetation which could move with the water

level. Because of these effects, spring maxima in periphyton biomass were low and annual maxima of

up to 650 g/m² were achieved in autumn, and often persisted until early winter when flows were

minimal. Similar conclusions on the correlation between periphyton abundance and the flood cycle

90

were reached by Iltis 1982, in some West African rivers. Here the abundance of periphyton per unit

area followed the same fluctuations as those of the phytoplankton with the peak in relative abundance

being reached at the end of the dry season. However, the greatly increased area of substrate available

during the floods in the form of root masses of floating vegetation probably compensates for the low

density and absolute abundance may well be maximal at this time.

Colonization with diatoms such is Melosira or Oscillatoria produce biomasses of 2–15 /m² in the

muddy bottoms of the Volga and in the Dnieper biomasses exceed these of the zoobenthos. There are

indications that the density of epiphytes decreases towards the shaded interior of stands and mats of

vegetation along with the rest of the Aufwuchs community. For instance, in the Danube, Juris (1969,

1973) found 7.7 × 106 cells/cm² were present in shaded areas. Similarly standing crops of periphyton

are reduced with depth (Fig. 3.3) both because the lower light penetration limits growth and because

predatory consumer populations increase slightly with depth.

Figure 3.3 Biomass (B) and chlorophyll (CH) of periphyton at different depths in the littoral of the

Danube River (Ertl et al., 1972)

Because of the concentrated nature of the periphyton it obviously is a major locus of production in the

aquatic system. However, epiphytes may also perform subsidiary roles in the nutrient ecology of the

system. Heeg and Breen (1982), for example, found that epiphyton on Potamogeton crispus had a

considerable nitrogen fixing capacity of up to 23 mg N per 24 hours or 1.27 mg N/m² during the

inundation of the Pongolo floodplain. It also forms the base of a particular community which is

associated with the periphytic and perilithic habit, including the complex known as Periphytic Detrital

Aggregate (PDA). In samples from L. Valencia, Venezuela, Bowen (1979) has shown PDA to contain

42.7% dry weight of organic matter. Algae, on the other hand, only contributed a small portion of the

percentage (0.2 – 2.8%) by dry weight of the same sample even though they contribute a greater

volume (17–279 mg/ml of sample).

HIGHER VEGETATION

Distribution and zonation

Higher plants provide the major biotic structural elements in fluvial ecosystems. Not only does their

distribution depend on the geology and morphology of the environment, but the presence of

vegetation can itself modify the form of the system.

91

Although only rarely used directly for food by fish, vegetation has a range of ecological values for

fish communities. It provides refuge, shade, a substrate for spawning and a support for many

organisms which are of dietary importance for fish.

Longitudinal zonation of vegetation within rivers is based mainly upon the related factors of depth,

flow, and mechanical stress. In torrential headwaters liverworts and mosses are the earliest forms to

appear on rocks, both submersed and in the splash zone. This type of vegetation persists into the rocky

riffles of the rhithron, but in these reaches the pools increasingly support rooted, floating leafed

species in the slacks and emergent vegetation along the banks as slope decreases. As the river enters

the more mature potamon, the ideal channel is fringed with emergent plants and floating grasses

which give way to floating leafed plants and submersed species as the depth increases toward the

centre of the river. Such a stable state is rarely attained in natural rivers, where one or other of these

elements may be lacking. Similar successions can be seen in islands within the river channel where

sand banks laid down by the current became colonised by plants which fix the bank and lead to

further siltation. Such islands eventually form part of the seasonally floodable area taking the form of

internalized floodplains. Gosse (1963) describes the distribution of vegetation on such islands in the

Congo R. (Fig. 3.4).

The river floodplain shows much more complex lateral successions based on the degree of flooding

such as that proposed by Adams, 1964 which contains the following zones:

(a) permanently flooded waters with submersed vegetation only (open waters);

(b) permanently flooded areas with rooted or floating emergent vegetation;

(c) regularly seasonally flooded areas with rooted and floating emergent vegetation;

(d) areas that are occasionally flooded (between mean flood and highest flood levels;

(e) areas that are not flooded but whose water table is influenced by the flood regime.

In the Shire river Elephant marshes, Howard-Williams (in Hastings, 1972) distinguished the following

zones:

(a) aquatic plant zone: with floating sudd islands composed of Echinochloa pyramidalis, Ludwigia

stolonifera and Ipomoea aquatica, together with true floating plants such as Azolla nilotica,

Salvinia hastata and Pistia stratiotes;

(b) a swamp zone with water between 50 cm and 2 m in depth which consists of floating meadows of

Echinochloa pyramidalis, Vossia cuspidata, Leersia hexandra, Cyperus papyrus, and

Echinochloa stagnina;

(c) flooded grassland 1.5–6 m deep, mainly dominated by Oryza barthii;

(d) shallow flooded grasslands and levees with depths of flooding between 0.25 and 25 cm with

tussocks of Setaria avettae, Vetivaria nigritana;

(e) floodplain margin regions with Hyparrhenia rufa, Panicum coloratum, Vetiveria nigritana and

Setaria sphacelata.

92

Figure 3.4 Successions of vegetation on islands in the Congo River (A) Young Island; (B) Older

Island (from J. Louis, 1947, quoted by Gosse, 1963)

Zonation of vegetation fringing the Parana River and its floodplain lakes is similar and also varies

according to the type of terrain as shown in Fig. 3.5.

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Detailed analyses of the vegetational zonation of floodplains carried out by Schmid (1961) for the

Tonle Sap in Cambodia and Smith (1976) for the Okavango Delta, show relatively little difference in

the basic type of spatial zonation for tropical systems.

In the Danube the zonation of vegetation is based on a hydrographic index where hg = 0.1 (mean high

water level - mean low water level) and vegetation complexes are correlated with hg in the following

manner:

Hg 6–7 highest river banks forested with willows, poplars, and ash

Hg 5–6 water meadow pastures

Hg 8–5 emergent water plants, cat tails and reeds

Hg 3 well developed reed beds

Hg 0.3 backwaters, lakes with floating vegetation and submersed plants

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Figure 3.5 Zonation of vegetation fringing the Parana River: A. Sand banks; B. Bordering deep

lagoons and channels; C. Shallow lagoons ad marshes. (After Franceschi and Lewis,

1979)

Because of the regular seasonal variations in water level in many river systems there is a temporal

succession as well as spatial one. In the long term the colonization of river channels, backwaters and

the various types of floodplain water body by floating and emergent vegetation accelerates siltation

and tends to catalyze the transition of such waters to dry land. This type of succession has already

been described for the Danube River in considering the morphology of the floodplain. More detailed

profiles of the shorelines of two of the numerous lakes from the Riachuelo river of the Parana system

(Bonetto et al., 1978a and 1978b) (Fig. 3.6) show the difference between Lago Totoras, a clear,

moderately eutrophicated lake with prairie like marginal vegetation which is presumably young in

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temporal succession whereas L. la Brava, which is covered with floating emergent vegetation,

possibly represents an older and mature lake of the same system.

Figure 3.6 Zonation of littoral vegetation in two lagoons of the Riachuelo River A. Laguna La Brava;

B. Laguna Totoras. (After Bonetto et al., 1978a and b)

In the short term there is an annual sequence of replacement of one species by another in response to

the different degrees of flooding. In individual lagoons, vegetation increases in density and biomass

96

throughout the dry season until it occupies most of the water surface. As the water rises much of the

vegetation is washed out and biomass within the lagoon falls rapidly. Nieff (1975) has further defined

the changes in the relative abundance of the species found in the lagoon showing the progressive

occupation of the free waters by floating forms such as Azolla and Salvinia during rising waters. The

subsequent growth of Polygonium punctatum, Ludwigia peploides and Mycrophyllum brasiliensis

shades out the free floating forms and as water level falls, these plants are in turn replaced by

Nymphoides indica.

Riparian forest is an important element in the vegetation complex of rivers in that, in the natural state,

fallen wood structures the environment, leaf fall provides a major source of organic and nutrient

inputs and the overhanging vegetation gives a mosaic of light and shade which conditions the

distribution of many aquatic organisms. Originally many of the world's floodplains would appear to

have been forested, at least by a strip of gallery along the channels. However, the influence of man

and his domestic animals in colonizing the rich alluvial bottom lands has resulted in their clearance to

produce the type of agricultural or savannah plains so familiar throughout the world. Two examples of

this historical development are given by Yon and Tendron, 1981, for the alluvial forests of Europe

(Fig. 3.7) and by Van Leynsele (1979) for the Ngiri floodplain. The Ngiri, which lies in the densely

forested Congo system is kept free of trees by regular burning of vegetation.

Figure 3.7 Schematic historical development of the landscape and vegetation of a river valley in

Europe. Middle reaches (right); Upper reaches (left).

a. At the beginning of the Christian era; b. Circa 1000; c. Circa 1800; d. Circa 1900

1. beech groves; 2. mixed forest and oak stands; 3. alder stands; 4. afforestation conifers;

5. bushy willows; 6. other bushes; 7. water meadow; 8. fresh meadow; 9. dry meadow; 10.

crops; 11. slope loam; 13. more; 14. gravel; 15. other soil; 16. average level of ground

water; 17. average level at high water. (After Yon and Tendron, 1981)

Deforestation is still proceeding throughout much of the tropical world, but considerable areas still

remain in South America, Africa and Asia, where the floodplains are occupied by dense forests of

97

flood-resistant trees. These may be of two main kinds which are best seen in the Amazon system

where the ombrophilous lowland forests occupying the alluvial plains of the whitewater rivers are

known as várzea forests and the tropical evergreen peat forests occupying the floodable zones of the

blackwater rivers are known as Igapó forest. River-side or gallery forests tend to occupy the levees on

many wet savannah rivers and Bonetto (1975) described the Paraná as a corridor by which the

Amazonian forest is able to penetrate far to the south of its normal distribution. Profiles of forested

floodplains given by Sioli (1964) and Bonetto (1975) (Figs. 1.7a and b) describe the general

distribution of the major vegetational zones of the Amazon and the Parandá Dry savannah rivers are

frequently bordered by flood-resistant trees and scrub, mainly of the Acacia type, but also some palms

especially on dry terrain ridges and levees. In the Central Delta of the Niger, the boundary between

unshaded and gallery forested floodplain channels was distinguished by Daget (1954) as following the

1 000 m isohyet; to the north drainage channels were unshaded and to the south were forested. The

displacement southwards of the isohyet during the Sahelian drought has produced changes in the

distribution of the woodlands bordering the river channels.

Submersed vegetation

True submersed rooted aquatics form a high proportion of the macrophytes in temperate rivers and

according to the data summarized by Westlake (1975) there are numerous studies indicating dry

weight biomasses of around 5 000 kg/ha at fertile sites, such as backwaters and lagoons or within

stabilized river channels. They are however, rarer in tropical systems. This appears to be mainly due

to high turbidity or shading by floating meadows and other floating plants in the tropics, which

prevent the development of species with no aerial parts. Aquatics with floating leaves are commoner

in the slacks of pool systems, in quiet bays, openings and backwaters or just off the open waters fringe

of the floating vegetation. Thus various species of Ceratophyllum, Trapa, Naja and Nymphaea are

widely if sparsely distributed through quiet river channels and in most of the permanent waters of the

world's floodplains. They also appear temporarily in the seasonally inundated area where they are

concentrated in the major depressions and channels.

In the floodplain lakes of the Danube, Potomogeton perfoliatus, Valisneria spiralis, Ceratophyllum

sp. as well as Trapa natans and Nymphaea alba contribute a significant portion of the biomass and

total plant production (Academia Republicii Socialiste Romania, 1967). In Crapina lagoon, for

instance, Nicolau (1952) calculated a biomass of P. perfoliatus of 1 749 kg/ha². Estimates in

Academia Republicii Socialiste Romania (1967) indicated productions of between 2.52 and 4.55

gC/m². Production is high in June-July but had fallen by September. The lowest values were recorded

in December. In the Pongolo river considerable standing crops of Potomogeton crispus of up to 1100

kg/ha² (dry wt) were recorded at the end of the growing season by Heeg and Breen (1982) indicating

that submersed plants can also be important in semi-tropical systems.

Floating vegetation

One of the most conspicuous features of the tropical floodplain swamp communities are the vast areas

occupied by floating vegetation; this may take the form of free floating types or of Sudd and meadow

forming varieties.

Free floating forms. The same types of small free floating plant tend to recur throughout the world's

swamps. Principal among these are Eichhornia crassipes, Pistia stratiotes, Azolla sp. and Salvinia sp.,

which form extensive mats which may choke water ways and induce deoxygenated conditions under

them. They are influenced by the wind and current, and Bonetto (1975) has illustrated the manner in

which this type of floating vegetation can accumulate at the outlet of depression lakes, clogging

normal drainage until released into the main channel as “embalsados” (Fig. 3.8) Eichhornia crassipes,

can double in number every 8–10 days in warm nutrient-rich waters (Wolverton and McDonald,

1976), but normal production is possibly less than this in the nutrient-poor swamps of floodplains.

98

Dymond (in Westlake, 1963), for example, found a biomass corresponding to 1.4 kg dry weight/m²

which is equivalent to an annual organic production of 11–33 t/ha.

Salvinia sp. can also form a major nuisance when introduced into waterways from which they were

previously absent. In the Sepik River of Papua New Guinea, for instance, S. molesta occupied nearly

all the free water in the system after its introduction and attained densities up to 6.8 kg of living

material (fresh out) and 2.8 kg of dead and decomposing material/m² [equivalent to 96 t/ha of

biomass], Mitchell et al. (1980). The mats effectively cut off all light from the underlying waters and

also produce significant reductions in Dissolved Oxygen concentrations.

Floating meadows

Although floating vegetation fringes temperate river channels, it is in the tropics that it finds its

greatest expansion. In Africa four species dominate the more deeply inundated parts of the floodplains

and regularly form vast floating mats fringing river channels and floodplain lagoons. During the

floods portions of these mats are liable to break up to form floating islands or sudds. These are

Cyperus papyrus, Echinochloa pyramidalis, E. stagnina and Vossia cuspidata. Of these, only C.

papyrus is dependent on permanent water for its survival and reaches standing crops of between 10

and 34 t/ha dry (Thompson et al., 1979). In the Upemba basin production of C. papyrus ranged

between 50 and 94 t/ha/yr.

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Figure 3.8 Mechanism of release of floating vegetation masses “embalsados” during the flood cycle:

(A) ponds with “embalsados” in flood condition; (B) drainage clogged by vegetation after

flooding; (C) the same situation during the rains; (D) release of the “embalsados” into the

river. (After Bonetto, 1975)

In the Amazon basin the varzea grasses desiccate during the dry season, although some may be semi-

aquatic and have alternative dry season forms. During rising water there is an explosive growth phase

which culminates after 4–6 months in flowering, followed by senescence and death. During the six

month growing season Junk (1970) estimated Paspalam repens to attain 6–8 t (dry weight)/ha with a

production surplus of 3–5 t/ha. Marlier (1967) estimated a mean standing crop of 96 t/ha fresh weight

equivalent to 9.1 t/ha dry weight in Lago Redondo. The flora of Amazonian varzea lakes having high

oscillations in water level (Fig. 3.9A) are composed mainly of P. repens and E. polystachya which die

back completely at times of low water. In lakes with less extreme variations, more permanent islands

of Leersia hexandra form and these become secondarily colonized with Cyperus sp. and eventually

small trees and other non aquatic plants (Fig. 3.9B).

100

Figure 3.9 Schematic distribution of vegetation of two types of Amazonian varzea lake: (A) with big

oscillations in water level; (B) with small oscillations in water level. (After Junk, 1970)

After the floods stranded floating vegetation decomposes extremely rapidly with 40–50% of dry

matter being eliminated in the first 14 days and 60–70% within 50 days of desiccation. The nutrients

concentrated within it are thus rapidly made available to other elements of the community,

particularly to detritus feeders of various sorts. According to Howard-Williams and Junk (1976) the

nutrient levels and nature of the decomposition litter varies very little with plant type, which means

that, irrespective of origin, the detritus becomes chemically more and more uniform as decomposition

proceeds.

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Floodplain meadows

The majority of savannah floodplains are covered with various types of grassland which follow a

fairly typical annual course in all but the most highly cultivated plains. Junk illustrates such a

temporal succession for grasses from the Amazonian floodplain (Fig. 3.10) whereby the dominant

plants on the plain change annually in conjunction with the flood cycle. Most floodplain grasses are

rhyzomatous and after the floods subside the enormous elaboration of wet season growth is burnt off

either naturally or by man set fires. New growth of the dry season type is grazed by cattle or by wild

game and burning may occur at intervals throughout the dry season. Very high levels of production

have been recorded during this phase, of which the 23 kg/ha/day recorded by Heeg and Breen (1982)

for Cynodon dactylon on the Pongolo plain may be considered normal. Although much of this is

grazed some 825 kg/ha remain at the end of the dry season which means that in a single year some 34

tons of C. dactylon is submerged representing about 25 tons of wet organic matter as input to the

aquatic system. In the floods some of these grasses may take on a wet season form with floating nodes

which themselves develop roots, as in the case of Vossia and Echinochloa, or may remain rooted in

the bottom but increase their stem length. Oryza barthii for instance, stands about 50 cm above the

water surface irrespective of depth even when submerged under 2–3 m of water. Growth is very fast,

as much as 1 m in two weeks, and productions of up to 2.5 t/ha can be achieved in five weeks. Total

annual production can be reasonably high, and while dry grassland will not produce more than 2–3

t/ha/yr, productions of 10–20 t/ha/yr are not regarded as unreasonable for seasonally flooded

grasslands by Thompson (1976).

Figure 3.10 Temporal succession of grasses on Amazonian floodplains correlated with changes in

water level throughout the hydrological cycle. (After Junk, 1983)

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The floodplain grasslands are highly modified both by the natural flood regime, which selects for

flood-resistant forms, but also by burning and grazing which prevent the recolonization of the plain by

flood-resistant scrub bushes (Greenway and Vasey-FitzGerald, 1969).

Emergent vegetation: Emergent aquatic vegetation, such as Typha, Scirpus and Phragmites are

widespread but localized in shallow muddy areas, and also tend to colonize sheltered alluvial banks of

rivers in both temperate and tropical areas. Certain alkaline soils seem to favour these forms; the plain

of reeds of the Mekong system where Eleocharis equisetima appears in great abundance (Le-Van-

Dang, 1970) is a good example for this.

Stands of emergent vegetation, one established, are a major modifier of the ecosystem. By changing

flow patterns they increase siltation accelerating the filling of floodplains depressions and

contributing to the accretion of silt on islands on the main channel. They buffer the effects of scour

and with their root and rhizome masses stabilize the river channel and its islands. On the temperate

floodplain of the Danube, Bothnariuc (1967) recorded 1.2 kg/m² (dry weight) of plants from a marshy

pond, and characterized the succession from ghiol to japse as a progression from a primary production

based on phytoplankton in the open water “ghiol” to one based on higher emergent vegetation in the

marshy “japse”.

Westlake (1963) placed tropical reed swamps as one of the most productive communities of plants

with organic productions of up to 75 t/ha/yr. Analysis of Cyperus papyrus has shown it to have about

20 kg total biomass/m² in dense stands, the aerial portions forming 60–70 percent of the biomass

(Thompson in Westlake, 1957), although 3–5 kg/m² were considered more likely over larger areas.

Typha domingensis reaches a similar total biomass of 4.4 kg/m² with 52 percent underground.

However, freshwater macrophytes in Malaysia are reported as having a standing stock of 370–520

g/m² (Wassink, 1975), a tenth of the values for African swamps.

The role of higher vegetation in nutrient balance

There seems no doubt that the major part of the primary production of the floodplain is concentrated

in the higher vegetation and principally in the perennial grasses. These die through desiccation, and by

decay, burning or digestion by grazing animals are returned to the dry soil as nutrient, ash or dung

ready for solution and utilization during the next flood phase. Floating weeds and sudd islands, which

are often present in huge amounts, are also a mechanism for the translocation of nutrients within the

system, representing a very real loss as they are swept downstream.

The remarks of Howard-Williams and Lenton (1975) summarizing the role attributed to higher

vegetation in the littoral flood zone of lakes, can equally be applied to the floodplains and swamp

vegetation:

(i) the vegetation provides a diverse habitat for animals and plants;

(ii) it acts as a filter and trap for allochthonous and autochthonous materials which in turn serve

as nutrients for the plant communities themselves or for the associated aufwuchs and fish

communities;

(iii) the nutrient pump effect of the emergent vegetation reputedly increases the concentration of

elements in littoral areas of lakes and almost certainly does so in newly flooded waters of the

floodplain;

(iv) it contributes to the autotrophic production in that as it decays it forms a rich detritus which

is utilized as food by may organisms;

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(v) furthermore, Howard-Williams and Junk (1977), suggested that the aquatic plants of the

Amazon, and presumably of other nutrient poor systems, act as nutrient reservoirs and play a

major role in the biogeochemical cycling of nutrients within the aquatic (floodable

ecosystem) of the varzea. The vegetation which proliferates during the flood is deposited on

the floodplain, on the exposed banks of the channels or in the lagoons where it decays. This

material is then subjected to leaching and elution by tropical rainstorms and is stored in the

soil until the next rainy season. This results in the conservation of salts in the floodplain

rather than their being swept downstream dissolved in the main water mass.

In forested rivers and streams, riparian or floodplain vegetation contributes considerable amounts of

nutrients to the system. This has been best studied in low order temperate rivers where the autumn-

shed leaves form an important source of allochthonous organic matter as well as food for

invertebrates. Kaushik and Hynes (1971) have documented the decomposition of such leaves to show

the build up of protein, nitrogen and phosphorus in water where the decomposing leaves serve not

only to nourish invertebrates but to provide food for detritivore fish. Indeed in nutrient poor rivers the

leaf fall may be the biggest single source of nutrients to the aquatic system.

CHAPTER 4

SECONDARY PRODUCTION IN RIVERS

ZOOPLANKTON AND DRIFT

Two groupings of small animal forms are found within the flowing waters of main river channels; the

true zooplankton and the drift. It is often very difficult to distinguish between the two and in many

cases the organisms comprising them are fairly similar. In torrential and low order streams the

majority of the drift arises from the dislodging of aquatic benthic organisms and from terrestrial

insects dropping from overhanging vegetation or swept into the channel with surface flow. In larger

slow flowing potamonic rivers the role of these dislodged and entrained organisms diminishes, and

the role of a true zooplankton increases accordingly. Kammeyn and Novotny, (1977) identified four

components of the drift in the Missouri river: (i) particulate organic matter (POM); (ii)

macroinvertebrates; (iii) larval fish, and (iv) zooplankton. Concentrations of POM and detritus ranged

from 11 mg/m³ in anabranches through 783 mg/m³ at the main channel border to 2331 mg/m³ in the

main channel in channelized reaches. Unchannelized reaches had less of the material with 18–135

mg/m³ in anabranches, 64–111 mg/m³ at the main channel border and 111–505 mg/m³ in the main

channel. Although concentration of POM may increase in channelized reaches of the Missouri, Morris

et al., 1968 recorded an opposing trend where mean values of drift organisms in unchannelized

reaches were 1980 mg/m³ and only 230 mg/m³ in channelized reaches. These estimates indicate that

about 526 kg of drift organism flow past a fixed point during the day in an unchannelized reach and

204 kg/day in a channelized reach. As with other workers Morris et al. found the species composition

of the drift was very dissimilar from that of the benthos in large river channels and more closely

resembled the composition of the aufwuchs, although about 20% of the drift by weight were terrestrial

(allochthonous) forms. Numbers of drift organisms increased at night when densities were often

double those during the day, and a marked diurnal periodicity in the drift has previously been

described from many temperate rivers (see Hynes, 1970). In tropical waters too the number of

organisms increases at night and particularly at nightfall (Elouard et Leveque, 1977). The abundance

of larval and juvenile fish in the drift, which shows no such periodicity may, in part, be attributed to

the downstream movement of yolk-sac and post yolk-sac larvae, and in part to the presence of

individuals feeding on the drift organisms.

104

Information on zooplankton in rivers, particularly those of the tropics, is rather sparse but existing

studies indicate that similar factors influence zooplankton densities as those influencing

phytoplankton. The abundance of phytoplankton itself is possibly one of the major determining

features and peaks in zooplankton abundance are apt to follow peaks in phytoplankton as exemplified

for the Paranà river by Bonetto (1976) (Fig. 4.1).

Variations in zooplankton abundance have, however, been attributed primarily to differences in flow,

with a number of other factors including turbidity, dissolved oxygen concentration and conductivity

also playing a minor role. Under normal flow regimes only low densities of zooplankton are present in

the main channel of rivers. For example only about 5 250 individuals/m³ of protozoa, crustacea and

rotifers have been recorded from the Mekong river (Sidthimunkra, 1970). Rzoska (1974) also

remarked on the small quantities of zooplankton found in the Sudd although Holden and Green (1960)

had noted higher numbers of up to 16 000 individuals/m³ in the Sokoto river. In fact considerable

differences are to be found between the planktonic fauna of the main channel and that of the lentic

waters of side arms and floodplain and the zooplankton of both lotic and lentic waters fluctuates in

abundance according to season. Thus whilst only 1 000 individuals/m³ of zooplankton were found in

the Upper Paranà River at high waters 10 000 individuals/m³ were present at low water. Similarly in

the Blue Nile at Kartoum planktonic Crustacea were present at about 20 000 individuals/m³ during

periods of moderate flow but these rose to up to 100 000 individuals/m³ at low water (Tailing and

Rzoska 1967). In the Nile Sudd densities were also higher in the dry season (mean of 4 460

individuals/m³ for four sites) than in the wet (mean of 2 070 individuals from the same four sites). The

abundance of zooplankton in temperate rivers may be seasonally high relative to that in tropical

waters, as in the Danube where Enaceanu (1964) recorded only 550 individuals/m³ in winter which

rose to over 1 million individuals/m³ in the September peak. This superabundance of organisms may

have been due to enrichment of the Danube waters by domestic and industrial contaminants but there

is no doubt that, were flow conditions unfavourable, no such development would have occurred.

105

Figure 4.1 Variations in abundance of phyto- and zooplankton in the Parana River at the level of

Parana relative to temperature and river level. (After Bonetto, 1976)

Zooplankton is much more common in the floodplain pools and backwaters where its abundance per

unit volume is usually inversely correlated with the amount of water in the system. Vranovsky (1974),

for instance, found the average biomass of zooplankton to be 14–15 times higher in the Baka side arm

of the Danube than in the main stream (mean of 6.75 g/m³ for two years in the river). During periods

of low or nil flow, greatly increased biomasses were present, especially in the isolated side arms

where values of up to 30 times those of the main river were recorded. Similarly Sidthimunka (1970)

was able to record 131 000 individuals/m³ in the Mong river, while only about 5 000 were present in

the main stem of the Mekong. In the standing waters of tropical floodplains too, there is a widespread

tendency for greater concentrations of organisms to be present in the dry season, as is the case with

phytoplankton. Like phytoplankton, however, the decrease in relative density may only be a dilution

effect and the number of organisms over the floodplain as a whole may in fact be higher during the

wet season than during the dry. Bonetto (1975) recorded increases in numbers of zooplankters per unit

volume even during the floods of the Parana, although elsewhere in the system the lowest values were

found during the same period (Bonetto and de Ferrato, 1966). Increases in specific diversity often

occur simultaneously with increases in absolute number. Rivers subject to marked annual thermal

variations tend to have peaks of zooplankton abundance in the spring and summer months (Bonetto

106

and de Ferrato, 1966; Bonetto, 1975), or in the spring and autumn (Ertl, 1966; Osmera, 1973) and

these do not always coincide with the period of lowest water. The abundance of organisms varies and

considerable numbers are occasionally found in permanent lakes. A summary of estimates of

zooplankton abundance from floodplain standing waters is shown in Tables 4.1 and 4.2.

Table 4.1

Estimates of biomass of zooplankton organisms in standing waters of floodplainsa

River Biomass g/m³ Authority

Danube:

Dyje:

Locality 1 2.0–4.6 (471.6 max.) Osmera, 1973

Locality 2 7.75

Locality 3 7.5

Baka:

Side arms 6.75 (mean of 2 years) Vranovsky, 1974

Amazon:

Lago Redondo 1.61 Marlier, 1968

Nile Sudd:

Aljab 0.002–0.001 Monakov, 1968

Shambe 0.002–0.40

Jor 0.021–0.049

No 0.138–0.107

Atar 0.246

Magdalena: 0.92 (mean for 4 lagoons April)

1.95 (mean for 5 lagoons June) Mikkola & Arias, 1976

1.53 (mean for 6 lagoons August)

a These figures are often based on only a few observations and do not therefore necessarily reflect

the full range of variations of the parameters measured

Flow is also slowed where waters are impounded behind dams and concentrations of zooplankton

reach high levels in the reservoirs thus formed. This high concentration of organisms forms a locally

increased concentration of zooplanktonic organisms at the point of discharge from the dam which

diminishes slowly as the water mass moves downstream, Dzyuban in Mordukhai-Boltovskoi (1979)

was thus able to state that flow regulation following the construction of the Volgograd reservoir has

brought about great changes in the composition and abundance of the zooplankton in the lower Volga.

These are now determined by zooplankton discharged from reservoirs together with the tendency of

the river to restore a rheophilic type community. Thus zooplankton decreases in abundance

downstream of the discharge point. Similar changes have occurred in the delta, mainly because of the

decrease in the overall volume of water and the suppression of the spring flood. High concentrations

of zooplankton were much more common in the floodplain pools of the upper delta where species

with high capacities for parthenogenetic reproduction attained up to 3 g/m³. The life of these pools has

been shortened and many have disappeared with a consequent reduction in overall zooplankton

abundance. By contrast an increase in emergent vegetation has provided favourable habitats for

phytophylic, periphytic and benthic forms. Extensive work on the zooplankton of the Nile showed

similar changes below the various dams. For instance Brook and Rzoska's (1954) data for river

plankton above and below Gebel Aulia showed a change in dominance from Copepoda upstream of

the lake to Cladocera below the dam. With the addition of the Roseires and Aswan dams the Nile has

undergone multiple regulation turning it into a cascade system in which the zooplankton community

has changed from a riverine species dominated fauna to one more typical of lacustrine systems. In the

lower Nile below the Aswan dam zooplankton density varied between 12 000 and 133 000

107

individuals/m³ with a more or less steady increase in density downstream. Since the closure of the

dam, this lowest river reach has been converted to a semi-lenitic body of water.

Table 4.2

Population estimates of zooplankton organisms in standing waters on floodplainsa

River No. of individuals/m Remarks Authority

Danube:

Erec lagoon 110 000 Minimum: December Ertl, 1966

4 006 000 Maximum: April

Husie lagoon 616 000 Minimum: February

8 493 000 Maximum: June

Dyje: locality 1 10 000 Minimum: February Osmera, 1973

500 000 Maximum: May-July

locality 2 2 000 Minimum: Spring flood

10 000 000 Maximum: June

Missouri:

Backwaters 6 670 Mean: April-October Kallemeyn & Novotny, 1977

Dvina: 247 999 400 000 Zhadin and Gerd (1970)

Oka: 950 000 Spring

1 281 000 Summer

Parana:

Don Felipe lagoon 17 000 Minimum: June Bonetto & De Ferrate, 1966

1 200 000 Maximum: October

Los Espejos lagoon 26 000 Minimum: July

277 000 Maximum: December

Flores lagoon 12 000 Minimum: January

830 000 Maximum: September

Laguna Totoras 80 000 Minimum: Falling water

November Bonetto et al., 1948

930 000 Maximum: High water

August

Laguna Gonzalez 6 330 000 Minimum: Falling water

October Bonetto et al., 1948

24 000 000 Maximum: Low water

December

Laguna La Brava 536 000 Minimum: High water

May Bonetto et al., 1978

761 000 Maximum: Low water

Laguna Sirena 12 000 Minimum: Low water CECOAL, 1977

536 000 Maximum: High water

Magdalena:

Los Ponches 5 380 000 Maximum: Low water Arias, 1977

Machado 4 360 000 Maximum: Low water

Pinalito 25 300 Maximum: Low water

Sokoto: 27 000 Maximum: Dry season Holden & Green, 1961

Mekong:

Nong pla pak swamp 38 500 Random sample Sidthimunka, 1970

Ping and Nan Rivers 136 234 Outflow Junk, 1976

Bung Borapet 122 279 Inflow

Amazon:

Lago Preto de Eva 738 000 Mean Marlier, 1967

108

Lago Calado 107 087 Junk, 1976

Nile:

Aljab lagoon 180 January Monakov, 1968

3 000 May

Shambe lagoon 600 February

3 000 May

Jor lagoon 2 700 February

12 400 May

No lagoon 8 000 February

53 000 May

Atar lagoon 59 000 April

a These figures are often based on only a few observations and do not therefore necessarily reflect

the full range of variations of the parameters measured

Kallemeyn and Novotney (1977) also found that numbers of zooplankters declined downstream of the

major impoundments in the Missouri river where the presence of zooplankton is strongly correlated

with discharge from the Lewis and Clark lakes. A 51% decrease in abundance 16 km downstream of

the point of discharge, 61% at 38 km and 70% at 145 km was shown by Hesse et al. (1982). High

concentrations of zooplankton also arise from the injection of organisms into the main river channel

from chutes and backwaters. In the Missouri this resulted in greater numbers in unchannelized section

of the river where 6 670 individuals/m³ were present in backwaters, 12 110 individuals/m³ in chutes,

10 864 individuals/m³ in the main channel adjacent to the river bank and 12 245 individuals/m³ in the

main channel itself. There was a strong peak of abundance in May at 370 mg/m³ when the rains

flushed out the backwaters, diminishing to less than 20 mg/m³ in winter. Vranovsky (1974) noted the

same phenomenon in the Danube where maximum values of zooplankton biomass (300–600 mg/m³)

occurred in autumn and the average amount of zooplankton drifting past a fixed point reached as high

as 260 g/s in 1967 due to the wash out of organisms from the side arms.

Sudden seasonal pulses in total zooplankton numbers seem to arise mainly by increases in rotifers,

although the other major components of the zooplankton, the copepods and cladocera, also have

characteristic peaks. Nauplii are common and are often washed out of the standing waters into the

main channel during rising floods. Rhizopods have also been noted as an important element of the

plankton at this time (Holden and Green, 1960; Green, 1963).

Some correlation between conductivity and zooplankton numbers has been found in vegetation

flanking the main channel of the Zambezi River. Here maxima of both conductivity and zooplankton

numbers (110,000 Cladocera and Copepoda m³) occurred during the dry season. Both parameters

were minimal during the peak flood when zooplankton was nearly absent.

The degree of vegetation also appears critical and the greatest number of zooplanktonic organisms is

commonly found in waters with at least sparse vegetation (Bonetto, 1975). Their density also

increases locally at or near the open water/vegetation interface, under mats of floating vegetation and

associated with submersed plants. This implies that there is a succession of planktonic organisms

within the standing waters of the floodplain, which corresponds to the general evolution of such

bodies of water, from the open water to the heavily vegetated state, as discussed by Botnariuc (1967).

Comparative studies that demonstrate this are few, although Green (1972 and 1972a) has

demonstrated such a succession in the floodplain lakes of the River Suià Missú in Brazil. As the

vegetated areas are generally more productive than open water areas, there is presumably a

comparable evolution in productivity per unit area. This would, however, be compensated for by the

diminution in size of the individual water body as it progresses through silting from the open water to

the marshy state. Work on lagoons of the Danube floodplain by Ertl (1966) and the Elbe floodplain by

Novotna and Korinek (1966), indicates that the density of the fish population influences the

composition and abundance of the zooplankton. Detailed work on ecological inter-reactions of this

109

nature is unfortunately lacking in tropical lagoons, although it is to be suspected that similar

relationships may also be found there.

Zooplankton densities are usually considered to be low in the main mass of water on the floodplain,

but the shallow littoral of the flood zone may support considerable quantities f planktonic forms. The

higher conductivity and temperature, and local availability of oxygen can support blooms of

zooplankton that can be readily observed at the water’s edge. Unfortunately little quantitative

information supports this as such areas have been relatively little investigated, but the high values for

the zooplankton from the Baka and Dyje backwaters of the Danube in Tables 4.1 and 4.2 may be

attributed to the fact that these water bodies are mainly littoral in nature (less than 1.5 m deep) with

considerable submersed vegetation. In a comparison between littoral and pelagic zones in backwaters,

Osmera (1973) showed that mean biomass from the littoral zone was 7.75 mg/1 as compared with

1.64 mg/1 in the pelagic zone of the same lagoon.

Survival of the adverse conditions of the flood poses certain problems for planktonic organisms which

are readily washed away by the increased flows. Nevertheless, zooplanktonic organisms reappear

rapidly when more favourable low flow patterns are re-established. This has been attributed to

reservoirs of organisms in upstream habitats, but Moghraby (1977) considered the 50–100

organisms/m present in riverside pools of the Blue Nile as inadequate to serve as sources for the

regeneration of the population. Instead he produced evidence that the adults or the eggs of many

species enter a diapause as temperatures are lowered and silt concentrations increased during the

earlier part of the flood. He found abundant pockets of diapausing individuals in various types of

bottom deposit and showed experimentally that they were only released when silt concentrations

dropped and temperatures rose to the dry season norms.

ANIMAL COMMUNITIES ASSOCIATED WITH FLOATING AND SUBMERSED

VEGETATION

Floating vegetation supports rich and varied animal communities either as a free living mesofauna in

the open water among the plant stems or as an epifauna in the root masses of floating plants (i.e.,

pleuston). These have been studied in most detail in the root masses of the floating meadow grasses

Paspalum repens and Echinochloa polystachya of the Amazon system from where many species of

crustacea, insect nymphs, oligochaetes and molluscs were identified. Junk (1973a) distinguished three

major biotopes according to fauna and flora. The first of these, the flowing whitewater biotope-

consists of the stands of vegetation bordering the main river channel. Here faunal densities on the

exposed outer fringe were low, possibly due to current, which sweeps organisms away, and to large

amounts of inorganic sediment, which hinder feeding. Faunal abundance increased from the fringe

towards the centre of the stand or mat and total abundance at the centre could be as-high as 100 000

individuals/m² although the norm was around 50 000 individuals/m². Biomass increased from 0.3

g/m² (dry weight) or 1.5 g/m² (wet weight) at the fringes to 4.2 g/m² (dry weight) or 20 g/m² (wet

weight) in sheltered places within the stand. A second biotope, well-oxygenated, sedimented

whitewater lagoons, undergoes fairly wide annual fluctuations in level which periodically destroy the

aquatic vegetation. During the optimal growing period large numbers of individuals occurred,

abundance usually ranged between 100 000 to 300 000/m², although densities of up to 700 000/m²

were recorded. Faunal density was evenly distributed and biomass varied between 2.5 and 11.6 g/m²

(dry weight) or 12–62 g/m² (wet weight). Marlier (1967) found similar values ranging from 3.9 to

13.9 g/m² (dry weight) or 18.9–39.7 g/m² (wet weight) from three samples of floating vegetation in

another lake of this type. The third biotope - sedimented whitewater lakes - had thick stands of

vegetation supporting little or no dissolved oxygen. Water regimes are more stable than those of the

former type of lagoon. In the peripheral zone of the vegetation mats, faunal abundance was about the

same as in the well-oxygenated lake, but various groups of organisms rapidly disappeared due to

oxygen stress and abundance was very low (0.16–0.29 g/m² - dry weight) even only a few metres

from the outer edge.

110

This community is also very important in the middle Paraná River where the abundance and density

of Eichhornia, Pistia, Salvinia and Azolla supplies an sample substrate (Bonetto, Dioni and

Pignalberi, 1969). As examples Bonetto et al. (1978) found 17 000 individuals/m² of molluscs,

oligochaetes, leeches, mites, crustacea and insects in Salvinia herzogii during the summer

(corresponding to high water) and 9 200 individuals/m² during winter in Laguna La Brava of the

Upper Parana; Varela et al. (1978) describe in detail the floral and faunal associations in the floating

islands or “embalsados” of that lagoon; CECOAL (1977) found a maximum value of 20 000

individuals/m² in summer falling to 4 000 individuals/m² in winter for Azolla and Pistia. Also in the

Parana system over 100 taxa at specific or generic level were found in the submerged stoloniferous

masses of Eichhornia (Poi de Neiff and Neiff, 1980) where seasonal changes in the dominance of

organisms and differences between the fauna associated with plants in river and lagoon were also

observed. Here maxima reached 262 000 individuals/m² within the Paspalum repens/Salvinia

hertzogii complex (Poi de Neiff, 1981). Also in Latin America, Zarate and Cubides (1977) found a

range of biomass from 6.56 and 130.7 gm/m² wet weight (Mean 35.14 g/m²) for 17 stations on

lagoons of the Magdalena system. The authors identified 67 taxa of invertebrates in the root masses of

Eichhornia. Similar distributions of organisms have been found in African papyrus swamps where the

zone close to the interface with the open water was considered the richest habitat in the system by

Rzoská (1974). Many species of oligochaetes, Bryozoa, Protozoa, Crustacea, insects and molluscs

have been found among the papyrus, reeds, Echinochloa and Eichhornia of the Nile Sudd, Monakov

(1969). Few quantitative samples have been taken of this habitat, but 5 minute pond net sweeps

captured as many as 547 individual Crustacea (Rzoska, 1974). Monakov (loc. cit.), who also

attempted to quantify the biomass of the Entomostraca, found that the fringe was 10–100 times richer

than the open water of the Sudd.

Petr (1968) found between 5 000 and 16 000 individuals/m² in the roots of Pistia from Volta Lake. In

the Sudd, Rzoská (1974) found up to 300 individual animals per Pistia plant, although densities

increased in quiet stretches of the river, notably in the swamps and reached a peak in river lagoons.

Fewer individuals were noted in floating vegetation in Bangula lagoon by Shepherd (1976). Here a

mean of 388 individuals of insect, oligochaete, crustacea and molluscs were present per m² of

Nymphaea and a similar assemblage at 210 individuals/m² was found in Pistia.

The weight of animals associated with the roots of Eichhornia was also high in the cienagas of the

Magdalena river, where Kapetsky et al. (1977) reported a mean of 35.1 g/m² from 17 sample sites in

10 different water bodies. Values ranged from 6.9 to 130 g/m².

Investigations of submersed vegetation in two lakes of the maritime delta of the Danube show the

considerable differences that can arise in faunal densities depending on relative distribution of support

plants. In Porcu lake Ceratophyllum demersum covers about 64% of the bottom and Nitellopsis obtusa

about 4% whereas in Rosu lake C. demersum occupied 14% and N. obtusa 53% of the bottom. Both

plants form a similar type of substrate and host similar species of Oligochaeta, Trichoptera and

(Chironomida) yet total quantities in Porcu lake, at 1.8 kcal/m² (0.36 g/m²) in May and 93.4 kcal/m²

(18.7 g/m²) in August exceeded the 2.2 kcal/m² (0.44 g/m²) in May and a maximum of 18.7 kcal/m²

(3.74 g/m²) in June. The differences in biomass were attributed to the greater density of organisms

supported by Ceratophyllum (Izvoranu, 1982).

A particular community has been identified in the umbels of Cyperus papyrus (Thornton, 1957). This

consists principally of terrestrial forms which pass their lives in this specialized habitat, but which

contribute to the allochthonous food supply of the swamp waters.

BENTHOS

The literature on the benthos of running waters has been reviewed by Hynes (1970) who concluded

that similar elements of the fauna of hard substrates are common to streams and rivers all over the

world. The benthic fauna of hard, stony runs and riffles is richer than that of the silty reaches and

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pools of the rhithron in both number of species and in total biomass. The communities of Soviet rivers

have been classified on the basis of both substrate and current velocity (Zhadin and Gerd, 1970) as

follows:

- Lithorheophilic communities inhabit solid substrates in flowing waters and consist mainly of large

insects.

- Psammorheophilic communities occur over sand bottoms in flowing waters and consist of small

arthropods and protozoons living in the interstices of the sand.

- Argillorheophilic communities inhabit clay substrates and are mostly sessile or burrowing forms.

- Pelorheophilic communities live in silt in flowing waters.

- Pelophylic communities are found in silt in still waters.

- Phytophylic forms live in backwaters rich in plants.

Benthic fauna is, as a rule, better developed on stable bottoms. On mobile bottoms sessile insect

larvae with their sensitive ducts or tissue structures and also moving molluscs are readily displaced.

Other more mobile forms such as Asellus or Gammarus may equally be at risk through the risks of

mechanical damage.

Biomass was thus found to be closely related to the type of community. For instance, in the Dnieper

Lithorheophils had a standing crop of about 1 kg/m², Pelorheophils about 10 g/m² and psalmophils

about 1 g/m² although isolated pelophils in cut-off lakes attained over 100 g/m².

In the Danube pelorheophilic (with 16.5–99.2 g/m²), psammopelorheophilic (with 22.7–69.2 g/m²)

and Argillorheophilic (with 71.6 g/m²) communities all and considerably higher standing crops than

the psamorheophilic community.

Russev (1967 and 1981) has also described the distribution of biomass according to location and

bottom type in the Danube with mean values of 72.5 g/m² over leaf litter, 47.4 g/m² over mud, 26.4

g/m² over clay and 0.24 g/m² over sand which is far less stable a substrate. Furthermore water speeds

in excess of 2–3 m/s¹ inhibit the development of benthos in the centre of the channel forcing it to

concentrate mainly in protected shingle areas was of the littoral (Russev, 1982). The mean biomass on

the Bulgaria reaches of the river was 35.22 g/m² of which 34.21 g/m² were molluscs. Similar mean

values of 38.9 g/m² (total) and 34.21 g/m² (molluscs) were obtained in the autumn. Biomass per unit

area varied considerably with water level. The mean biomass (without molluscs) was only 0.76 g/m²

at high water (April-August), whereas at mid-water it was 1.94 g/m² and at low water 6.34 g/m². The

much higher values obtained as water level decreased were attributed to the dislodging and transport

downstream of benthos at times of high flood, and the dispersion of benthic organisms over a wider

area during the flood period.

In tropical rivers Bishop (1973) commented on the similarity of rheophilic rainforest communities in

Africa, Sri Lanka, tropical S. America and the Gombak River Malaysia. The lowland reaches of rivers

share many features, but relatively little information is available3 on the benthic fauna of the slow-

flowing silt-laden rivers associated with floodplains.3 Such that does exist indicates a generally poor

fauna consisting of a relatively small number of species consistent with Zhadin and Gerds conclusions

of the low biomass of pelorheophils in Russian rivers. Bonetto and Ezcura (1964) reached similar

conclusions based on observations on the Parana River which showed the benthic fauna to diminish in

diversity and, abundance as the current slows. Maximum values may reach over 71 000

individuals/m² in the region of Yacireta as compared with 17 000 individuals/m² at Posadas, 23 000 at

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Ituzaingo and 13 000 at Ita bate (CECOAL 1979). Data from the Amazon (Junk 1971) also indicate a

poor fauna in the main river, although experiments with artificial substrates where 12 900

individuals/m² settled in four days showed populations in blackwater rivers to be higher than

expected. Monakov (1969) also concluded that the bottom fauna is monotonous in the main channels

of the main channels of the Sudd where only small groups of Chironomids and Oligochaetes inhabit

the sand and mud bottoms. In general biomass ranged between 0 and 0.2 g/m² although Rzoska

questioned this conclusion on the basis of the small number of samples taken. Further downstream,

between the Aswan dam and the sea, the macrobenthos is dominated by Molluscs although some

Polychaetes and Oligochaetes were also present. Quantitatively biomass varied between 1–9 g/m³

rising to the unusual value of 53.61 g/m³.

Differences in species composition have also been recorded from some rivers, for instance the

mollusc fauna of the Niger consists mainly of abundant Aetheria elliptica, Aspatharia, Mutela dubia

and Viviparus unicolor in muddy reaches, contrasted with rare individuals of Corbula fluminialis,

Mutela rostrata, Caelatura aegyptica and Cleopatra bulminoides in sandy reaches (Blanc et al.,

1955). Large aggregations bivalve molluscs also occur in the Volga, Dneister, Dneiper and Don. In

the Volga Dreissena contribute local standing crops of several kg/m². Otherwise the benthos of the

river is poor in the main channel. A high benthic biomass of 400 g/m² has also been recorded from the

many tributaries of the Mekong (Sidthmunka, 1970) which again was almost entirely due to a

concentration of molluscs.

In the Missouri river there are no data to show the benthic populations before impoundment, but

studies by Morris et al. (1968) indicated that there is relatively little difference between the benthic

biomass per unit area in channelized and unchannelized reaches. In the first case mean biomass was

calculated as 0.08 g/m² and in the second at 0.07 g/m². These means, however, conceal considerable

variations with locality as within the river biomass varied between 0.019 (0.048) 0.99 g/m² in

unchannelized reaches of the main stream whereas in channelized reaches it varied between 0.002

(0.007) 0.022 g/m². On mud banks the biomass was higher in channelized reaches at O.058 (0.803)

2.678 g/m² than in unchannelized reaches where it was 0.111 (0.267) 0.349 g/m². The general scarcity

of benthos was attributed to the general nature of the Missouri river in that it has shifting substrates,

high silt load, fluctuating water levels and absence of aquatic vegetation, all of which militate against

high production of benthic organisms. However an overall loss of benthos occurred in channelized

reaches where the area available for colonization was reduced by 67% over the unchannelized

reaches. Earlier Berner (1951) had found similar values ranging between 0.024 g m-2 in the middle of

the channel to 0.258 g/m² nearer the edge, and had constrasted the low values from the Missouri with

the clearer, slower flowing Illinois where a mean of 29.23 g/m² was found by Richardson (1921) at a

time when the river still had its floodplain lakes. Schramm et al. (1974) summarized the data for

benthos from the Middle Mississippi which show similar differences between channelized and

unchannelized reaches. On the basis for this they emphasize the importance of the channel area for

benthic production.

Information obtained from backwaters and standing waters of the floodplain indicate somewhat

higher standing crops than in the main river. In the Romanian portions of the Danube floodplain a

mean biomass of 20.06 ± 9.35 g/m² was calculated from 12 lakes (Academia Republicii Socialiste

Romania, 1967) and in lakes of the Volga delta values of between 7–48 g/m² have been found.

Monakov (1969) recorded biomasses of zoobenthos consisting mainly of Oligochaetes, chironomids

and molluscs ranging from 0.58–4.7 g/m² from five of the floodplain lakes of the Sudd. The mean

values of 2.95 g/m² for the period of high but falling water and 2.07 g/m² for low water compared

with the 0–0.2 g/m² found for the main channel. Benthic biomass quoted for the Nong Pla Pak

swamps of the Mekong floodplain (Sidthimunka, 1970) were of the same order at 1.75 g/m² at a time

when, benthic biomass in the main river was only 0.12 g/m². Similarly a mean of 1.51 g/m² (range 0–

6.51 g/m²) of benthic organisms was found in cienagas of the Magdalena system by Mikkola and

Arias (1976). Here a mean of 1.9 g/m² was found in the open waters, of the Cienagas whereas the

more sheltered bay habitats had a mean density of 3.1 g/m². Arias (1977) found values of 1.76±1.81

113

g/m² for the open waters of 18 cienagas in this system when the bay areas had values of 2.96 ± 1.51

g/m². These variations in biomass were attributed to the often deoxygenated conditions in the deeper

waters of the cienagas.

The tendency for littoral waters to support more benthos than the deeper waters has been confirmed

by numerous other workers. Bonetto et al. (1978) furnished detailed information about the benthic

fauna of two lagoons of the Riachuelo River, a tributary of the Parana system. In the Laguna Totoras

benthos was maximal during low water when total numbers exceeded 114,000 individuals/m². In

summer, during high water, maximum abundance was much less at 46 500 individuals/m². In this

lagoon too, there were considerable differences between the various zones of the lake with maximum

abundance in the sub-littoral zone and the least in the centre. Similar differences between the littoral

and open water zones on one hand and between seasons on the other, were found in Laguna Gonzalez

which, because of its great eutrophication, supported less organisms. In Laguna La Brava the meso-

and macro-benthos contained between 17 500 and 95 300 individuals/m² at low water and 1 700–57

000 individuals/m² at high water depending on location in the lake. Particularly significant in this

water body were thecamoebans which numbers sometimes exceeded 1.5 × 106 cells/m² at low water

but only 0.6 × 106 cells/m² at high water.

Three temporary lagoons in the savannah floodplain of the Rio Branco, N. Brazil, supported mean

weights ranging from 0.16–0.88 g of benthos/m². There were some variations with depth and

individual samples gave values as high as 2.52 g/m². The 0.25 g/m² (dry weight) obtained from the

Lago de Rodondo appears consistent with the range of biomass found in the lagoons, despite Marlier

(1967) opinion that these were very low. Nevertheless higher values have been reported for some

Amazonian lakes, Eittkau et al. (1975) for example, found a mean annual weight of between 0.14 and

6.20 g/m² of benthos, and studies by Reiss (quoted in Sioli, 1975) also revealed biomasses up to 6.2

g/m² in the centre of Amazonian Varzea lakes. By contrast, samples from Lake Tupe, a typical river

lake, and some blackwater várzea lakes, led Reiss (1977) to conclude that the profundal zone of such

bodies of water supports the poorest benthic fauna of any Amazonian lacustrine biotope. Values from

the littoral zone are however much higher, reaching up to 104 g/m².

The range of densities from both African and Latin American floodplain lagoons suggests extreme

values ranging from 0 to about 6 g/m² and mean values of about 2 g/m² for tropical rivers. Rzoska

(1974), however, considered that values from such areas should be treated with caution as the cyclic

emergences of Ephemoptera, Chaoborus and chironomids can influence the standing crop and

biomass profoundly. Very dense localized populations of other organisms can also arise where

conditions are particularly favourable. Botnaruic (1967), for instance, recorded densities of molluscs

of up to 2 657 g/m² in the deep water of Crapina Lake of the Danube floodplain. Values are not

normally this high, and Russev (1967) quoted Russian work on the 8 580 km² Kiliya arm in the

Danube delta, where the mean biomass was 9.45 g/m². The same work indicated an annual production

of 19 235 t (224.19 g/m²) from the whole arm. Work by Lellak (1966) on the benthos of

Czechoslovakian backwaters showed this to be strongly affected by the presence or absence of fish.

Where fish were absent, population densities of Chironomus and Chaoborus larvae were very high.

Similarly, Enaceanu (1957) showed benthic biomass to be higher in enclosures from which fish were

excluded (226.12 kg/hm² mean July-October) than in bottoms freely accessible to fish (70.98 kg/hm²

for the same period) in the same lagoon. In floodplain pools where the composition and density of the

fish populations isolated by receding flood waters are likely to vary, differences in benthos are liable

to arise as a result of diversity in predation pressures. Although the authors quoted above have noted

seasonal fluctuations in the benthos of the standing waters of the floodplain, Bonetto (1975)

considered this the least affected by differences in water level of all communities inhabiting such

habitats.

Submersed vegetation, where present, also acts as a centre of concentration for benthic invertebrates.

Examination by Academia Republicii Socialiste Romania (1967) of 1 550 g of Potomogeton

perfoliatus, collected from an area of 0.5 m², showed 10.489 individuals of insect larvae, Crustacea

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and molluscs to be present. The total weight of 87.3 g equivalent to 175 g/m² is reasonably high for

the benthic fauna of this system, which normally varies around a mean of 20 ± 9 g/m². Values from

the Bangula lagoon ranged between 0 005 g/m² under Pistia, 0.06 g/m² in open water, 0.378 g/m² in

Ceratophyllum and 0.977 g/m² under Nymphaea. The numbers of individuals present in

Ceratophyllum was still relatively low, 4 385 individuals/m² but was significantly greater than the

densities found in Nymphaea and Pistia. Very large concentrations of invertebrate, including acarine

mites, snails and phytophagous species of insect and mollusc were present in Egeria naias of the

Totoras lagoon where between 10 000 individuals per 1 000 g of plant were present in winter and 320

000 individuals per 1 000 g of plant were present in summer.

On the floodplain itself, where seasonal desiccation occurs, flood season populations of macro-

invertebrates are less well studied. Personal observations have shown enormous densities of

pulmonate snails to be present on the bottom and at the water surface of inundated areas. Carey (1967

and 1971) reported Ephemoptera nymphs, Trichoptera larvae, chironomid larvae, Hemiptera and

molluscs to be very abundant and widely distributed in the inundated zones of the Kafue River,

especially in submerged banks of Najas and Ceratophyllum. The profusion of molluscs on the

floodplain has also been noted on the Central Delta of the Niger by Blanc et al. (1955). In the Parana

River, Bonetto, Dioni and Pignalberi (1969) commented on the restricted number of species which

contributed to the great biomass. Particularly abundant were Unionacea, which made the most

important contribution, despite the low calcium content and relatively low pH. The fact that molluscs

can make a significant contribution to the benthos under these conditions is emphasized by their

presence in Amazonian blackwaters. Certain of these are, however, so poor in minerals that only

small Ferrissidae with conchiolin cases are present (Fittkau, 1967).

In the forest floodplains of the Amazon inundated with whitewaters, benthic organisms are common

and have two production peaks, one at the beginning of the inundation and a second one after

maximum high water in June/July. Reiss' (1977) work on the blackwater river lake Tupe, also

indicated two peaks of abundance for littoral benthos. One peak occurred on the rising flood when 2

559 individuals/m² were recorded, and a second peak occurred at low water when 1 248

individuals/m² were present. Minimum densities during falling water were about 623 individuals/m².

The faunal composition changed completely between the minimum and rising water level peaks. At

low water chironomidae dominated the fauna with minor representation by oligochaetes, Acari and

Corixidae. During rising water Chaoboridae and Ostracoda became steadily more important to the

exclusion of other groups. Blackwater flooded Igapo forests, however, are generally very

impoverished; Irmler (quoted by Sioli, 1975) found a benthic biomass of only 0.2 g/m² in such an

area.

In areas of rapidly fluctuating water level which are subject to seasonal desiccation, it is evident that

mechanisms must exist to survive the dry season. Fittkau et al. (1975) considered three possible

mechanisms: migration, dormancy and recolonization. Hynes (1975) also considered that, in view of

the high degree of adaptation required for resting eggs or burrowing and aestivating larvae,

recolonization is by far the easiest strategy for drought survival. This implies very rapid growth of

larvae, and in fact most larvae and nymphs in the Ghana River studied by him were fully grown after

a month. Whyte (1971) has even noted some chironomid species which reached full size in three

weeks. Studies on Rhodesian streams liable to seasonal desiccation have shown that the re-

establishment of the fauna after the annual resumption of flow can be very rapid (Harrison, 1966).

Oligochaetes, Crustacea and insect larvae appeared within ten days and there was a form of

succession whereby the species composition typical of pools was re-established and stabilizing within

a month. Harrison's conclusions also indicated that pulmonate snails and oligochaetes survived the

drought by aestivating. Some smaller crustacea may have had dormant eggs. Most insects recolonized

the area by the movement of flying adults from more permanent water bodies.

Decapod crustacea, which form part of the macro-benthic community, are a particularly important

element of tropical river fauna. Ecologically they may be considered together with fish on the basis of

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their size, position in the food chain, behaviour and economic importance as food organisms. Species

of Macrobrachium particularly form the basis for fisheries in many tropical rivers (see Kensley and

Walker (1982) for the Amazon; Robertson (1983) for the Sepik River, FAO/UN (1971) for the Oueme

River and Inyang (1984) for the Lower Niger River) and have also been widely adopted for fish

culture. Similar activities are pursued with Astacus, Procamberus and Pacifastacus in the temperate

waters of USA and Europe.

NEUSTON

The neuston “community” is little discussed by workers on tropical floodplain ecology. Nevertheless,

personal observations have shown the abundance of forms living at the air water interface. Mosquito

larvae and various forms of pulmonate snails are extremely common, as are water striders (Hemiptera

and Coleoptera), mites and spiders, particularly in the sheltered water among the stems of the floating

vegetation. It is to be supposed that this community expands considerably during the flood season,

although as yet there are no recorded observations to support this assumption.

VERTEBRATES OTHER THAN FISH

AMPHIBIA

Both larval and adult stages of various species of frogs and toads are plentiful on floodplains. Most

fringing areas of the plain and all permanent and temporary swamps are colonized and in the most

deoxygenated areas, tadpoles are often the only form of vertebrate life. Similarly amphibians rapidly

appear in the most isolated of temporary pools. Little work appears to have been done on the ecology

and dynamics of amphibian populations of rivers although they undoubtedly form an important

component of the fauna of the floodplain, and probably contribute to the rapid recycling of detritus

and mud by converting it into flesh which is useable by predatory species of fish, reptiles and birds.

Frogs are exploited commercially for their legs particularly in China and Romania. However the

resources have been either over-exploited, or the stocks have been reduced by pollution in some areas,

for instance France, where they are mow protected.

REPTILES

Several families of reptiles have remained associated closely with water, and of these, four groups

(Crocodiles, monitors, iguanas and turtles) are regularly found in and around rivers and the permanent

and temporary standing waters of their floodplains. Certain species of snake too have adopted aquatic

or semi-aquatic habits. Various forms of crocodile are distributed around the tropical world, and have

been widely associated with flood rivers in the past. They have, however, been subject to widespread

slaughter and are now virtually absent from many parts of their former range. The economic

importance of the crocodile as a consumer of fish has been discussed, principally by Cott (1961).

Although crocodiles eat a large quantity of fish, they also prey on other organisms which themselves

are predators of fish. Their status as major competitors of man for fish is therefore somewhat obscure.

A second function of crocodilia in the ecology of some types of tropical water has been suggested by

Fittkau (1970 and 1973). In the nutrient-poor rain forest the larger elements of the community serve as

nutrient sinks, which slowly accumulate the few minerals available (Fittkau and Klinge, 1973). Such a

system depends much on the abundance of its species which are able to maximize the storage and

recycling of nutrients of allochthonous origin. A similar process is thought to occur on the equally

nutrient-poor blackwaters of the rainforest zones. In such aquatic systems the low level of nutrients

does not permit much primary production and the food chain originates mostly in the rain of

allochthonous material. Fish moving into the mouth lakes are allochthonous to that particular

ecosystem and are fed upon by a variety of large predators, of which caimans are perhaps the most

significant. A medium-sized caiman can eat between 0.6 and 0.8 percent of its body weight per day

and excrete about 0.20 to 0.27 percent of its body weight of nitrogen, phosphorus, calcium,

magnesium, sodium and potassium ions per day. Their contribution to the nutrient balance of such

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environments has been estimated by Fittkau to be locally superior and complementary to the nutrients

derived from rain water, itself the major source of nutrients in the system as a whole. In places where

crocodiles have been eliminated, declines in fish production have been noted, possibly because of a

drop in the primary production based on the excreted nutrients.

BIRDS

Birds are a very conspicuous feature of the floodplain ecosystem. As with most wetlands various

kinds of waterfowl are extremely common, but it is doubtful whether most of these react directly with

fish populations. Piscivorous predators are also abundant, however, and represent possibly the greatest

source of pressure from outside the aquatic system. Reizer (1974) for instance listed 37 species of

ichthyopredator in the Senegal River and 19 species are listed by Shepherd (1976) from the Shire

River. Birds are capable of taking a wide range of species of all sizes and the piscivorous bird

community appears to be specialized toward the taking of particular sizes and types of prey.

In general, the life cycle of water birds is closely linked to the floods. Avian breeding seasons

coincide with those of the fish and the fledglings are being reared at just that time when small fish

suitable for their feeding are most abundant. There is a particular heavy predation during the period of

receding water when many fish are stranded in temporary pools (Lowe-McConnell, 1964; Bonetto,

1975).

The impact of birds on the fish population is potentially very large, and studies from Africa indicate

that the amount of fish taken by them can surpass the amount taken by the fishery. In the Senegal

River, for instance, Reizer (1974) presented figures by Morel of between 100–200 000 herons and

cormorants and 2 000 pelicans in the delta alone. Fish consumption was estimated at between 500–1

000 gm per day for the darter Anhinga rufa and for a heron, 1 000–2 000 gm per day for a pelican and

250 gm for the kingfisher, Ceryle rudis. On the basis of this birds take about 70 000 t/yr of fish as

compared to a fish catch of about 50 000 t/yr.

Bowmaker (1963) gave an estimate for the daily ration of the cormorant Phalacrocorax africanus of

78 gm/day leading to a consumption of 286 t/yr from Lake Bangweulu. This estimate may be low by a

factor of 10 in view of Reizer data and that of McIntosh (1978) who found that the European

cormorant (Phalacrocorax carbo L.) eats some 650 to 700 gms of fish/day. Bowmaker concluded that

the African cormorant is not harmful to the fishery, but could in fact be beneficial by its use of non-

commercial species, and by its excreta which fertilize the water. Other figures from cold water

streams indicate that kingfishers eat about 14 kg of fish/km, of stream terns consume about 63 kg of

fish/km of river and Mergansers take some 25 kg/km. These are equivalent to 2.3%, 14.1% and 4.2%

of total mortality respectively (Alexander, 1979).

MAMMALS

Several aquatic or semi-aquatic mammals figure in the ecology of rivers although their role is by no

means always clear. Beavers, which are confined to the north temperate zone, have had an enormous

influence on the aquatic system. Their habit of building dams resulted in the more or less permanent

inundation of large areas of floodplain, creating widespread swampy conditions in the valley bottoms.

The dams themselves affected flow characteristics of streams, and the organic debris increase habitat

diversity and cover for fish. Beaver populations have been drastically reduced in parts of Europe and

North America and their dams have been cleared from rivers in an attempt to improve drainage. This

process, together with the clearance of dead wood, stumps and other naturally occurring obstructions

from the stream bed, has produced much dryer conditions in the valley bottoms and has effectively

removed considerable areas of floodplain from lower order temperate streams. Otters are widespread

throughout temperate and tropical systems. They apparently prey heavily on fish, although population

densities are far from high so the impact of such predation is, in most cases, probably relatively low.

They, do, however, have a bad reputation with fishermen for their habit of robbing and damaging set

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gear such as traps or gill nets. One record by Alexander (1979) shows otters to take about 13 kg of

trout, km of cold water stream against 25 kg/km taken by mink. These figures were equivalent to

3.0% and 5.6% of total mortality respectively.

In African rivers hippopotami contribute a large amount of fertilizer to the aquatic system by cropping

terrestrial vegetation and excreting it into the water in a form readily accessible to phytoplankton.

Densities of hippopotamus were at one time very high in most African inland waters and they must

have made a considerable contribution to nutrient inputs and recycling. They have, however, been

eliminated through much of the continent and their numbers have been considerably reduced even

within the confines of protected zones.

In Latin America and elsewhere, members of the family Sirenidae appear to play a similar role in

nutrient balances particularly in the black water rivers and mouthlakes of the Amazon. The dugong

(Dugong dugon) is distributed throughout the Indo-Pacific region (Husar, 1975). The African manatee

Trichechus senegalensis is distributed throughout the rivers of West Africa and is present in

considerable number in Cameroon, Gabon and in the Niger River System, (Nishiwaki et al., 1982).

Trichechus manatus is distributed from Florida to North Eastern Brazil (Maranhao) and Trichechus

inguinis is found throughout the Amazon River system. Studies on the feeding of Sirenidae (Best,

1981) show manatees to consume about 8% of their total body weight in aquatic plants daily whereas

the dugong eats about 14%. Their selected foods are higher plants not normally eaten by other

organisms and they are thus not in active trophic competition with other elements of the community.

Many large river systems have species of dolphin of the family Platanistidae. Platanista gangetica is

found in the Ganges, Brahmaputra and other rivers of the Ganges basin. Platanista minor inhabits the

Indus River and enters the Zambezi, Lipotes vexellifer occurs in the Yangtse, Inia geoffrensis is

distributed throughout the Amazon system together with Sotalia fluviatilis, the only freshwater

Delpinid, which is also found in the Orinoco River. Pontoporia blainvillei inhabits the lower reaches

of the Parana and Uruguay rivers, the estuary of the Rio de la Plata and the adjacent coasted waters.

Orcaella breviostris, which primarily inhabits shallow coastal waters of the S.E. Asian region, enters

many including the Irrawaddy, Ganges, Brahmaputra and Mekong as well as minor water bodies of

the Indonensian archipelago (Watson, 1981). All these dolphins feed on fish and Crustacea using their

long beaks to root in the bottom. In the Amazon, the two coexisting species are separated by very

different ecological characteristics (Ferreira, 1983) Sotalia is a social dolphin inhabiting the open

waters of the river channel and feeding on small sized schooling fish eaten whole. It consumes about

6.6% of its body weight daily. Inia is a more solitary animal that depends on large solitary bottom

dwelling fish and which are ripped apart before being consumed. It consumes 3.1% of its body weight

daily. There is some overlap of diet at the edges of the channel. Inia feeds mainly during rising river

levels as fish spread out onto the floodplain and has been recorded many kilometres from the river in

the Igapo. Sotalia feeds principally during low waters as pelagic schooling fish are concentrated into

the river. Extrapolating from what is known of the ecological role of other freshwater aquatic

mammals, dolphins may play two major roles within the ecosystem. Firstly, they may contribute to

the maintenance of the nutrient balance especially in nutrient poor rivers in much the same way as that

described for the Amazonian caimans. Secondly, they may consume significant amounts of fish thus

affecting the dynamic of the populations within the river. Little information is available at present

upon which to base any conclusions as to the role of these species, although indications from Ferreira

(1983) are that large quantities of fish are consumed seasonally. This may be especially significant for

fisheries in the case of Inia, which, by taking large and commercially preferred species, is in direct

competition with fisheries. The smaller schooling fishes preyed upon by Sotalia have are not yet

formed the subject of a fishery and are thus of little or no direct economic important. Despite this

competition Inia, in common with most dolphins in freshwaters, is strongly protected by tradition, and

is neither harmed nor eaten.

The floodplain is also used during the dry phase by a number of wild and domestic animals, but these

will be considered separately in a later chapter of this review.

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CHAPTER 5

RIVER FISHES AND THE RIVERINE SYSTEM

NUMBERS OF SPECIES IN RIVER SYSTEMS

DIFFERENCES IN NUMBERS OF SPECIES BETWEEN SYSTEMS

The considerable differences in the numbers of species inhabiting the various river systems are largely

attributable to the size of the river as represented by its basin area or some correlate of it such as

length of main channel or stream order. A relationship of the form N = fAb (where N = Number of

species and A = basin area in km2) can be fitted to the dispersion in Fig. 5.1. When all points are

included a regression, N = 0.297 A0.477 is obtained. In fact there are differences between different

geographical areas which become apparent when individual blocks of data are analysed separately as

follows:

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Figure 5.1 Number of species of fish present in different river systems plotted according to their

basin areas: (•) South America; (o) Africa; (▭) Asia; (x) Europe; ( ) North America

North flowing Siberian and European Soviet Rivers:

N = 2.76A0.19 (N = 6, r = 0.91)

South flowing European rivers:

N = 0.6A0.14 (n = 11, r = 0.72)

Africa:

N = 0.449A0.434 (n = 25; r = 0.91)

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South America:

N = 0.169A0.552 (n = 11; r = 0.95)

Daget and Economidis (1975) have independently fitted similar log-log regressions to date from sets

of small rivers in Portugal and Greece obtaining:

Greece:

N = 2.319A0.24 (n = 12; r = 0.94)

Portugal:

N = 1.786A0.19 (n = 12; r = 0.92)

which agree well with the regression established for Europe. Similar scatter plots for Australian rivers

also conform to these general principles with the distribution for nine North flowing tropical rivers

being located above the world mean regression line and five of the six points for temperate rivers

being located below the line. (Bishop and Forbes, in press). These relationships enable us to conclude

that while species diversity increases with basin area at all latitudes, it does so faster as one

approaches the tropics - as indicated by the larger exponent at lower latitudes. Because a log-log

relationship does not completely describe the distribution of points, the intercept, which Daget uses as

a relative index of faunal richness, is probably not a reliable predictor for basins of extremely small

area. Many reasons have been advanced for the increased diversity at low latitudes and these are

discussed more fully by Lowe-McConnell (1975). As to the influence of size, the number of

ecological niches is probably greater in larger river systems than in small ones. Meandering creates a

regular series of habitats in the main course of the river and some floodplain lakes are often isolated

from the rest of the system for long periods of time. Furthermore, similar habitats in the sub-systems

are often separated by considerable distances of inimicable biotope, leading to the formation of

distinct groups of species adapted to similar conditions along the length of the river. Roberts (1973)

has described the influence of such factors on the faunas of the two largest rivers of the world, the

Amazon and the Congo. However, such differences do not occur solely in the larger systems for they

have also been noted in small rivers and streams of sixth order or lower by Kuehne (1962); Barrel,

Davis and Dorris (1967), Whiteside and McNatt (1972), and Platts (1979) from the United States, and

Bishop (1973) from the Gombak river in Malaysia. Gorman and Karr (1978) also found that fish

community diversity is correlated with stream habitat diversity in selected Indiana and Panama

streams, and Horwitz (1978) demonstrated similar relationships in various types of rivers in Illinois,

Missouri, Ohio and Wyoming. Furthermore, diversity of habitat increases with increasing stream

order.

The aquatic components of river basins resemble island fauna in their relative isolation one from

another. The mathematical relationship between the size of islands and the number and diversity of

species inhabiting them has been examined by McArthur and Wilson (1967), who concluded that the

equilibrium between the extinction of species and the colonization with new species differs according

to the size of system; larger systems favour higher diversity perhaps because of the type of

biogeographical factors mentioned above.

Although total numbers in large systems tend to be high, groups of species are often located in very

different portions of the system, thus only a certain percentage concerns the floodplain or its fishery at

any one point. Some species also are limited to the turbulent waters of rapids and headwater

tributaries and are rarely, if ever, found in the slower flowing floodplain reaches. Nevertheless

individual fisheries in larger rivers do have many more species available to them than those of smaller

streams and catches consisting of over fifty species in one type of gear are not uncommon in the

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biggest systems. Figure 5.2 illustrates the diversity in form of representative species of various genera

of tropical river systems.

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123

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Figure 5.2 Representatives of genera of fish from tropical river systems

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RELATIVE ABUNDANCE OF SPECIES WITHIN ONE SYSTEM

In his review of data covering a number of areas and taxa, Preston (1962 and 1962a) found that within

a particular taxon and region, the relative abundance of species followed a lognormal distribution (i.e.,

the logarithms of the species' abundance were normally distributed). These distributions were of

course truncated at very high and very low levels of abundance, but over the observed range

distribution of this type described most of the sets of data well. In theory there are an infinite number

of lognormal distributions that could describe the relative abundance of a given number of species,

but Preston found a very close empirical relationship between the number of species in an assemblage

and the precise form (i.e., the variance) of the observed lognormal distribution of relative abundance.

He proposed that distributions obeying this relationship be called “canonical distribution”. Daget

(1966) concluded from two samples from the flooded plain of the Benue River, that the distribution of

numerical abundance and rarity of fishes in the sample locality did indeed conform to the canonical

type. A similar conclusion was reached by Loubens (1970) on the basis of 15 samples from various

parts of the Lake Chad-Chari river complex. Here samples were distributed log normally, both when

analysed by number and by weight, although in the latter case the correlations were less exact. With

smaller sample areas a simpler exponential relationship between ranked species (R) and number (N)

of the form N = abR was equally applicable. The extension of this type of analysis to other series of

data from other water bodies throughout the world demonstrates the wider application of these

principles, both to areas sampled by complete fishing and to samples obtained with individual types of

gear. Preston (1969) expanded the idea of relative abundance of taxa in one place at one time to

suggest that commonness and rarity are also distributed log normally in space and in time.

These conclusions are of interest when considering the community dynamics of floodplain

ecosystems insofar as they affect the dominance of species especially favoured by the fishery. They

also imply that the fish catch will tend to be dominated by only a few species even in communities of

high specific diversity and abundance. For this reason changes in population structure under

exploitation deserve further investigation. Particularly interesting is Preston's contention that the

distribution of commonness in any sample of a population is a truncated lognormal, having the same

modal height and logarithmic distribution as the “universe” from which it is drawn. The abundance

structure of fish isolated within floodplain depressions and lagoons may thus be linked by similar

relationships to the abundance structure of the fish community in the river as a whole.

SUB-POPULATIONS

There is very little information on the fine structure of species distribution of fishes inhabiting rivers.

From the behaviour of freshwater fish elsewhere it might be expected that any one species is far from

homogeneous and the sub-populations or stocks might be set up within the system. In the Volga River

the existence of such populations was indicated by the continued segregation of a species (Abramis

brama) into a number of local stocks after impoundment to form a reservoir of the riverine type. The

individual stocks (see Fig. 5.3) maintained distinct, isolated reproductive areas and areas for feeding

and over wintering. The feeding sites were separated from the breeding sites by considerable distances

(Poddubnyi in Mordukhai-Boltovskoi (1979). In rivers such as the Niger, Congo or Zambezi where

floodplain reaches alternate with stretches of rapids, conditions seem particularly suitable for the type

of genetic isolation which leads to the setting up of separate breeding populations.

In the Mekong, Sao-Leang and Dom Saveun (1955) noted that populations north of Khone falls

migrate downstream of them, whereas fish from the southern reaches often move upstream of the

falls. Blackfish species, particularly those whose longitudinal migrations are minimal, seem likely

candidates for the evolution of sub-populations along the length of the river. The more mobile

whitefish, however, have much more opportunity for dispersal and mixing of genetic strains, but even

here there are some indications that sub-populations are formed. There are hints that some species

have a homing instinct. Bonetto found tagged individuals of Prochilodus platensis in the same

floodplain pool in successive years in the Parana River, as did Holden (1963) with individuals of

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Lates niloticus and Hydrocynus lineatus in the Sokoto. This, of course, may mean simply that the fish

have not moved at all in the intervening flood period, but it does imply a certain degree of

territoriality in the species concerned. Furthermore, Godoy (1959 and 1975) removed tagged

individuals of Prochilodus scrofa from one site on the Mogi Guassu to place several hundred

kilometres away and even located on different branches of the same river system. Some fish returned

rapidly to the site from which they were captured originally, indicating the type of homing ability

which is often associated with distinct stocks of migratory species.

Figure 5.3 Feeding and breeding locations of different stocks of bream (Abramis brama) in a river and

riverine reservoir (Volga system). (After Poddubnyi, 1979)

The existence of separate sub-populations of species in river systems has also been proposed on other

grounds. According to Durand and Loubens (1969), Alestes baremoze has two populations, one of

which is migratory within the Chari and Logone rivers, the other resident in Lake Chad. A similar

separation has been attributed to Basilichthys bonariensis on ecological evidence. Here there is an

estuarine population in the Rio de la Plata, and a riverine population in the Parana. The riverine

population can also be distinguished by its faster and more consistent growth rate (Cabrera, 1962).

From the results of their tagging experiments, Bonetto and Pignalberi (1964) suspected the existence

of an upstream and downstream population of Prochilodus platensis. Prochilodus platensis also has

two body forms, ‘longilineas’ and ‘brevilineas’ which may depend on the early nutritional history of

the individual fish (Vidal, 1967). Studies on a more restricted area of the La Plata estuary have also

shown this species to have sub-populations with different growth characteristics (Cabrera and Candia,

1964), whereas Parapimelodus valenciennesi, a blackfish species from the same region, only has a

single homogeneous population (Cabrera et al., 1973; Candia et al., 1973). Bayley (pers.com.) has

also collected meristic evidence of the existence of several distinct sub-populations of Prochilodus

insignis in the Amazon. Separate sub-populations of migratory fish have also been identified with

differing life-history strategies adapted to the range of physical and climatic conditions in the streams

in which the fish spawn. The anadromous Alosa sapidissima studied by Legget and Carscadden

(1978), for example, shows a gradation in fecundity and migratory distance in sub-populations

associated with East Coast American rivers. These authors' arguments can probably be extended to

other species and may apply to other systems which are sufficiently geographically or spatially

diverse as to have a range of climatic conditions over their length.

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There are indications that many riverine species exist in two behavioural phases which may

correspond to sub-populations. One phase is migratory showing characteristic separation of breeding

and feeding grounds. The other is static and is more inclined to form territories. Such behaviour has

been described for Hilsa ilisha (Pillay and Rosa, 1963) in India and for Gobio gobio and Rutilus

rutilus in Europe (Stott, 1967). The existence of such alternative forms would explain the ease with

which some species adapt to changes in flow within rivers and readily adopt more sedentary habits

after flood control or impoundment.

There appears to be good circumstantial evidence that geographic sub-populations exist in rivers and

there is support for this from other inland waters such as the Great Lakes of N. America (Loftus,

1976). However, there are only few cases where such subpopulations have been positively identified.

Whether or not a population of fish is disassociated into separate stocks at a sub-population level is of

great significance to the management and conservation of fisheries for that species. Where separate

subpopulations exist, local depletion of the fish fauna by overfishing or by other environmental

pressures is less likely to be compensated for by recolonization from larger populations

SIZE OF SPECIES

The species of fish inhabiting floodplain rivers cover a wide range of size as is illustrated by the

maximum recorded lengths of the individual species from communities inhabiting three typical rivers

in Fig. 5.4 in which the histograms represent the number of species classed according to their total

lengths. Sizes normally span about 3 orders of magnitude (i.e., from about 1.5 cm to 1 500 cm). There

tends to be a high proportion of fish of very small adult size (less than 10 cm) in rivers both in feeder

streams or rocky headwaters and in the potamon reaches of the river. Small size is advantageous both

in the riffles of the rhithron and in most floodplains, as pygmy species can mature more rapidly (often

within one year) and can seek refuge in the root masses of vegetation in the interstices of pebble

masses and other small crevices. Equally they can colonize the surface area of the water and more

readily exploit the neuston or allochthonous food sources found there. Most river systems have a few

species of truly gigantic size. In Latin America the characteristic giant species are Arapaima gigas,

Pseudoplatystoma spp. and Brachyplatystoma spp., in Africa, Lates niloticus and in the Mekong

Pangasianodon gigas, although these are by no means the only fish attaining lengths of 1.5 m or

more.

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Figure 5.4 Histograms showing the proportion by number of species of different maximum lengths in

three typical tropical river systems

DISTRIBUTION OF SPECIES IN RIVER SYSTEMS

DISTRIBUTION IN SPACE AND ZONATION

Zonation

Many attempts have been made at establishing general principles regulating the zonal distribution of

living organisms in rivers. Illies and Botosaneanu (1963) summarized the schemes proposed by a

number of authors and themselves suggested a separation of the river into creon, rhithron and

potamon zones based on faunistic criteria: A specific rule describing the distribution of Northern

European species was advanced by Huet (1949) on the basis of the slope and width of any particular

reach of the river (Fig. 5.5). More recently Vannote et al. (1980) suggested the river continuum

concept which posits an orderly downstream progression of organisms as discussed in Chapter 3.

These attempts mostly originated in the North Temperate Zone and pre-suppose an idealized river

form with a continuously decreasing gradient from source to mouth. As such they are useful within

areas where the original climatic or morphological conditions prevail, but are transposed to the tropics

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only with difficulty. The temperate pattern of zonation has been demonstrated by Bishop (1973) for

the Gombak river (Malaysia) and in other Malaysia rivers the cyprinids, at least, show a clear

succession from Tor tambroides and Acrossocheilis hexagonolepis in upper rapids reaches,

Leptobarbus hoeveni, Puntius bulu and P. daruphani in intermediate streams and P. jullieni only in

the lower potamonic reaches (Tan, 1980). Watson and Balon (1984a) also found highly structured

communities to exist in the Baram River of North Borneo. Here fish partitioned first into layers in the

water column, surface, pelagic, benthic and substratum niche types. A tendency was noted for there to

be an increase in the number of species inhabiting the bottom zone with increasing distance

downstream. Furthermore a related reduction in niche size with the increasing numbers of species was

indicated by a decrease in mean life span and adult size of the individual species.

Figure 5.5 Graphical representation of the relationship between slope of river bed and channel width

and fish community in European rivers. (After Huet, 1949)

In Africa the Illies-Botosaneanu classification has only been applied successfully in some South

African rivers (Harrison, 1965) or in the Luanza, a high altitude tributary of the Congo River system

(Malaisse, 1976) where there is a succession of rhithronic and potamonic reaches similar to those in

Europe. In most parts of the continent zonation is much more obscure as is shown by examples from

West Africa (Ogun river, Sydenham (1978); Bandama river, Merona (1981); Ebo Stream, Ghana,

Lelek (1968)) or from Central Africa (Kalomo river, Balon and Coche (1974) and the Luongo river,

Balon and Stewart (1983)).

Provided there are no geographical discontinuities such as large waterfalls, the increase in diversity

along many rivers tends to be by addition of species to those present rather than by the replacement of

species. This is particularly true of the tropics although Horwitz (1978) remarks on this aspect of the

increase in diversity of fish faunas in streams of the mid-Western United States. In Africa and parts of

Asia most large river systems have such discontinuities so that there are sufficient important

exceptions for this not to constitute a general principle. For example, the Zambezi River is divided

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into three faunistic zones, the middle of which between the Cahora Bassa rapids and the Victoria Falls

is the poorest, or in the Nile, the Kabalega Falls between Lake Kyoga and Lake Mobutu, marks a

discontinuity between the generalized nilotic fishes downstream and a distinct riverine and lacustrine

fauna upstream. Instead it is possible from the work that has been done, to separate the causes of

species distribution in water courses under two main headings: geographical factors and

geomorphological factors.

Geographical factors influence the taxonomic differences that may be encountered within a river

basin. For instance, the isolation of sub-populations of species in small order streams can result in

specific divergence over a long time period. In this way it is possible to observe two similar species

occupying identical or similar niches in two separate streams of the same basin. This isolation is to a

certain extent reinforced by the behaviour of the species themselves, which are often small, sedentary

and have a short life cycle, which may contribute to a rapid rate of speciation. This tendency for

species to diverge in the lower order streams of a river basin could account for much of the increasing

diversity of species shown by larger river basins in Fig. 5.1 and for the relatively large proportion of

species of small size in river fish communities. A second mechanism influencing distribution is river

capture when species may be exchanged between systems. Mahon (1984) discusses some of these

mechanisms in connection with the differences between fish faunas of similar river systems in Europe

and N. America. In North America the more normal divergence has occurred with occupation of head

water streams by numerous small cyprinid and percid species. In Europe the rivers are still occupied

by larger cyprinid species which inhabit the potamon and only migrate to the rhithron to spawn. The

headwater streams there tend to be used mainly by the young cyprinids. This difference is attributed

mainly to the affect of glaciation.

Whereas geographical considerations may influence the distribution of fish species between river

systems, the distribution of the various types of species within any one system is more likely to be

controlled by the geomorphology of any particular river reach. Thus, it is useful to distinguish two

major fish communities:

(i) communities of rhithron-like or rapids zones; and

(ii) communities of the potamon,

Certain amount of interchange between these two communities may occur, particularly by certain

elements of the potamon fauna which enter rapids to breed. Rhithronic fishes tend to be relatively

static within their preferred zone, although juveniles of migratory potamonic species or even marine

anadromous forms, such as the salmon, form temporary elements of their communities. Potamonic

fishes can, however, be divided into two fairly distant groups on the basis of their behaviour.

(a) The first group of fishes avoid severe conditions on the floodplain by migration to the main river

channel and frequently by more extensive movements in the river beyond the floodplain area.

Members of this group are recognized a “Piracema” or “Subienda” species in Latin America, in the

Mekong system are termed “white” fishes, and in other river systems may be classified as rheophilic.

Species of Cyprinidae in Asia, Africa and Europe and Characoidei in Africa and South America are

conspicuously members of this group, sometimes undertaking very spectacular migrations. Some

siluroids and mormyrids are also migratory in behaviour. A few species are confined to the river

channel at all times and never penetrate the plain.

(b) The second group of fish consists of species which have considerable resistance to deoxygenated

conditions and which are termed “black” fish in Southeast Asia or limnophylic elsewhere. Their

movements are therefore more limited than those of the “white” fish. They frequently remain in the

standing waters of floodplain during the dry period and if they move to the river they remain within

the vegetated fringes or in the pools of the river bed as it dries. Most siluroids belong to this category

together with ophiocephalids (channids) anabantids, osteoglossids, polypterids and lung fishes.

131

Although it is very rough, the distinction between blackfish and whitefish is very useful as a first level

ecological classification of species and will be retained in this book for this level of discussion. The

response of the two groups to varying conditions on the floodplain is very different and has important

implications for the management of the ecosystem and the fish stock.

Because many tropical rivers pass through several successive alternations between calm and rapids

reaches, these two faunas likewise alternate along the length of the stream, which gives a type of

zonation based purely on the flow or bottom characteristics peculiar to the river in which they are

found as, for instance, in the Luanza River (Balon and Stewart, 1983). Species of several families

have become adapted to life in rapids and as Poll (1959; 1959a) points out for the Congo River, the

inhabitants of downstream rapids are generally representatives of families which are normally found

in the potamon, but which have occupied the rapids, rather than representatives of those families, such

as the Kneriidae or Amphiliidae which are native to the rapids of the headwater streams. Asian and

South American rivers tend to be of a more traditional form with steep mountainous headwater

streams giving way to flat lower courses which are sometimes of extreme length. In such rivers the

discontinuity between the rheophilic fishes of the headwater streams and the communities of the lentic

potamon downstream is apt to be particularly abrupt.

Conditions become increasingly estuarine toward the mouth of the river where saline waters penetrate

many kilometres upstream, particularly in lowland rivers of slight slope. A pronounced zonation of

species occurs according to their salinity tolerance, although the zones move up and down river as the

interface between saline and freshwater varies with the state of the tide and the flood regime of the

river. Three groups of fish are to be found in this transitional zone.

(i) Freshwater stenohaline species which enter the zone during the flood and retreat upstream at

low water according to the penetration of the saline tongue.

(ii) Marine stenohaline species which follow the influx of marine waters into the river for feeding.

(iii) Euryhaline species which move little but which adapt to the changing salinities of the water.

These may be of freshwater origin (e.g., members of Cichlidae, Cyprinodontidae and some

Siluroidae) or of marine origin (e.g., members of Clupeidae, Atherinidae, Mugilidae, Lutjanidae,

Sciaenidae, Ariidae, Pomadasydae, Gerridae, Carangidae, Centropomidae, Eleotridae and

Gobiidae).

Several species migrate between the river system and the sea either for breeding or feeding.

Anadromous forms, where the breeding cycle is completed in freshwater, include several estuarine

species of marine origin, the Eleotrids Batanga lebretonis, for example, which undertake limited

migrations upstream, as well as coastal marine species such as the clupeids which sometimes migrate

over long distances in the river. In the temperate flood rivers of Southern Europe the sturgeons

(acipenseridae) and salmonids are the main anadromous fishes. Truly catadromous species are

somewhat rarer in large tropical rivers, although eels are present in some systems, notably Anguilla

nebulosa in the Zambezi and its tributaries. Many normally marine species enter the lower reaches of

rivers to feed during the dry season and return to the sea during the rains.

Habitats of River Systems and the Accompanying Floodplains

A number of areas can be considered distinct habitats from differences in morphology, chemical and

physical conditions, presence or absence of vegetation, structured objects such as rocks, wood or other

cover and abundance and type of food. Table 5.1 based on such ecological characteristics, lists the

major habitats found in river systems.

Table 5.1

132

Major habitats of river systems

RIVER CHANNELS FLOODPLAIN 1. Main channels: rapid and turbulent 1. Flooded grassland

flow; fairly uniform; floating sudd

islands. A. Floating meadows - these are prob-

ably not uniform as there are slight dif-

2. Tributary streams ferences in bottom substrate and relief,

floral associations are variable.

A. Small rocky torrential streams

descending from unflooded terrace, or B. Open water.

upstream of main floodplain area.

C. Littoral fringes area at limit of

B. Small channels linking floodplain to advancing or retreating water, submerged

subsidiary marsh or lake areas above main grass; often low D.O. in sheltered areas;

floodplain level - Terra Firme lakes of higher D.O. in turbulent wave-washed

the Amazonas type or type 1 lakes areas.

(Svensson, 1933).

2. Lagoons and depressions

A. Open water

(i) mud bottom

(ii) sand bottom

B. Standing vegetation

C. Floating vegetation mats

D. Floating leafed vegetation

E. Submersed vegetation

3. Lakes (as above but with a greater

proportion of open water and deeper)

4. Flooded forest

A. Dense rain forest

B. Gallery or levee woodland

C. Acacia and bush scrub

5. Flood areas outside main flood area

Terra Firme lakes of the Amazonas or type 1 lakes

(Svensspn, 1933)

1. Rhithronic zone (may break up into an alternation of

pools and rocky riffles) 1. Floodplain pools

A. Pools which dry out completely

A. Pools (in extreme form pools become isolated and

deoxygenated)

B. Marshy pools (heavily vegetated with little

dissolved oxygen)

(i) mud bottom

(ii) sand bottom (i) surface film

(iii) leaf litter - forested or open (ii) deeper water

α floating vegetation fringe C. Shaded pools (in forested or wooded

β submersed vegetation areas)

γ emergent vegetation

(i) clear

B. Rock riffles (a variety of habitats under rocks or

on surface of rocks) (ii) with tree trunks and other debris

C. Tree trunks and other debris 2. Lagoons

2. Potamonic zone (meanders produce a A. Deeper open waters

regular succession of habitats of varying

depth and bottom type) (i) mud bottom

(ii) sand bottom

A. Shallows

B. Vegetated fringes

(i) mud bottom with no current

(ii) sand bottom or with slight α floating mats

(iii) leaf litter current β submersed vegetation

γ emergent vegetation

133

B. Deeps with slow or faster current-shaded by forest

or open

α floating vegetation 3. Large lakes (sub-habitats as for lagoons but more

inclined to set up permanent stratification, greater

depth, more open water relative to shoreline, often with

sheltered and exposed shores)

β emergent vegetation

BACKWATERS CONNECTED TO MAIN

CHANNEL DOWNSTREAM LARGER WATER BODIES

Lentic water regions, open to main channel but with

many of the characteristics of lagoons or lakes above,

may be:

Lakes, sea or larger river system.

A. Shaded - clear or with tree trunks and debris.

B. Open with

(a) deep water:

(i) mud bottom

(ii) sand bottom

(b) shallow water with:

α floating vegetation mats

β submersed vegetation

γ standing vegetation

δ floating leafed vegetation

(c) shallow littoral usually vegetated.

The main channel

Fish in the rhithronic reaches are distributed either among the riffles or in the deeper pools. Although

the riffles present a reasonably homogeneous environment they are structurally complex and require

specific adaptations either to resist the current and the turbulent conditions found on the surface or to

live in the crevices and holes in the rocks. Large, swift swimming, streamlined species sometimes

penetrate those turbulent waters but either pass through in an upstream migration or drop back to the

shelter of the pools. Riffles are also favoured environments for deposition of eggs of some species and

for the subsequent development of their larvae and fry. The adaptations to swift current also fit fish to

exploit the available food sources in the riffles. Swift swimming forms pick up allochthonous material

falling on the surface of the water or drift organisms. Fishes with adaptations to cling to the surface of

the stones often have sucker like mouths with which to feed on epilithic or epiphytic organisms

whereas the diminutive or elongated species that inhabit the interstices in the rocks are particularly

well situated to prey upon the numerous insect larvae crustacea that inhabit the bottom.

Pool habitats are much more varied and fish tend to segregate both by depth and by distance from the

shore. Three communities can be readily distinguished within the water column. A pelagic community

which tends to consist of small species, silvery in colour with upward facing mouths, a mid-water

community of larger silvery fishes streamlined with terminal mouths and a bottom living community

of drab coloured fishes with dorsally humped profiles and ventrally positioned mouths. These various

forms are well illustrated by the communities of cyprinids in a small Borneo stream as described by

Inger and Chin (1962) Fig. 5.6. The slacks near the bank are often heavily vegetated with floating and

emergent aquatic and ever-changing terrestrial plants. Here bottoms are usually composed of food

rich detritus from leaf fall and from silt deposition in the quieter waters.

The slack areas at the fringes of pools, the limited floodplains of the rapids reaches and perennial

streams of the lowland zone have similar faunas many of the elements of which recur within the

floodplains of the potamon. Because there are a variety of microhabitats in the slacks small size is a

great advantage. Spatial niche selection seems very highly developed and all levels of the water

column are occupied as in the example of the Borneo streams above. In a tropical African forest

134

stream several genera of cyprinodonts occupy the water surface, particularly Epiplatys and

Aphyosemion although some Characins such as Brycinus macrolepidotus are also found there. The

mid-waters support clouds of dwarf Barbus and Aphyosemion species which are more often than not

associated with the shade of floating lily leaves. The rich detritus of the bottom attracts several species

of small mormyridae, Neolebias, Barbus and Labeo together with various small or dwarf cichlids such

as Pelvicachromis, Thysia ansorgii, Hemichronis bimaculatus. Similar elongated fishes are found in

the rocks also occur in the floating vegetation at the fringes of the pools as the sinuous habit is equally

adapted to such conditions. Mastacembelus and Calamoichthys particularly are conspicuous in such

environments as they also frequent similar vegetation in the potamon.

Semi-permanent channels, which are more characteristic of low order streams, frequently become

broken-up into a series of rocky riffles, and pools which may become deoxygenated or totally anoxic

as the season advances. Very great densities of fish may be found in the pools, which essentially

represent the sump into which most of the population drains. Lowe-McConnell (1964), for instance,

found 870 fish belonging to 36 species in a pool of 19 m³. This she attributed to a three dimensional

use of space within the pool, together with an alternation of activity between two very different

nocturnal and diurnal fish faunas. The species composition of main river pools is rarely the same as

that of floodplain pools, and within the pools segregation appears to occur by bottom type. Lowe-

McDonnell recorded 44 species of fish in the pools of the Rupununi river, of which 37 (84%) were

found over only one type of substrate.

During the floods the main river channel is filled with rapidly flowing water, but at its margin the

floating vegetation fringe merges with the flooded banks of the plain itself. Large masses of

vegetation become detached and are swept downstream as floating islands. As a consequence the

channel is usually comparatively sparsely populated at this time, although some species, for instance

Petrocephalus bane or Hydrocynus forskahlii, never move out of the river on to the plain (Daget,

1954). In Latin American rivers, the upstream migration, “Piracema” or “Subienda”, of the major

characins during low water and the beginning of the flood also ensures that some species are present

in the channel during at least the earlier phases of rising water. The fringing vegetation and floating

sudd islands shelter juvenile fish or small vegetation dwelling species, which may become distributed

throughout the system in this way.

135

Figure 5.6 Segregation by depth of cyprinid species in a Borneo stream. (From data in Inger and Chin,

1962)

In the Potamon most species of fish move to the main channel during the dry season and settle in

different habitats along its length. Some dry season habitats can lie a considerable distance away from

the main floodplain, and may even be located in entirely different aquatic systems such as the sea or

major freshwater lakes.

In the permanent channels during the dry season, fish also separate by depth, type of bottom and

vegetation cover. Many smaller species inhabit the root masses of the floating vegetation at the edge

of the river, finding there both abundant food and shelter. Several species have specific adaptations to

this habitat. The “upside-down” swimming position of some Synodontis species enables them to

browse on the root fauna, and the serpentine shape of Mastacembelus or Calamoichthys enables them

to weave among the entangled stems of the plants.

136

Deeper areas of the main channel attract the larger fish species. Pangasius sutchi, for example,

migrates to the deepest reaches of the Mekong River between Sambor and Stung Tren during the dry

season (Sao-Leang and Dom Saveun, 1955). The larger individuals of Lates niloticus are reputed to

frequent the deeper parts of the African rivers within its range. Similarly Carey (1967a) noted that

individuals of several species were much larger in the Kafue River than in the adjacent lagoons.

Several species are pelagic within the main channel, and also in the larger permanent water bodies. In

African rivers, small groups of Brycinus macrolepidotus are to be seen cruising at the surface under

overhanging vegetation, and small clupeids characins or cyprinids occur in the surface waters of all

three continents. The surface film presents a specialized habitat occupied by small cyprinodonts

which are found in all quiet stretches of river and floodplain alike.

Three other special habitats have been noted by Lowe-McConnell (1964, 1967) and Mago-Leccia

(1970). Some fishes burrow into sand bottoms. Gymnorhamphichthys hypostomus of the Rupununi

and Venezuelan savannah rivers has an elongated snout which facilitates respiration while buried.

Potamotrygon hystrix and Xenogoniatus have also been recorded from the bottom of Venezuelan

streams. A somewhat similar habit has been noted by Daget (1954) and Cromeria nilotica which

burrows in sandy bottoms of the Niger River when alarmed.

Leaf litter on the bottom of forested streams and pools also yield a rich harvest of small species some

of which do not occur elsewhere in the system. Lowe-McConnell listed Agmus lyriformis and

Farlowella sp. to which Mago-Leccia added Aequidens, Apistogramma, Orinocodoras, Corydoras,

Agamyxis, Homodiaetus and Hyphessobrycon. The crevices and hollows of the mass of decaying

branches which also accumulate in such creeks are inhabited by several small types of fish. Lowe-

McConnell reported having collected 17 species and over 200 individuals in two hours from split logs

and branches. In the Rupununi these were mostly catfishes, such as Platydoras, Trachycorystes,

Pseudopimelodus, Hoplosternum or Ancistrus. In the Apure River some gymnotids also have been

recorded from the crevice dwelling habitat including Electrophorus electricus, Sternopygus macrurus

and Apteronotus albifrons, although catfishes such as Panaque nigrolineatus also occur there. Both

Lowe-McConnell and Mago-Leccia maintained that crevice dwelling is associated with nocturnal

habits. Specialized habitats of this type have only been studied in Latin America, but it seems

probable that they also exist in both African and Asian inland waters.

The blind arms and backwaters of the main channel remain in communication with the main river at

one end, but are otherwise lentic, having many of the characteristics of floodplain lagoons. They are

usually especially rich localities, as silt accumulates in them giving rise to plankton blooms and

increased primary production. They are sheltered and accumulate tree trunks, branches, rocks and

other types of cover providing material for shelter so many fishes enter them as a refuge from the

current of the main stream. As such backwaters are of particular importance as major concentrators of

ichthyomass, and in rivers such as the Danube and the Mississippi are the main surviving features of

the flood system. In the Mississippi particularly they are identified by Schramm et al. (1974) as

providing essential habitats for feeding and reproduction in at least 57% of the fish species recorded

from the reach between St. Louis and Cairo. Guillery (1979) also identified a movement of 15

mainstream species on to the floodplain coincident with flooding of which some adults and juveniles

remained trapped in sloughs and blind areas during low waters.

The floodplain

When inundated the plain has a rich mosaic of habitats although there is little information on the

distribution of fishes amongst them. A basic difference exists between forested and savannah plains,

although originally many plains that are now exposed supported greater tree cover. This means that

much of the present day fauna on the now denuded plains has become modified in recent times as

human activities have worked upon the environment. Flooded rain forests themselves appear to be

varied habitats.

137

Observations on the fish faunas of flooded forests are limited, but indications from Grand Lac of the

Mekong (Bardach, 1959), the Congo River (Matthes, 1964) and the Amazon (Roberts, 1973) attest to

the variety of species in such fish communities. Goulding (1980) particularly has investigated the fish

fauna inhibiting the Amazonian Igapo showing the importance of the fruit and seed bearing trees to a

variety of fishes, including Colossoma macropomum, C. bidens, Brycon spp., Myleus spp., and

Serrasalmus spp. Occupancy of the flooded forest by these species is closely allied to the distribution

of these favoured fruit bearing trees.

On floodplains with more restricted gallery forest or bush scrub, the submerged branches and roots

provide a feeding substratum and concealment for many species. Mago Leccia (1970) lists several

fishes from the Orinoco river which occur among flooded scrub. These include nocturnal armoured

catfishes such as Hypostomus and Pterygoplichthys and the gymnotids, Sternopygus macrurus,

Rhamphichthys rostratus, Adontosternarchus sachsi and Eigenmannia. Diurnal forms such as the

cichlids Astronotus ocellatus and a multitude of characids including Triportheus, Cynopotamus,

Astyanax, Moenkhausia and Thoracocharax were also found. The flooded scrub habitat is common on

most tropical plains, and as observed above, probably more nearly approximates to the original

condition of most of them. Thus, fish communities of this type are presumably widespread although

they have not been specifically described from other systems.

The floating meadows, which now seasonally cover many of the world's savannah floodplains, appear

at first sight to contain little variation. Closer examination reveals a fine texture allied to contour.

There are deeper places around lakes and depressions which are often free of vegetation or have a

flora more typical of permanently wet areas, and shallower places over the levees and terraces. Local

differences in vegetation and bottom type are associated with these features and it may be assumed

that fish segregate accordingly. There is no doubt that certain areas do have particular attraction for

characteristic species. The most important factors controlling such distribution are probably dissolved

oxygen concentration, depth, substrate and vegetation cover.

The littoral zone of the plain, that area of interface between land and water, is usually colonized by

young fish. In Africa these are almost entirely cichlids of the general Tilapia, and Sarotherodon or

Oreochromis and cyprinodonts, which have specific tolerance of the elevated temperature found

there.

On some of the most extensive plains, where sheet flooding is common, there is an intermediate zone

where the areas flooded by rainwater meet those inundated by the rising river level. This zone marks

the boundary between two types of water of different productivity and may limit the distribution of

fish on the plain. Blanc, Daget and D'Aubenton (1955) distinguished two zones on the Niger river

floodplain, a) a zone corresponding to the major bed of the river which was rich in fish and b) a

peripheral flood area which was less densely colonized. A similar distribution pattern has been

reported from Arauca-Apure flood system by Matthes (pers.com.), from the flooded forests of the

Mekong by Le Van Dang (1970) and from the Sudd of the Nile River (Mefit Babtie, 1983). Some of

the patchiness in fish distribution on other large floodplains may be attributable to such differences in

water quality.

In the dry season most of the plain is drained leaving only the network of depression pools, lagoons

and swamps, some of which dry out and some of which persist until the next flood. Most fish leave

the plain at this time, but a certain section of the community remains in these standing waters. Of the

fish remaining, a proportion is composed of species which would normally retreat to the river

channels, but which have become isolated in various depressions. The majority of these die through

desiccation of the water body in which they find themselves, through deoxygenated conditions, or

through exposure to excessive temperatures. Sane find their way to deeper lakes where they may

survive.

138

An assortment of fishes stay on the plain. Daget (1954) and Blanc, Daget and D'Aubenton (1955)

listed Marcusenius senegalensis, Pollimyrus isidori, Petrocephalus bovei (mormyrids), Gymnarchus

niloticus, Heterotis niloticus, Ctenopoma, Parachanna, Polypterus, Synodontis, Clarias, Hepsetus,

Auchenoglanis and Heterobranchus as comprising the characteristic fauna of the floodplain pools of

the Niger river. These genera recur on floodplains throughout their range in Africa, which in some

cases is very widespread. In the Mekong, the blackfish assemblage described by Sao-Leang and Dom

Saveun (1955) contains a similar group of fishes, many of which belong to the same families and

closely resemble the African species. Major examples such as Ophicephalus striatus, O. micropeltes,

Anabas testudineus and Clarias batrachus remain in depression pools throughout the dry season to

spread over the plain during the flood. Species recorded from the lagoons and pools of the Apure

river, Venezuela, by Mago-Leccia (1970) included Hoplias malabaricus, Serrasalmus notatus,

Callichthys callichthys, Hoplosternum littorale, Pseudoplatystoma fasciatum, Sorubim lima.

Pimelodus, Leporinus and Pimelodella. Similar assemblages, often involving the same species or

genera, have been described from the Rupununi by Lowe-McConnell (1964), from the Parana river by

Bonetto et al. (1969) and from the Magdalena by Kapetsky et al. (1976).

There are considerable differences in the species composition of fish population from floodplain pools

of the same system, as typified by the data given in Fig. 5.7. Attempts have been made to correlate

these with the number of variables. Welcomme (1975) traced the increased specific diversity of

populations with size of pool in the Ouémé system (Table 5.2). That size of pool can influence species

composition has also been found by Lowe-McConnell (1964) who found that larger species inhabited

the larger bodies of water. Holden (1963) found the same effect on the Sokoto River and noted that

individuals of the same species tended to be larger in the bigger pools.

139

Figure 5.7 Percentage representation of major fish groups in eight floodplain depression lakes of the

middle Parana system. (After Bonetto et al., 1969a)

140

Table 5.2

Differences in species composition of catches from permanent floodplain lagoon of

different areas of the Ouémé floodplain (FAO/UN, 1971)

Species

Percentage of species in lagoons

smalla mediumb largec

Normally swamp

dwelling species

with auxiliary

breathing organs

Clarias ebriensis 72.2 20.0 1.3

C. lazera 5.0 13.6 3.4

Habitually found

only on flood-plain

Ctenopoma kingsleyae 0.9 7.2 P

Gymnarchus niloticus P 2.1

Heterotis niloticus 26.0 2.6

Parachanna obscurus 23.8 27.2 1.6

Polypterus senegalus P 0.3 0.7

Protopterus annectens P 0.8 0.9

Xenomystus nigri P 0.2 P

Occasional swamp

dwelling species

without auxiliary

organs

Citharinus latus 0.1 1.2

Distichodus rostratus 0.7 8.1

Hepsetus odoe 2.3 2.6

Found in flood-

plain or river

Chromidotilapia guntheri P P P

Hemichromis spp. P P P

Tilapiine cichlids 1.6 2.2

Synodontis spp. 15.2

small mormyrids 18.4

Species normally

found in river

Hyperopisus bebe P 5.4

Mormyrops deliciosus 18.4

Labeo senegalensis P

Schilbe mystus 6.0

Lates niloticus 10.1

a up to 500 m² b 500–5 000 m² c over 5 000 m²

The influence of size of pool may depend on the heightened dissolved oxygen concentration arising

from lessened vegetation cover and improved aeration by wind. Such relatively favourable conditions

allow many more species to survive, including some which would normally return to the river channel

in the dry season. Greater area al so leads to a greater diversity of habitat, with more varied bottom

types, submersed, floating and emergent vegetation, and open water in the place of the densely packed

stands of aquatic plants found in the smaller pools. On the basis of samples from a selection of

floodplain pools from the Sokoto River, Holden (1963) concluded that several species were

distributed according to substrate. Although no species was confined to any one bottom type, certain

species, such as Alestes dentex, were more common over sand. Oreochromis galilaeus was found

more frequently over mud and Tilapia zillii over intermediate bottoms. There is little explanation for

the variability in species composition found by Bonetto et al. (1969) (Fig. 5.7) in the pools of the

Parana river, either in terms of vegetation cover or size of water body, although Cordiviola, de Yuan

and Pignalberi (1981) attribute the variations in these and other lagoons to a number of factors.

141

Firstly, geographical location in the river valley determines overall species composition with

Triportheus paranensis, Moenkhausia dichroura, Acestrorhynchus falcatus and to a certain extent

Prochilodus platensis being dominant to the North (Corrientes area) and P. platensis alone being

dominant to the South (Santa Fe area). Elsewhere Bonetto (1975) has stressed the abundance of mud

eating fish in lagoons whose bottom is of fine particulate organic material. Secondly specific content

of the catch may be generally related to a complex of topographical and biological factors related to

the biology of the species. Thirdly, while some species are of constant occurrence in the ponds over at

least five years the presence of other species appears much more haphazard. Depth and exposure may

also influence the distribution of species composition of the shallow sheltered bay habitat of

Magdalena river lagoons, where Potamotrygon and Pseudoplatystoma dominate, as opposed to the

open water where Hemiancistrus, Triportheus and Prochilodus are more cannon. Other species such

as Plagioscion are distributed more or less indifferently.

How much the distribution of species among the dry season habitats is a matter of chance, with only

those specially adapted species surviving adverse conditions, and how much is a matter of deliberate

selection of habitats is not clear. It seems likely that those blackfish species, which always remain on

the plain, seek out, or maybe never leave, the vicinity of the depressions where they pass their lives,

and the year-to-year recurrence of such species in any one lagoon would support this. With migratory

species, on the other hand, there would appear to be a considerable element of chance and the precise

composition of the fish fauna of the larger lagoons is probably determined by the haphazard trapping

of such individuals. The shoaling habit of many of these fishes would lead to them being either

isolated in large quantities, or not present at all, giving skewed distributions of abundance.

There is some evidence from the Rupununi swamps that species do select their dry season habitats

very closely. In Lowe-McConnell's list of 129 species, 79 percent were found in only one habitat

(excluding channels draining the floodplain which would be likely to have a high proportion of

transient species). More generally, however, distribution patterns do not appear to be so exclusive and

preferences are shown by greater concentrations in one locality rather than another. Such selection

may be by species, although different life stages or age groups of the same species may also segregate

in this way. Intraspecific resource partitioning of this type has been shown for fish from headwater

streams in U.S. rivers such as the pearl dace, Semotilus margarita where in summer the 0 year class

occupies shallow pools and deep channels and age 2+ fish occupy deep channels and pools. This

segregation is also reflected in differences in feeding pattern which may reduce competition between

age classes in this omnivorous species (Tallman and Gee, 1982). This type of segregation is especially

common in those species which utilize the rhithron either as a full time habitat or periodically for

breeding. There the use of riffles for depositing spawn, for the development of the fry and the growth

of juveniles is widespread, whereas the pools serve to house the larger individuals of the resident

species. Oreochromis species also have different habitats for feeding of juveniles (nurseries in the

littoral zone) and adults (the bottom in deeper water). They also segregate for breeding, with the

mature males being found over distinct nesting sites on the bottom, non breeding adults, both male

and female, staying in mid-water adjacent to the breeding sites, and brooding females carrying eggs in

their mouths being located in sheltered brooding sites near the nurseries. Not all fishes show such

complex distribution patterns, but in many, some phase of the life cycle (usually, but not always the

immature stage) is passed in a habitat other than that frequented by the other age groups of the

species.

DISTRIBUTION IN TIME AND MIGRATION

TYPES OF MIGRATION AND MOVEMENTS

Migrations of Adult Fish

Because the best breeding habitat rarely coincides with the best feeding habitat most species have two

distinct centres of concentration and fish have to travel, sometimes over long distances, between the

two. Two components of such movements have been recognized by Daget (1960) for tropical African

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species. His categories of longitudinal and lateral migration are of general application to flood rivers

everywhere. Longitudinal migrations are those which take place within the main river channel, and

lateral migrations are those whereby fish leave the main channel and distribute themselves over the

floodplain.

Four phases of fish movement have been identified separately by Blache et al., (1964) and Williams

(1971) from the Chari and Kafue rivers respectively. Combining their two classifications, six main

phases in the distribution of fish emerge:

(a) longitudinal migrations within main channel: these are usually upstream, but not always so;

(b) lateral migrations on to the floodplain;

(c) local movements on the floodplain and distribution among flood season habitats;

(d) lateral migration from the floodplain towards the main channel;

(e) longitudinal migrations within the main channel: these are usually downstream but not always so;

(f) local movements within the dry season habitat: this may be the river, adjacent lake or in sane

cases the sea.

Although these are broadly applicable to the majority of individuals of most fish species inhabiting

flood river systems, some species are confined to one habitat only. It is also not certain that all

individuals of the mobile species do in fact undertake migrations every year. Within the above pattern

three distinct groups of freshwater species can be distinguished.

(i) The “blackfish” species, whose migrations between dry and wet season habitats are restricted and

which, at the most, undertake lateral migrations to the fringes of the main channel. These species are

more normally confined to the plain spreading over it during the floods from the residual pools and

lagoons which are the dry season habitat;

(ii) Those species which undertake moderate movements within the river, but which spawn on the

floodplain. The major migrations are for dispersal to dry season habitats such as those of the African

Brycinus leuciscus described by Daget (1952), or Crossocheilus reba (Tongsanga and Kessunchai,

1966), and other Mekong species (Bardach, 1959). Migrations to the favoured breeding places on the

floodplain may be either upstream or downstream and rarely involve the formation of large shoals.

Migrations of this type appear to be mainly for the avoidance of unfavourable conditions on the plain;

(iii) The “whitefish” species which undertake an upstream migration during the dry season or early in

the wet season. Such migrations are usually linked both to reproduction and to the need to escape the

adverse conditions of the downstream river channels and lakes in which water levels and dissolved

oxygen concentrations may become dangerously lowered for sensitive species.

The migrations of freshwater fish in the tropics are best known from the South American continent

where a wide variety of migrations occur. A number of medium sized systems seem to have migratory

fish populations showing similar characteristics with a single seasonal movement to and from a down

river feeding zone and an up river feeding zone. Such patterns have been reported from the Rupununi

river with Boulangerella cuvieri, Hydrolicus scomberoides and Myleus pacu (Lowe-McConnell,

1964), the Pilcomayo river with Prochilodus platensis (Bayley, 1973), the Sao Francisco river with

Prochilodus sp. and Duopalatinus emarginatus (Paiva and Bastos, 1982) and are presumably repeated

throughout most moderate size rivers of the continent which carry characin and siluroid populations.

The most complete studies of this pattern of migration are from the Mogi Guassu in which a

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succession of fish species moves up-river at rising water (Godoy, 1975) (Fig. 5.8). A similar but more

complicated sequence of movements is shown by several species from the River Magdalena. Here

there are two seasons of activity, the first the major “Subienda” involves a migration from the

extensive deltaic downstream floodplain to the more shallow upstream reaches in February and

March. Prochilodus reticulatus is the main species involved, although it may be accompanied by

others including Brycon moorei, Pimelodus clarias and Pseudoplatystoma fasciatum. This is followed

by a downstream movement in April to June and a second, minor, upstream migration the “mitaca” in

July to September with a final downstream movement in October-December. Prochilodus reticulatus

is also involved in similar migration upstream from the floodplains of the Catatumbo River at its point

of inflow into L. Maracaibo.

Figure 5.8 migration of fish within the Mogi-Pardo-Grande river system. (After Godoy, 1975)

In larger rivers the migration patterns tend to become more complex as migrations within the tributary

rivers at the main river channel are mixed with movements between these various elements of the

144

system. Thus different but equally complicated patterns have been reported from the Parana River

system and from the Amazon with some observations from the Orinoco indicating an intermediate

level of complexity.

Two main species Prochilodus platensis and Salminus maxillosus has been studied in the Parana and

Uruguay rivers by Bonetto and Pignalberi (1964), Bonetto et al. (1971) and Bonetto et al. (1981)

although several other migratory fishes were also investigated by them. The studies which combined

observation and tagging experiments show the movements of fish in the Parana and its tributaries to

be elaborations of the simple upriver and downriver translocations between feeding and breeding

sites. Prochilodus shows fairly straight forward movements in the peripheral rivers such as the

Pilcomayo but in the main stream individual fish appear to move upstream or downstream more or

less indiscriminately. This seeming confusion arises from the size of the system, the thermal regime in

the South which is severe enough to influence behaviour, the assumed existence of separate upriver

and downriver populations and to the presence of several major tributaries. A further example,

Salminus maxillosus, moves downstream in the Parana River in October whereas the same species is

ascending the Bermejo River at the same time. In an attempt to resolve these complex patterns

Bonetto et al. (1981) proposes the diagram in Fig. 5.9 to represent current thinking on fish movements

in this river. In at least one species from the Parana, Luciopimelodus pati, migration occurs in late

summer on falling water when fish travel upstream for 400–600 km to arrive at the spawning sites just

before the next rise in water level. The curious feature of this migration lies in the behaviour of the

male fish which tend to remain upriver to an increasing degree as they grow older so that females

between 40–70 cm in length come to dominate the downstream populations.

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Figure 5.9 Summary diagram of principal movements of fish within the Parana River. (Bonetto et al.

1981)

Migrations of fish in the various components of the Orinoco system are still very imperfectly

understood, although observations have been made on Semaprochilodus laticeps in the lower reaches

of the river from Caicara to the delta (Novoa, 1982). Behaviourally this species resembles P. platensis

in that eggs and young fish drift downstream from an upriver spawning site, move laterally into

lagoons where they grow and subsequently emerge to migrate up the main channel to spawn some

two to four years later. This pattern of movement produces the type of upstream increase in mean

adult size observed by Goulding (1981) for the Madeira and for male Luciopimelodus pati in the

Parana and would suggest that in the Orinoco at least one discrete stock of S. laticeps is involved.

In the Amazon several types of migrations appear to be undertaken by the same species. Many of the

larger characin genera, Colossoma, Brycon, Mylossoma, Triportheus, Leporinus, Schizodon,

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Rhytiodus, Prochilodus, Semaprochilodus, Anodus and Curimatus show a pattern of migration

originally described as “Piracema” by Ihering (1930) and Geisler et al. (1973). Recent investigations

by Goulding (1980) in the Madeira river, Goulding and Carvalho (1982) in the Amazon and Ribeiro

(1983 and a) in the Rio Negro have shed more light on the nature of these movements which can be

broken down into three components (Fig. 5.10).

Figure 5.10 Diagram of assumed migration patterns of prochilodontid fishes in the Amazon and

Madeira rivers

A. The cycle starts with a migration downstream in a tributary river on the rising flood which

culminates in breeding at the mouth of the tributary. Eggs and young fish then drift downstream in

the main channel until of sufficient size to move into marginal vegetation of the floodplains or

other suitable nursery sites.

B. Adult fish then re-ascend the same tributary and disperse into the flooded forest to feed.

C. As water levels fall vast shoals of fish move down the tributary, up the main channel and into the

next tributary upstream. It is this phase of the migratory cycle which is referred to as the true

“Piracema”.

This type of migration results in a lack of juvenile fish in the tributaries and an accumulation of larger,

older fish as one moves towards the headwaters and is probably adaptive to the very poor trophic

conditions for juvenile fish in the flooded forests fringing the black-water tributaries.

In Africa characins and some siluroids are conspicuous among the migratory species, but the

cyprinids also show this type of behaviour. Potamodramous migrations, whereby adult fish leave

lakes and ascend rivers during the floods to spawn in the upstream swamps, have been described by

Whitehead (1959) for 18 migratory species in the Lake Victoria/Nzoia river systems, and De Kimpe

(1964) for some mormyrids, characins and cyprinids of the L. Mweru/Luapula river system.

Carmouze et al. (1983) distinguish three groups of migrants in the Lake Chad basin. True migrant

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species, Alestes baremoze, Brachysynodontis batensoda, Distichodus rostratus, Marcusenius

cyprinoides, Petrocephalus bane and possibly Hemisynodontis membranaceus, Labeo senegalensis

and Hydrocynus brevis made large-scale longitudinal movements for breeding. Such movements were

usually between the lake and the Yaeres floodplain, but a further species A. dentex appeared to

reproduce in the upper reaches of the Chari and Logone rivers and hence made even more extensive

migrations. Mixed migrants made similar large scale longitudinal movements whose motives were

less certain as both adult and juvenile fish were involved. These species included Schilbe

uranoscopus, Synodontis schall, Hyperopisus bebe, Mormyrus rume and Eutropius niloticus. Finally a

well defined class of lateral migrants was present whose movements between the river channel and

the floodplain were mostly associated with feeding.

A similar group of species is involved in an annual migration up the Senegal River from the lower

reaches around Dagana to as far up river as Dioulde-Diabe where further movement is asserted by a

shallow rocky sill across the river bed. The fish then move laterally across the plain during the floods

and return to the river in falling water. The downstream movement was found by Reizer (1974) to

take the form of a drift produced by the failure of fish to combat the current in the main channel. As

larger fish are generally more capable of combating the current there is a tendency for them to be

swept less far downstream and a noticeable gradation in mean size results in smaller fisher nearer the

mouth of the river.

Long distance spawning migration has also been recorded in S.E. Asia where Probarbus jullieni

migrate up the Mekong River to spawning sites having shallow water, moderate current and sandy

bottoms. Sritingsook and Yoovetwatana (1976). Day (1958) notes that there are several components

of the fish communities of Indian rivers which move long distances to the spawning sites in the hill

tributaries of the major rivers. Species of the genus Tor are particularly famous for their ascent of the

Himalayan streams. Several cyprinid species migrate during the early part of the spring flood in the

Tigris/Euphrates system (FAO/UN, 1954). Notable amongst these are Barbus xanthopterus, B. grypus

and Aspius vorax whose adults spawn on upstream gravel beds, and whose young later drift

downstream to occupy the flood lands in the lower reaches of the river complex. Other cyprinid

species remain downstream to spawn in the river channel or in the swamps, e.g., Barbus sharpeyi or

B. luteus.

Not all migrations are for breeding, however, as Thynnichthys vaillantii appear to move mainly to

avoid adverse conditions in their downstream habitat during the dry season. This species migrates out

of the flood lakes of the Mahakan system of Borneo during falling water and moves progressively

upstream as water levels and dissolved oxygen concentrations fall downstream. The movement is

accompanied by gonadial ripening but spawning is not reported to occur until the fish have returned to

the lakes during the rising flood (Saanin, 1953). The abandoning of downstream lacustrine habitats

due to unfavourable conditions has also been recorded among the Mesopotamic fishes which also

move out of the marshes and floodplains of the Tigris/Euphrates at low water.

In European rivers many species are normally long distance migrants to the extent that Mahon (1984)

comments on the prevalence of this behaviour in European rivers as contrasted to the relatively low

percentage of migrant species in equivalent North American waters. Most of these species still

undertake seasonal migrations although the highly modified state of most waters has limited these

severely. Many records exist of migrant species which have disappeared following the damming of

rivers but Zambriborsch and Nguen Tan Chin (1973) cite examples of continuing semi anadromous

migrations by Aspius aspius, Blicca bjoerkna and Abramis brama in the Kiliya arm of the Danube,

and Belyy (1972) attributed similar behaviour to Lucioperca lucioperca from the Dneiper.

In estuarine deltas where sea water penetrates upstream in the dry season, freshwater species also

undergo local movements to avoid the saline conditions. Reizer (1974) noted that the more sensitive

species move as a wave for up to 100 km in front of the saline tongue. In the Senegal River these are

Hyperopisus bebe, Mormyrus rume, Mormyrops deliciosus, Marcusenius senegalensis, Alestes dentex,

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Citharinus citharus, Labeo senegalensis and Schilbe mystus. Similar migrations undoubtedly occur in

all other rivers having a long zone of interaction between sea and fresh waters.

Marine and brackish water species also show a variety of migration patterns. Entry into the lower

reaches of the river with the saline waters during the dry season may be regarded as an extension of

the estuarine environment. However, penetrations into the freshwaters further upstream are also a

common feature which has been described from many systems. Fish of marine origin regularly move

many hundreds of kilometres up such rivers as the Niger in Africa (Reed et al., 1967), the Mekong in

Asia (Shiraishi, 1970) or the Magdalena in South America (Dahl, 1971). These penetrations of the

freshwater habitat may be for feeding or for breeding. Some notable anadromous migrations also

occur, for instance that of Hilsa ilisha, which moves up many Indian and Southeast Asian rivers to

spawn (Pillay and Rosa, 1963), the Golden perch, Macquaria ambigua, which migrates up the Murray

river in Australia (Butcher, 1967). The salmonids of Europe of N. America or the many migratory

species of the Volga river which moved into the headwaters of the river each year prior to the

construction of the dams there (Poddubnyi in Mordukhai-Boltovskoi (1979). Other less spectacular

breeding migrations are those of the eleotrids, such as Batanga lebretonis or Dormitator latifrons,

which enter rivers to spawn on the flooded vegetation fringes of the lower reaches.

Movement of Juveniles

Originally it was suggested that only the longitudinal component of migration was active, being

directed by physiological stimuli, and that lateral migration was more of a passive affair with fish

being swept by the rising floods on to the plain. It has since become apparent from the orderly

succession of the migrations, and from the way in which fish often swim against the current to gain

access to the plain, that this is not generally the case. In fact it would appear from the available but

rather circumstantial evidence that most movements of healthy adult fish are directed rather than

passive. Passive drifts do occur, nevertheless, where the eggs and larval forms of several species

profit by the current for transport from the spawning site. Observations from the Danube and the

Tigris/Euphrates systems and from various rivers in the United States (e.g., Muth and Schmulbach,

1984) have shown the earlier phases of return movements of younger stages to be by passive drift

with the current. Similarly Godoy (1959) noted that the eggs of Prochilodus scrofa are semi-pelagic

and are carried downstream by the current from the spawning sites which are in mid-channel. The

eggs, however, develop rapidly and the fry soon take refuge in flooded oxbows only to be washed out

later by falling water when they swim downstream to the major feeding grounds. In the Pilcomayo

river, lateral floodplains are absent in the upper courses and Bayley (1973) supposed that the eggs and

fry of P. platensis are swept directly downstream. Similar drifts are suggested by Novoa (1982) for

Semaprochilodus laticeps in the Orinoco, several species of characins in the Amazon (Goulding,

1980) and Hilsa ilisha in the Indus (Islam and Talbot, 1968). Reynolds (1983) also draws attention to

the differences in migration patterns in the Murray-Darling River as a correlate of reproductive

strategy. Here only those fish with buoyant eggs such as the golden and silver perches undertake

extensive upstream movements in order to allow the eggs space for their downstream drift. Some light

is shed on the nature of the passive larval drift through work in the Missouri river (Harrow and

Schlesinger (1980) where a rich and varied ichthyoplankton was composed of larvae of Aplidonatus

grunniens, catastomids, cyprinids, centrarchids, sturgeon and paddlefish. The larvae disappear from

the drift abruptly at about 8–12 mm in length either by catastrophic mortality or because they leave

the drift to occupy more protected and food rich habitats. The latter view is supported by the rapid

build-up of post-larval and juvenile fish in the backwaters of the river coincident with this

disappearance and would also indicate purposeful movements beyond a certain size. Similar

conclusions were reached by Nezdoliy (1984) for Lucioperca lucioperca, Perca fluviatilis and

Abramis brama fry in the Ili River when the initiation of active migration was marked by schooling

behaviour.

The mechanisms for “passive drift” movements of juvenile fish may be complex as Pavlov et al.

(1977) found in the Volga and Kuban rivers where juveniles of several cyprinid, clupeid and gobiid

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species move downstream from the spawning grounds to the feeding grounds by drift. This occurs at

night in clear water rivers probably due to the loss of visual fix by the young fish, thus there is a

marked diurnal migration pattern. In the Kuban, a turbid river, the drift continued around the clock.

Vertical distribution of the young fish differ considerably during the 24 hour period and horizontal

distribution patterns appear to be species specific with some species located in the middle of the

channel and others along the banks. Vertical distribution is also apparently highly biased in some

species such as Catastomus commersoni which tend to occupy the surface of the water rather than the

bottom, particularly as individuals increase in size (Clifford, 1972). Such distributions are regarded by

Clifford and by Pavlov et al. as an active process controlled by definite behavioural reactions.

Therefore the downstream movement, although passive in the sense of propulsion, may involve some

active participation thereby differing from the distribution and transport pattern of inert matter or

plankton within the stream.

The larvae of some species which lay their eggs in floodplain lagoons congregate on the bottom and

show the peculiar behaviour of frequently ascending to the surface and then sinking back down again.

Daget (1957) noted this behaviour in Heterotis niloticus and interpreted it as a respiratory adaptation.

However, similar behaviour in Labeo niloticus (Fryer and Whitehead, 1959), and Lucioperca

lucioperca (Belyy, 1972) and several cyprinid species from the Lli River (Nezdoliy, 1984) is thought

to be to catch currents for transport to the river and eventually downstream. In fact Belyy found that

fry were moved considerable distances in this manner and it could be that the same behavioural

pattern serves the two functions equally.

The downstream movement of juvenile fish within the main channel is an important phase of the total

migratory pattern of long distance migrants. It ensures the return of stocks to areas vacated by the

adults, and in many cases, where the feeding grounds differ from the breeding grounds, enables the

young fish to reach sources of food. However, the passive drift type of behaviour contrasts with that

of fry spawned in the mountain headstreams of Indian rivers, which according to Day (1958) take

their time migrating downstream and are often halted for one or more dry seasons in the residual pools

of the main river channel. This type of behaviour would seem better adapted to continual survival in

torrential streams where the loss of control inherent in the transport by drift may rapidly carry the fry

outside the range of the preferred habitat.

DISTANCE AND SPEED OF MOVEMENT

The main long distance migrations of tropical freshwater fish are those undertaken by the South

American characin species. Tagging experiments, particularly those undertaken on the Parana system,

have yielded considerable information on the extent of such movements. Bonetto and Pignalberi

(1964) tagged 40 000 fish of which 70% were Prochilodus platensis. A second and third series of

tagging (Bonetto et al., 1971, 1981) gave more details of migration distances and routes for this and

other species. In this series of experiments the maximum distance travelled downstream by P.

platensis was 650 km at 3.3 km/day; although the mean migration speed was 7 km/day. In 1962 the

maximum distance covered downstream was 500 km at a mean speed of 5.8 km/day. A considerable

proportion of the fish did not migrate but stayed near the point of release. Salminus maxillosus tagged

in the same series of experiments travelled further, 1000 km in 60 days (16.7 km/day). This and other

results are summarized in Table 5.3.

Table 5.3

Upstream and downstream migration rates in km/day of some species from Parana River

(from data in Bonetto et al., 1981)

Species Maximum recorded distance Migration rate

travelled (km)

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Upstream Downstream Upstream Downstream

Ageneiosus brevifilis 657 3

Hemisorubim platyrhynchus 635 7.3

Luciopimelodus pati 600

Paulicei lutkeni 706 14.4

Prochilodus platensis 455 939 0.5(3.2)8.77 0.3(2.8)18

Pseudoplatystoma coruscans 821 281 4–6 1

Pterodoras granulosus 308 1054 1–3 1–2

Rhaphiodon vulpinus 270 265 2 2.3

Salminus maxillosus 737 1000 0.9(9.3)21.5 0.4(2.5)3.9

Serrasalmus nattereri 260 2

In the Mogi Guassu and Rio Grande sub-system of the Parana, 17 species of characin migrate every

year over a total distance of 1228 km between the rhithronic upriver spawning sites and the potamonic

feeding sites downstream. Recorded speeds upriver ranged from 5–8 km/day for Prochilodus scrofa,

2.5–10.0 km/day for S. maxillosus and 3 km/day for Leporinus copelandii. After spawning fish moved

downstream at between 3–5 km/day.

Bayley (1973) also recorded migrations of P. platensis in the Pilcomayo tributary of the Parana. Here

he assumes the fish to travel at least 450 km from the downstream floodplains to the upriver spawning

site. Migrations of the same type were described by INDERENA (1973) from the Magdalena River,

Colombia, where the “Subienda” migration of Brycon moorei, Pimelodus clarias, Prochilodus

reticulatus and Pseudoplatystoma fasciatum, among other species, from the cienagas of the floodplain

to the breeding grounds in small Andean tributary streams is about 500 km.

Tagging experiments in the Sao Francisco River conducted by Paiva and Bastos (1982) showed that

distances travelled by different species ranged from 3 km (Hoplias malabaricus) to 530 km

(Duopalatinus emarginatus). Daily migration rates ranged from 0.03 km/day in Leporinus to 13.9

km/day in Duopalatinus and one specimen of Prochilodus travelled 250 km downstream in 30 days

(6.6 km/day).

In Africa dry season migrations are associated mainly with dispersal of fish in the river system. In the

Niger River, for instance, Daget (1952) studies Brycinus leuciscus which travelled at 1–1.5 km/hr up

to a maximum of about 9 km/day. Total distances traversed were as great as 400 km before the

construction of the Markala dam. Also in the Niger fishermen from the middle reaches between

Ayourou and Niamey in Niger maintain that many species move upstream at the beginning of the

rising flood. Fish are full of eggs at this time and it is suspected that the main destination is the

floodplain of the Central Delta some 400–650 km distant. The floodplains in the middle reaches of the

Niger are very poorly developed and probably do not provide an adequate area for breeding in those

species requiring this type of habitat. Many migratory species including Alestes baremoze and A.

dentex are indicated by Blache, Miton and Stauch (1962) and Carmouze et al. (1983) as undertaking

potamodromous movements of up to 650 km between Lake Chad and the upstream swamps. Reizer's

(1974) observations in the Senegal River also indicate maximum distances of 400 km for the main

migratory species which here include Alestes, Brycinus, Citharinus, Distichodus, Labeo, Lates and

Clarias. Other recorded African breeding migrations are of shorter, but still impressive lengths. The

ascent of species from Lake Victoria up to Nzoia river in Kenya during the flood were classed by

Whitehead (1959) as long duration (80 km or more), Barbus altianalis; medium duration (15–25 km),

Labeo victorianus and Schilbe mystus and short duration (up to 8 km), Brycinus nurse. Another

Labeo, L. altivelis migrated up to 150 km up the Luapula River from Lake Mweru at the beginning of

the floods. Williams (1971) tagged several species of fish as they left the Kafue river plains and found

that most undertook movements of up to 60 km upstream and 120 km downstream.

Among Asiatic species the Indian major carps appear to move only locally and mainly laterally on to

the floodplains within the Ganges and other river systems (Jhingran, 1968, for Catla catla, and Khan

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and Jhingran, 1975, for Labeo rohita). Other species such as the Mahseers (Tor spp) undertake

extensive migrations within the Gangetic system running up the major tributaries to the foothills of

the Himalayas to spawn. In the Mekong several species have been recorded as covering large

distances. Shiraishi (1970) quoted migration distances of up to 1 000 km between the delta and

Vientiane during the wet season. There appears to be little evidence to support this, however, and

more reasonable estimates were given by Bardach (1959) for the movement of Pangasius sutchi from

the Grand Lac to the Khone falls, a distance of some 3–4 00 km. Pantulu (1970) did mention that

Pangasianodon gigas is suspected of spectacularly long migrations, although other species in the

Mekong, including P. pangasius, P. sanitwongsei, Cirrhinus auratus, Probarbus jullieni and

Thynnichthys thynnoides only undertake medium to long-range migrations. Another species of

Thynnichthys, T. vaillanti has been described as migratory up to 450 km upstream of its wet season

habitats at low water (Saanin, 1953). Long distance migrations also occur in Ctenopharyngodon idella

and Hypophthalmichthys molitrix in the Amur River which cover up to 1 200 km from downstream

feeding grounds to upstream breeding sites (Krykhtin and Gorbach, 1981).

The Golden Perch (Macquaria ambigua) of the Murray-Darling river system of Australian are

recorded by Butcher, (1967 and Reynolds (1983) as undertaking migration of 800 to 1 000 kms from

the lower reaches of these rivers to the headwaters to spawn. The average speed of the fish was 3

km/day and a maximum speed of 15.7 km/day. A fourth species from this system, the Silver Perch,

Bidyanus bidyanus was also found to move up to 520 km upstream.

Many anadromous fish travel considerable distances within freshwater for instance Huso huso and

Acipenser gueldenstaeti are both recorded to have moved up to 3000 km up the Volga river from the

Caspian sea. Such journeys within the rivers may represent only a small part of the total distance

travelled as species such as Salmo salar may have a preliminary route of several thousand kilometres

in the sea.

TIMING OF MIGRATION

The timing of the initiation of longitudinal migration varies considerably according to various groups

of species.

Movements of fish in the Mekong River, recorded from the commercial fishery at Khone falls

(Chanthepha, 1972) showed two separate groups. The first, consisting of cyprinids, passed upstream

in November to February and probably represented dispersal migrations after leaving the floodplain.

The second group, mainly of siluroids, passed up through the falls from mid April to July and were

possibly pre-spawning migrations. Alternations between dispersal and spawning migrations in the

same species have also been recorded in Latin America. For instance, Prochilodus mariae in the

Orinoco shows two phases of movement, one reproductive at low water (February-May) and one for

dispersal from September to December just after peak flood. In this the species is accompanied by

Brycon sp. Curimatus sp. and some predatory siluroids such as Pseudopimelodus and Sorubimichthys.

Other species adapt different timings in the same river. Thus Colossoma brachypomum migrates at

low water, while Semaprochilous laticeps and S. kneri emerge from the lagoons and migrate up the

main channel to breed early in the rising flood May to July. Similar plurimodality of migration is

demonstrated by the Prochilodus and Semaprochilodus of the Amazon. In most other systems in Latin

America the Prochilodontidae and certain siluroids complete their major upstream movements at low

waters. Thus in the Parana, Mogi Guassu and Magdalena rivers the fish time their movements so as to

arrive at the upstream spawning sites as the flood begins to rise. Goulding (1981) also describes the

migration of many species of catfish up the Teotonio cataracts of the Madeira River at low water, and

here only one species, Goslinia platynema, was observed to migrate at high water.

The pattern also appears in the Lake Chad basin where Alestes baremoze move up the Chari and

Logone rivers at low water, arriving at the entrance to the Yaeres floodplains at the beginning of the

flood (Stauch, pers.com.), in the Senegal river where the longitudinal migrations of Alestes during the

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dry season bring them to the channels opening on to the floodplain as the floods rise and in the

movements of Brycinus leuciscus and B. nurse within the Central Delta of the Niger, although in these

species the migration is for dispersal rather than spawning (Daget, 1952).

Other families of migratory fish, particularly the mormyrids in Africa, some siluroids and most

cyprinids in Africa and Asia and the Golden perch in Australia normally initiate their riverine

spawning migrations as the floods appear. In some cases, for example, Labeo victorianus and other

small cyprinids in Lake Victoria, riverine migration may be preceded by a preparatory movement to

the river mouth in the lake itself (Cadwalladr, 1965). However, the arrival of the first freshets of the

floods in the river seems to trigger longitudinal migratory behaviour in the majority of species. This

migratory urge appears to be transformed into a general impulse for lateral movements as the bankfull

stage is reached and floods spill on to the floodplain. The observations of several workers that many

fish migrate actively against the current up channels where water is flowing out of the plain, rather

than to enter passively on inflowing currents, points to the physiological orientation of such

movements.

Migration on to the floodplain seems to develop as an ordered sequence of species. Sao-Leang and

Don Saveun (1955) described the movements of fish in the Mekong as a series of waves with siluroids

entering early in the sequence and whitefish later. In the Niger (FAO/UN, 1970), swamp tolerant

species which often stay in the larger lagoons over the dry season, tended to enter first from the river.

Earlier entries included species of Clarias, Distichodus, Citharinus and Labeo, whilst Brycinus,

Tilapia, mormyrids, Schilbe and Synodontis entered second. In the Kafue flats, Williams (1971) has

observed a different sequence that, to a certain extent, contradicts that observed above. Hare, Clarias,

Schilbe, Barbus and Tilapia migrated on to the plain while the water was still quite low and

Serranochromis and Haplochromis only moved on to the plain later. The early movement of Tilapia

and the clariids is confirmed by the University of Idaho et al. (1971) for the same area. Because of the

lack of information, which is difficult to collect, the situation with regard to movement during rising

water is far from clear, although the concept of phased migration of species on to the plain is probably

sufficiently well-established.

During normal flooding there appears to be adequate space on the floodplains to accommodate both

the resident black fishes and the migratory individuals of the more mobile species. Benech and

Quensiere (1983), however, concluded that, during periods of drought, the sedentary stocks can

restrain the annual colonization of the floodplain by riverine fishes. In the Yaeres, the disproportions

between resident and immigrant species increased in those years when floods were less intense and

was minimal during the period when virtually no wetting of the floodplain occurred. The effect may

be due in part to the prior occupation of such living space as is available on the plain, and in part to

the reduction in possible ingress channels with lessened flooding.

Movements off the plain are more easily studied and these too show that fish leave in a distinct

sequence. This appears to be roughly the reverse of the sequence with which fish enter the plain. In

the Mekong, studies by Blache and Goosens (1954) and by Sao-Leang and Dom Saveun (1955) from

trap catches of over 100 species showed that movement at the level of Quatre Bras occurs between

October and February. Here the sequence of migration was apparently conditioned primarily by the

size of the fish, the larger fish and larger species leaving first. Similar observations on size sequence

in migration have been made by Saanin (1953) in the Mahakan system where the larger individuals of

Thynnichthys vaillantii left the floodplain lakes and migrated upriver before the smaller fish and in the

Mogi Guassu where smaller species always precede the larger ones in the upstream movement.

Lunar phase also influenced the timing of the migration of whitefish in the Mekong which takes place

only during the second quarter to full moon each month. Thus there were monthly waves of migration

in which a few of the largest fish moved in October and November. In December to January mixed

groups of large fish including Cirrhinus auratus, Osteocheilus hasselti, Pangasius sutchi, Pangasius

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larnaudi, Belodontichthys and Cyclocheilichthys were caught. In February smaller species such as

Thynnichthys thynnoides, Cirrhinus jullieni and Botia modesta dominated in the catch.

Studies by Durand (1970 and 1971) on the succession of species passing through the El Beid River

which drains the Yaeres, also pointed to there being a definite sequence of species which correspond

to different water masses. The first group, which moved at high water and had a very clear peak in

November and December, consisted of Marcusenius cyprinoides, Hyperopisus bebe, Alestes dentex

and Labeo senegalensis, preceded by a group of accompanying species, Alestes baremoze, Polypterus

bichir, Hydrocynus brevis and Lates niloticus and followed by Distichodus rostratus, Oreochromis

aureus, Pollimyrus isidori and Distichodus brevipinnis. A second well-defined group consisting of

Oreochromis galilaeus, Brienomyrus niger, Barbus and Clarias species was very abundant in January

and Ichthyoborus besse, Synodontis auritus, S. schall and Schilbe mystus appeared most strongly in

February. The first group corresponded to fish migrating at the end of the Logone floods, the second

and third groups move with the water draining off the flooded plain. More detailed descriptions of this

phenomenon by Benech and Quensiere (1982,1983 and 1984) confirmed the various species groups

and their order of migration and linked these to flood conditions on the plain. A strong diurnal pattern

was also demonstrated with diurnal, nocturnal and crepuscular migrants. Lunar phase influenced the

nocturnal migrants but this was considered of secondary importance to the hydrological phase.

Certain species may change their lunacity depending on the flood period. Thus Hyperopisus bebe,

which is purely nocturnal on the floodplain during the rising flood, is active both day and night at the

beginning of the flood and as the water is draining from the plain, i.e., at times of migration.

This example of the migrations from the Yaeres serves to illustrate the probable complexity of

migration patterns elsewhere and in some ways similar to the phased appearance of young fish on

shoreline nursery habitats in a North American stream studied by Floyd et al. (1984). Here an ordered

succession of species, from cottids through percids, catostomids, centrarchids to ictalurids, occupied

the habitats. The phasing of arrival on the shoreline was linked to the time of breeding of the species

in a somewhat extended season. Nevertheless, sufficient numbers of species were present in the

habitat at the same time as to call for their partitioning of the resource. Residence time on the

shoreline ranged from three to sixteen weeks depending on species and represented a way station

between the spawning habitat on the shoreline or the riffle systems and the adult habitat.

That adult fish leave the floodplain before the young-of-the-year is borne out by many authors. For

example, Motwani (FAO/UN, 1970) remarked that older age groups leave the swamps of the Niger

and Benue River first. The juveniles remain on the floodplain until the later stages of its emptying.

The University of Idaho et al. (1971) and Williams (1971) noted the same phenomenon in the Kafue

River. In the Yaeres, the breeding fish often do not return through the El Beid River, preferring to re-

enter the lake via the main channels of the Chari and Logone rivers. Thus Durand (1970) was able to

record that young fish make up 95 percent of the El Beid catch by number and weight. This

phenomenon is mainly caused by the peculiar flow patterns of this system (Fig. 5.11) which only

allow the adult fish to fight against the currents entering the plain through the levees and thus to

regain the main channel of the river. The young fish are not strong enough to do this and are more

strongly influenced by the current which directs them down the El Beid as it drains the plain. More

extreme examples of delayed migration are shown by the juveniles of Prochilodus platensis, which

remain in the floodplain pools of the Parana River during an entire dry season before entering the

river after the next flood (Bonetto, 1975). Prochilodus species in the Apure River show similar

behaviour (Matthes, pers.com.). That return migrations by juveniles are more complex than just a

passive movement under the influence of the movement of water as it leaves the plain is indicated by

the different behaviour of two very similar species of small cyprinid in a stream system flowing into

Lake Victoria (Welcomme, 1969). Here the adults of both species left the upstream swamps towards

the end of the flood. The juveniles of one species, B. kerstenii remained only a short time in the

swamps, moving quickly to the drainage channels and the river and migrating shortly after to the lake.

B. apleurogramma on the other hand tended to stay in the swamps until they were almost dry, moving

to the drainage channels and later to the river where it remains throughout the dry season. These

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movements occurred at quite characteristic sizes. By contrast a third sequence of movement is shown

by a small cichlid which inhabited the same system and did not enter the lake.

External factors which might stimulate spawning migrations have been discussed for many years,

somewhat inconclusively. In view of the great differences in timing both of the initiation of

movement and of spawning among the many species inhabiting tropical river systems, it is likely that

there are a whole range of fine tuning stimuli which influence the various species in different ways.

Because the end of the flood tends to be less predictable than the beginning, and the behaviour of the

fish at this time is not apparently subject to a strong physiological impulse such as reproduction, it

seems reasonable to suppose that simple mechanical and chemical factors would be sufficient to

initiate movement out of the floodplain habitats. It is important to the survival of the species that such

signals should anticipate the onset of conditions that would be lethal to the majority of the population.

Nevertheless, it does seem that stimuli for return migrations are often less than totally effective in this

respect, as huge quantities of fish are stranded and die every year.

The generally noted tendency for both the larger species and the larger individuals of each species to

leave the floodplain earlier than the smaller fish is probably indicative that depth is one of the major

factors controlling this. The fact that the large fish often fail to move out of deeper floodplain pools

would also tend to support this supposition. Dissolved oxygen concentration and temperature also

seem to be of major importance in determining the distribution of fish within the system, and changes

in these factors are liable to provoke fish into leaving regions where conditions are less than optimal.

155

156

Figure 5.11 Migration patterns within the Yaeres-Lake Chad system. (Adapted from Durand, 1970)

Light plays a very strong role in regulating the time of migration. Several families move mainly at

night. In Asia these include siluroids and ophicephalids, in Africa siluroids, ophicephalids and

mormyrids, and in South America siluroids and gymnotiids. Other fishes including cichlids, cyprinids

and some characins, tend to move by day, yet others, such as the small mormyrids which move up the

Nzoia River from Lake Victoria to spawn concentrate their movements at dawn and dusk (Okedi,

1969). There is, in this manner a possibility for round-the-clock utilization of both the migratory

pathways and the slack water refuges and resting places. That diurnal sequences of migration may be

quite complex is evidenced by Lowe-McConnell (1964) observations on the Rupununi system. Here,

the periods for peak migration were more closely defined as follows: dawn and early morning -

Cichla ocellaris, Osteoglossum; daytime - Serrasalmus nattereri, Metynnis sp., Geophagus jurupari,

Cichla ocellaris, Cichlosoma severum, Leporinus friderici; late afternoon - Brycon falcatus, Metynnis

sp.; evening (just after dark) - Prochilodus insignis, Schizodon fasciatum; night - Pimelodus and other

catfishes.

The strong influence of lunar phase on the timing of the migration of whitefish in the Mekong has

already been mentioned. A similar phenomenon has been described for Brycinus leuciscus in the

Central Delta of the Niger by Daget (1952). This characin species forms extensive shoals during

moonlit nights for their upstream dispersal migrations. When there is no moon the shoals dissociate.

Migration is also initiated as soon after the draining of the floodplain as is consistent with lunar phase,

and because of the topography and drawdown regime of the central delta, four different migratory

groups are formed according to the successive coincidence of full moon and low water as the flood

recedes downriver. Other characin species, Brycinus nurse, Alestes dentex and A. baremoze also

migrate upstream but are not ordered by lunar phase. Upstream movements of B. nurse in fact always

precede movements of B. leuciscus from the same portion of floodplain.

Potamodramous migrations would appear to have several advantages for the fish species undertaking

them. Many species may migrate primarily to avoid unfavourable conditions in the lower reaches of

the river, but the majority of migrations seem directed at reaching localities suitable for reproduction

or feeding. By placing the young fish nearer the headwaters, or where appropriate in the main stem of

the river, their journey downstream can coincide with the flood wave over several hundred kilometres.

Juvenile fish can thereby arrive at floodplain reaches suitable for feeding grounds, which may differ

from the rocky, sandy and turbulent areas chosen for breeding, and thus ensure maximum exposure to

zooplankton in the floodplain pools. Furthermore, in forested nutrient poor rivers, such as the

Amazonian tributaries, the fringing flood forests are so lacking in nutrients that the young need to be

transported downstream to find suitable feeding and nursery habitats. This opinion is also advanced

by Krykhtin and Gorbach (1981) for Ctenopharynogodon idella and Hypophthalmichthys molitrix

from the Amur River. The eggs of these species originate from spawnings on upstream grounds more

than 1200 km from the mouth of the river, which distance rules out the possibility of eggs or larvae

being swept out to sea. Spawning at the earliest phase of the rising flood also ensures that the eggs

and larvae drift downstream and that the juvenile fish are thus positioned opposite suitable floodplain

foraging areas at the time of changeover from zooplankton to mixed feeding some four and a half

days after hatching. At the current velocity of 4.7 km/hr pertaining in the Amur even this short time

lapse may mean a drift over 500 km between hatching and seeking refuge in the side arms of the

floodplain.

A similar strategy, that of placing the young on the floodplain as early as possible also is followed by

these upstream migrants who time their arrival at the channels opening on to the floodplain to

coincide with the first flow of water on to the plain. In addition to the benefits for feeding noted

above, upstream migration by adult fish prior to spawning must have a role in counteracting

downstream drift of larvae and fry in those species that are obligate main channel spawners. Although

not analysed in detail by any studies so far it would appear that migration distances upstream as

described for the Pilcomayo (Bayley, 1973), the Parana (Bonetto, et al. 1971, 1981) or the Amur

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(Krykhtin and Gorbach (1981) are of the same order as the distance drifted downstream by larvae and

fry given the current velocities of the rivers and development times of the fishes concerned.

Obviously during drift type movement there must be considerable dispersal of the population which

may ensure mixing of stocks over such a long river as the Parana. In other systems, such as the

Orinoco, it is, however, clear that separate sub-populations are maintained in different reaches of the

river.

There has been much discussion of the role of this potomodromous habit in African species, Jackson

(1961) maintaining that it is principally a device to protect the young from predation, whereas Fryer

(1965) considered it mainly as a mechanism to secure dispersal over the whole river course. As in so

many such arguments both participants are probably partly correct and there is no doubt that the use

of upstream swamps and spawning habitat does have the double advantage of presenting the young

fish with a rich habitat in which to start life, while at the same time giving considerable shelter from

the predation to which they would be exposed in the adult lacustrine habitat.

ADAPTATIONS TO EXTREME ENVIRONMENT CONDITIONS

Many of the habitats within river ecosystems have extreme physical or chemical conditions which call

for special adaptations on the part of the fish inhabiting them. Many of the adaptations are

behavioural, involving migrations or local movements whereby the adverse conditions are avoided. A

certain section of the fish fauna, however, has specific anatomical or physiological adaptations which

permit the species concerned to survive low dissolved oxygen concentrations or even complete

deoxygenation, high temperature, desiccation, poor light conditions or strong current.

LOW DISSOLVED OXYGEN CONCENTRATIONS

One of the main factors determining the distribution of fish within the river and floodplain system is

the availability of dissolved oxygen. As has been shown above low dissolved oxygen concentrations

or completely anoxic conditions are common on floodplains at certain times of the year. Fish have

become adapted to these conditions in a variety of ways but some species which inhabit the lentic

waters of the floodplain are strangely sensitive to low dissolved oxygen levels. Serrasalmus nattereri

and S. rhombeus for instance show the first symptoms of asphyxia when the oxygen falls below only

20 percent saturation. Such species are especially sensitive to sudden drops in dissolved oxygen and

are among the first to suffer catastrophic mortalities. There are two sources from which fish may

obtain supplementary oxygen in poorly aerated waters. These are:

(a) the air above the water, and

(b) the thin, but well oxygenated surface layer which is often only a few millimetres deep.

Many of the blackfish species inhabiting swamps have modifications which allow them to benefit

from one or another of these sources. For example, Carter and Beadle (1931) found that eight out of

twenty species occurring in the Paraguayan Chaco had anatomical respiratory modifications for air

breathing, whereas the rest used the surface layer as a source of oxygenated water.

Adaptations for Air Breathing

The development of specialized organs which enable fish to breath air has occurred independently in

many taxa and in all zoogeographic regions. Respiratory modifications have been centred on three

main anatomical systems, the mouth and digestive tract, the gills and branchial chamber, and the lung

or swim bladder. These are discussed in detail by Carter (in Brown, 1957), Norman (1975) and

various contributions in Hughes (1976) particularly that of Dehadrai and Tripathi.

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Several species have become so dependent on air that they die if prevented from reaching the surface.

Thus the lung fishes are obliged to breathe at frequent intervals, and the paiche (Arapaima gigas)

needs air every 10–15 minutes when adult and more frequently when young (Sanchez Romero, 1961).

Modification of the digestive tract for air breathing is made possible by the stopping of feeding during

the dry season (Lowe-McConnell, 1967), at just those times when deoxygenation is most extreme.

Most parts of the alimentary canal have been modified in one family or another. The mouth cavity and

pharynx are highly papillated and well supplied with blood in Electrophorus electricus which surfaces

to gulp air. Air bubbles are passed backwards to lodge inside the heavily vascularized stomach of

Ancistrus, Corydoras, Plecostomus spp. and Hypostomus, the intestine of Hoplosternum or

Callichthys, or the rectum of Misgurnus fossilis.

The branchial or pharyngeal cavity has become modified by simple vascularization in Hypopomus and

in particular in the synbranchid eels such as Monopterus and Symbranchus marmoratus.

Mastacembelus spp. secrete a protective slime over the unmodified gills, which permits a limited

amount of aerial respiration. Three different families have developed diverticula of the branchial

cavity (Fig. 5.12). These are least developed in the Channidae whose supra-branchial chambers are

simply lined with a richly vascularized epithelium. The Anabantidae have labyrinth organs elaborated

from the first gill arch. In the clariidae the II and IV gill arches have become modified into

arborescent organs in many genera, and in Heteropneustes (Saccobranchus) fossilis the branchial

chamber is extended backwards along the body.

Figure 5.12 Adaptations for air-breathing in (A) Channidae, Channa; (B) Anabantidae, Ctenopoma;

(C) Clariidae, Clarias; (D) Heteropneustidae, Heteropneustes

Only the lung fishes (Dipnoi) and bichirs (Polypteridae) have true lungs, but several physostomous

families have modified swim bladders which act in an almost identical manner. These included

Osteoglossidae (Arapaima gigas), lepisosteidae (Lepisosteus sp.); Gymnarchus and several species of

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Mormyridae, Erythrinidae (Erythrinus and Hoploerythrinus), Notopteridae, and Umbridae (Umbra).

Young forms of Lepidosirenidae, Polypteridae, Osteoglossidae and Gymnarchidae have external gills

which are resorbed during development when the lungs or swim bladder take over the main

respiratory function.

The ability to breathe atmospheric oxygen enables fish to colonize waters which would be otherwise

uninhabitable thereby reducing interspecific competition. Furthermore as comparatively few

ichthyophage predators have developed these mechanisms fish colonizing such deoxygenated waters

are relatively free of predation (Junk, Soares and Carvalho, 1983). It has the additional bonus that it

permits overland movement. Migration of fish over dry land, or at least over damp, swampy grounds

have been recorded in several species, particularly various clariids in Africa and Asia, but also in the

erythrinids and perhaps the callichthyids and loricariids of South America (Kramer et al., 1978). Such

mobility permits fish to escape desiccating water bodies and also to colonize pools isolated from the

main water mass.

Dehadrai and Tripathi (in Hughes, 1976) mention the energy cost of air breathing. Young

Ophicephalus punctatus kept in 40 cm of water surfaced 1 879 times per day at a cost of 161 cal/day.

In 2.5 cm of water the same species surfaces 482 times per day at a cost of 92 cal/day. Fish living in

deeper water consume about 1.5 times as much food as those from the shallower waters presumably to

compensate for the additional energy required. Another disadvantage of the air breathing habit is the

increased vulnerability to predation of the fishes by both aquatic and airborne predators during their

journey to and from the surface and at the surface itself (Kramer et al., 1982). Kramer and Graham

(1976) suggested that, in many species from many families this is to a certain extent mitigated by the

social behaviour of synchronous breathing by a number of individuals which space their breath

randomly. This type of behaviour has been observed in the laboratory in Ancistrus chagresi

(Loricariidae), Hoplosternum thoracicum (Callichthyidae), Piabucina festae (Lebiasinidae) and

Trichogaster leeri (Anabantidae), and in the field in Piabucina and Hoplosternum. Other species from

other families (Polypteridae, Lepisosteidae, Notopteridae and Megalopidae appear to behave in a

similar manner from descriptions in the literature, indicating that the adaptation may be widespread.

Adaptations for Using the Surface Layer

In species lacking specific physical or physiological adaptations the practice of utilizing the better

oxygenated surface layers of the water is common. For instance Kramer (1981) recorded 93% of non-

air breathing fishes in Panama as breathing at the surface layers and later Kramer and McClure (1982)

extended these observations to the Amazon where 31 species belonging to 14 families made

increasing use of the surface layer as oxygen concentrations dropped. Thus aquatic surface respiration

was considered to be a specific adaptation to hypoxia in fishes inhabiting shallow tropical waters.

Anatomical modifications which enable certain small species of fish to use the oxygenated surface

film of the water particularly efficiently have been described by Lewis (1970). These adaptations

include the small dorsally-oriented mouth and dorso-ventrally flattened head found in most

cyprinodonts. Lewis carried out experiments to show that in deoxygenated conditions Fundulus,

Poecilia and Gambusia all adopted a characteristic posture at the water surface where they can

survive indefinitely, provided a critical population density is not exceeded. Kramer and McClure

indicate similar morphology and behaviour in species of cichlids (Pterophyllum) and osteoglossid

(Osteoglossum). Some larger fishes too have adopted this strategy and in addition to their posture at

the surface Colossoma macropomum and Brycon melanopterus develop dermal enlargements of the

lower jaw during hypoxic conditions which facilitate the influx of surface water into the buccal

cavity. These enlargements are lost in oxygen rich water (Braum and Junk, 1982).

Physiological Adaptations

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Physiological mechanisms in some species allow the fish to withstand low dissolved oxygen

concentrations, although not necessarily to survive under complete anoxia. For instance Blazka (1958)

found that Carassius carassius can tolerate anoxia for at least two months at low temperature. At

higher temperatures tolerance time is lessened. Much of this is due to the composition of the blood

which in some species, notably those inhabiting oxygen-poor waters, is relatively unaffected by

changes in 02 uptake produced by increases in CO2 tension (Lagler et al., 1977). There is in fact a

great diversity of response by fish to respiratory stress as described by Fry (in Brown, 1957). Work by

Powers et al., (1979) on Amazonian fishes has particularly drawn attention to differences in the

affinities of the blood of different species of fish for oxygen. Their work was summarized by a series

of curves (see Fig. 5.13) which show fish from running waters where there is much dissolved oxygen

to have blood of lower affinity than fish from still waters with low dissolved oxygen tensions. Fish

from the margins of rapids occupy an intermediate position. By extension fish occupying rapids which

dry up during the dry season may change the O2 affinity of their blood to correspond to prevailing

conditions.

ADAPTATIONS TO RESIST HIGH TEMPERATURE

Temperatures at the fringes of the flooded plain, and in shallow water bodies, may rise as high as

40°C. Certain species of fish, particularly the juveniles, tend to prefer such areas, using them as

nurseries. Here they benefit from the higher temperature and greater availability of food to grow

faster, and also as a partial refuge from predation (Welcomme, 1964). There is experimental evidence

that species inhabiting these warmer water areas have a much greater physiological resistance to the

effects of high temperatures than most other species. For example, Fig. 5.14 illustrates the difference

between the survival of Tilapia zillii and Haplochromis spp. under experimental high temperatures. T.

zillii survive indefinitely at 38°C, a temperature at which half of the sample of Haplochromis would

die within two minutes.

There are indications that such thermal tolerance may be cyclic, at least in some species. Johnson

(1976) has shown that there is a daily rhythm of thermal tolerance in Gambusia affinis which rises

some 3 degrees between morning and mid-day and falls away again towards evening.

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Figure 5.13 A comparison of O2-Hb affinity for water breathing fish from lotic habitats in the

Amazon system. Thick solid lines enclose the range of blood equilibrium curves for species

from slow flowing zones; Thin solid lines those for species from rapidly flowing water;

Dashed lines those from rapid waters but which inhabit streams or river margins. (From

Powers et al., 1979)

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Figure 5.14 Survival of Haplochromis species and Tilapia zillii in high temperatures when

acclimatized to 27°C

ADAPTATIONS TO RESIST DESICCATION

Very few species are adapted to survive desiccation and annual losses of fish trapped in temporary

water bodies is enormous. Some species however survive the dry period by cocooning. The African

lung fishes Protopterus annectens and P. aethiopicus burrow into the bed of a drying pool and secrete

a cocoon of hard slime in which they rest, coiled, so the mouth is upwards and connected to an air

passage. Fish have been recorded as having survived over a year of aestivation. Some murrels

(Ophicephalidae) are reputed to survive short periods of drought in a similar manner as do the

synbranchid eels of the Amazon (Kramer et al., 1978). Dehadrai and Tripathi (in Hughes, 1976) also

report that Clarias and Heteropneustes take refuge in soft mud in drying pools, and Donnelly (1978)

summarizes reports from Africa of Oreochromis mossambicus and Clarias gariepinus survival in wet

sand. Bruton (1979), in his review of the literature on survival of habitat desiccation by air breathing

clariids, supports the idea that Clarias species can survive for some time in burrows in damp mud or

wet sand. This ability, however, does not seem to extend to survival under totally dry substrates as in

the case of Protopterus to which the construction of a cocoon is unique.

Several species of cyprinodont in Africa and South America can maintain permanent populations in

temporary aquatic habitats. These are annual fishes which complete their life cycles in as little time as

a few weeks and which have several adaptations including drought resistant eggs, embryonic

diapause, rapid hatching responses to rainfall all of which adapt them to withstand prolonged

desiccation (Simpson, 1979).

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The lowering of the water level in their environment, and possibly the associated physical and

chemical changes in water quality, appear to act as the stimulus for reproduction. The eggs are shed

on the bottom where they settle or are pushed into the mud by the parent fish which die shortly

afterwards. The eggs and growing embryos may stop their development for variable periods and such

arrests (diapauses) may occur at three stages, described by Wourms (1972) as Diapause I (dispersed

cell phase), Diapause II (long somite embryo) and Diapause III (pre-hatching). In some species the

arrest is facultative at Diapause I and II, but obligate at Diapause III. The different combinations of

Diapause can generate 8 different distributions of total development time as shown in Fig. 5.15. In

this way a single egg population can give rise to several sub-populations which allow for the repeated

loss of individual eggs under conditions which can start development but do not allow maturation and

successful reproduction. This “multiplier” effect guarantees that some portion of the egg stock will

survive to reproduce.

Figure 5.15 “Multiplier” effects of various combinations of diapause on egg stock of annual fish.

(After Wourms, 1972)

ADAPTATIONS TO POOR LIGHT

Levine et al. (1980) and Levine and MacNichol (1982) have drawn attention to the necessity for

fishes to have eyes which are fully adapted to the environments in which they live and to their own

particular position in that environment. The quality and quantity of light within river systems varies

considerably according to habitat. Thus in some mountain streams or clear water rivers the limpidity

of the water permits great light penetration and only a gradual modification of the spectrum. However,

more commonly, the highly turbid conditions of rivers and their floodplains leads to distinctly murky

conditions. Equally, the poor penetration and high spectral distortion of blackwaters requires

specialized visual adaptation. Differences in visual structures have been found among various species

from the Amazon which are directed towards life in the diverse habitats (Menezes et al., 1981).

Equally, specializations are found which correspond to the fish’s accustomed position in the water

column or to its preferred period of activity (Levine and MacNichol, 1982) which may be termed

“visual riches”. Some behavioural quirks may be explained in this manner although at present there is

164

insufficient evidence for or against such hypotheses. For instance, the migration of prochilodontid

characins from the turbid floodplains and white water rivers to the mouths of blackwater rivers for

breeding might be explained by differential visual requirements.

Many species either supplement, or completely supplant, vision with other organs, particularly in

those fishes which live at or near the bottom of the river. Thus barbels are found in most cyprinids and

siluroids, some of which are eyeless (e.g., the cetopsid catfishes). An alternative system is the

elaboration of electric organs to send impulses which presumably serve not only as communication

but also to define the environment. The gymnotids of South America and the mormyrids of Africa are

noted examples of this adaptation which in the mormyrids is coupled with greatly reduced eyes.

ADAPTATIONS TO RESIST STRONG CURRENT

Species of fish inhabiting the rocky riffles of the rhithron or the rapids reaches of larger rivers are

highly adapted to the turbulent conditions. There are three main groups of such fishes; (i) Those

which cling to the surface of vegetation and rocks, (ii) Those which take refuge from the current in the

crevices and holes between rocks and (iii) those which can swim sufficiently fast as to resist the

current.

The first group is particularly well represented by members of the family Amphiliidae in Africa

whose several genera are all adapted in various ways to life in strong currents. They are all elongated,

streamlined and with the slightly humped form that results in the fish being forced on to the bottom by

the flow. In addition the various genera possess a variety of suckers or enlarged fins with which they

cling to the substrate (Fig. 5.16a). For example, Amphilius spp. have sucker like mouths and stiff

pectoral spines. Doumea spp. have enlarged rigid pectoral spines and Phractura clauseni has been

observed to cling to the edges of leaves with suffered maxillary barbels. Similar structures are found

in the Mochokidae where Chiloglanis spp. have elaborate pectoral spines and in the Bagridae with

some Auchenoglanis spp. Sucker like mouths are also found among the Cyprinids in both Garra and

Labeo species. Numerous other families and genera have species with similar modifications

throughout the world - among which, the Gastromyzontidae, Cyprinidae, or Sisoriidae in Asia and the

Astroblepidae in South America.

Two particular adaptations fit fish for life in the interstices of rocks and anchored vegetation, sinuous,

serpentine form and small size (Fig. 5.16b). Both are found in rapids fauna where a number of genera

from various families, for instance the clariidae, with Gymnallabes and Clariallabes, the mormyridae

with Mormyrops, the mastacembelidae with Mastacembelus and the cichlids with Gobiocichla in

Africa. The cyprinids and the Gobiidae with a wide variety of species in Asia, Europe and N.

America, the Trichomycteridae in Latin America and the Galaxiidae in Australia have all developed

elongated or pygmy species.

165

166

Figure 5.16 Adoptions to swift current in some African rheophilic species.

(A) illustrates mouth suckers, stiffened barbels and stout pectoral spines;

(B) illustrates slim or serpentine form

CHAPTER 6

THE PRODUCTION BIOLOGY OF RIVER FISH

FEEDING

SOURCES OF FOOD

The richness and variety of riverine habitats provide a wide range of possible food organisms and

substrates. These originate either from within the aquatic system itself (autochthonous food sources)

or from outside the system (allochthonous food sources), although they are all ultimately dependent

on materials of external origin in the form of alluvial silt, dissolved nutrients, material washed into the

system with surface flow or decomposition products on inundated ground. These nutrients form the

basis for numerous sources of food as follows:

Autochthonous

Plankton community - phytoplankton

- zooplankton

- drift organisms

Benthic community - mud and associated microorganisms,

- coarse detritus, decomposing vegetable or animal remains

- insects and small crustacea

Plant community - plants including filamentous algae and submersed, floating or emergent

higher vegetation

Epilithic - Epiphytic - epiphytic or epilithic algae

community (“Aufwuchs”) - associated microorganisms, insects, crustacea, etc.

- this category can include the root flora and fauna of floating vegetation as

well as some detrital aggregate, that slimy coating found on submerged parts

of plants or rocks which consists of detritus, bacteria and algae.

Neuston community - surface living insects and larvae at the air/water interface

Fish - including eggs, larvae and juveniles

Other vertebrates - amphibia, reptiles, birds or small aquatic mammals

Allochthonous

Vegetable matter - leaves, roots, flowers, fruit and seeds of plants growing near the water or

overhanging the water course which contribute to the surface drift and to the

detritus

Animal matter - insects, arachnids, worms, etc., falling on to or washed into the water from

terrestrial environment

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In rivers primary production is located more in the proliferation of higher plants than in the

phytoplankton. Epiphytic or epilithic algae are abundant only at the fringes of the vegetation mass or

on rocks and other supports. Higher plants are themselves useful for food only when young and

tender, although fruit and seeds do figure in the diet of some species. The major contribution of higher

plants to the nutrient flow is by decay and the consequent enrichment of the detritus. Because

acceptable primary plant foods are not common in river floodplain habitats, purely herbivorous

species are relatively rare. Species that do eat higher plants or phytoplankton usually have an

alternative source of food. In the case of higher plant browsers such as Tilapia zillii, the fish often

have recourse to vegetable detritus for the consumption of which they are equally well-adapted. The

scarcity of phytoplankton in flowing water systems means that this source of food, so common in

lakes, is also of minor importance.

The general absence of primary feeders means that other types of food dominate in the diet of riverine

species. Four categories emerge as of particular importance depending on locality within the river

system. These are:

Benthos which is particularly important in the headwater streams or in rejuvenated reaches. Benthic

organisms are particularly abundant in the rocky and often torrential low order streams but decline in

abundance downstream until, in the mesopotamon, they form a relatively minor part of the diet. Thus

food supply in such reaches tends to alternate between a variety of forms living in the bottom, usually

in the interstices of the rocks, in the riffles, and a drift of such autochthonous benthic organisms and

of allochthonous material in the pools. The supply of microbenthos in these spaces between the rocks

favours small size thus the young of many species of fish pass the earlier stages of their development

within the riffles.

Mud and detritus: Bottom deposits really represent two rather different kinds of food. The detritus

feeders rely on coarser decomposing plant material together with associated micro organisms and

animal communities. These comprise a high proportion of species particularly in headwater stream

and forested habitats where leaf fall accumulates in the slack of the pools or close to floating

vegetation where litter is also abundant. The resulting coarse detritus tends to be a feature of low

order streams and it becomes finer with progress downstream until, in the potamon, it forms fine

organic mud.

Mud itself contains amino acids and other organic products of decay which can be used by fish in

combination with the saprophytic bacterial and protozoan microorganisms. Bakare (1970) has

analysed this element of the diet of Citharinus and Labeo in the Niger River. The finer the particle the

greater its alimentary value, and the preferred particle size was between 0.10 and 0.05 mm, although

grains as large as 0.18 mm were taken. The finer fractions contained relatively larger amounts of

carbon and nitrogen than did the larger particles. The size of particle and the food content of the

deposit, which the fish seemed able to detect, appeared to be the major factors limiting the distribution

of these species. At the time of sampling about 70 percent of the bottom deposits were suitable for

food. Bakare noted that bottom deposits became progressively depleted of C and N during the flood

when C. citharus was actively feeding. Periodic drying of the mud may recharge the organic content

through the incorporation of dung and other decaying animal and vegetable matter. Similarly studies

by Quiros et al. (1981) in the artificial lake of Salto Grande, a river type reservoir on the Uruguay

river, showed strong correlations between both the total organic matter and organic nitrogen

concentrations in different locations throughout the lake and the catch of such iliophagous and

detritophagous species as Prochilodus platensis, Pseudocurimata gilberti, P. nitens, Curimatorbis

planatus, as well as Plecostomus and Loricaria anus. Further evidence for the ability of certain

species of fish to feed on amino acids present in bottom mud was produced by Bowen (1980) who

found that the substances were readily absorbed along the intestine of Oreochromis mossambicus

living in Lake Valencia, Venezuela. Unusual gastric juices are required to liberate protein from this

form of detritus and O. mossambicus is recorded as having gastric acid at an uncommonly low pH

(<1.5) for fish (Bowen, 1981). Bowen (1979a) had earlier established that benthic detrital-aggregate

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contained rich organic residues (up to 45.7% carbohydrate and 1.8–14.2% protein) a large proportion

of which was in the form of non living amorphous material. Both Bakare and Sandon and Tayib

(1953) found a high proportion of mud feeders in the fish populations of the Niger and Nile rivers. In

the Niger 10 percent of the species feed exclusively on this source of food and 10 percent more

include it as a major element of the diet. The number of species, however, is little guide to the true

abundance of mud-eating fish. In the La Plata system, for instance, 60 percent of the ichthyomass of

the floodplain pools is located in the main mud-eating (iliophagous) species Prochilodus platensis

(Bonetto, Dioni and Pignalberi, 1969). Fish of the genus Prochilodus are widespread mud-eaters in

Latin America and are met in equal abundance in other systems such as the Mogi Guassu (Godoy,

1975) and the Magdalena (Kapetsky et al., 1976). Goulding (1980) also confirms that mud and

detritus eating fishes of the genus Prochilodus, Semaprochilodus and Curimatus account for a large

part of the biomass in the nutrient poor forested floodplains of the Amazonian rivers he studied.

Allochthonous material: Many workers here have remarked on the quantity of allochthonous

material consumed by fish in river systems. Not only is such external material one of the two major

food sources in headwater streams but on forested blackwater river floodplains the rain of animal and

vegetable matter from the overhanging vegetation is the only appreciable source of food and all food

chains start from it. Typical of the latter are the Amazon and Congo rivers from which Geisler et al.,

(1973), Roberts (1973) and Goulding (1980 and 1981) have noted that food from terrestrial sources is

particularly important. Similar observations have been made in the Mekong basin especially in the

flooded forest surrounding the Grand Lac. Most species in these habitats show great flexibility in the

type of allochthonous food taken, but some fish, such as the frugivores of tropical forest rivers have

specialized in using particular food items. Indeed the most exhaustive study carried out on such

species (Goulding, 1980) indicates that some Amazonian species of Characidae, Cynodontidae,

Anostomidae, Pimelodidae, Doradidae and Auchenipteridae specialize in fruit or seed eating in the

Amazon during high water to the extent that over 87% of the total food consumed by Colossoma,

Mylosoma, Myleus and Brycon in the wet season was fruit or seeds. The various species even show

preferences for particular types of fruit or seed correlated with their dentition. Most such species

either cease feeding during the dry season or turn to alternative food sources which in many cases

may consist of other allochthonous material such as leaves or flowers, but may as in the case of the

piranhas be of living animal origin. The frugivorous habit has been described, or hinted at, by workers

from other forested areas. Fruit, seeds and flowers form a component of the diet of Notopterus

notopterus, Paralaubuca typus and Clarias batrachus in the Mekong (Bardach, 1959); Distichodus

atroventralis and D. sexfasciatus of the Congo (Matthes, 1964); Leptobarbus melanotaenia, Puntius

bulu, P. binotatus, P. bramoides, P. sealei, Botrachocephalus mino and Chonerhinus modestus of

North Borneo (Inger and Chin, 1962); Leptobarbus hoeveni of Sumatra (Vaas et al., 1953). Tan

(1980) records Tor tambroides, Acrossocheilus hexagonolepis, Leptobarbus hoeveni, Puntus bulu and

P. daruphani as gathering around Ficus variegata, Eugenia sp., Diptocarpus oblongifolius,

Dypoxylon angustifolium and Elateriospermus tapus to eat the ripe fruit as it falls into the water.

Other species have become adapted to taking organisms from outside the aquatic system, the most

extreme example of this being found in the Archer fish (Toxotes) which shoots water droplets at

insects which settle on overhanging vegetation so as to knock them into the water. Fittkau (1973) has

illustrated the type of simplified nutrient-food-consumer cycles that are found in the mouth lakes of

the Amazonian tributaries (Fig. 6.1). The role of food coming from outside the aquatic system is not

confined to forested rivers, as some species inhabiting savannah plains also rely heavily on this source

(Kelley in FAO/UN, 1968a). Adaptations to this are such that Brycinus spp. have been recorded as

deliberately jumping against the stems of rice plants to bring down seeds for consumption (Matthes,

1977).

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Figure 6.1 The nutrient cycle in the mouth lake of an Amazonian tributary. (After Fittkau, 1973)

Predation on fish and large invertebrates by other fish is relatively unimportant in small streams.

Indeed the smaller and more unstable a water course, the less likelihood that predation will

significantly affect its population structure. (Moyle and Li, 1979). In some small coldwater streams

more stable conditions allow predation by fish such as trout to play a more significant role. Trout for

instance accounted for 15% of the total mortality of other fish in the water course studies by

Alexander (1979). The relative importance of predatory species tends to increase downstream until

the point where Bayley (1983) found piscivorous predation to account for most of the productivity of

small and medium sized fishes in the forested Amazon. On savannah floodplains there is a tendency

for bottom-feeding forms to dominate by weight, although there are usually only a few species at this

trophic level. Furthermore in some systems decapod Crustacea perform the primary scavenging role

and fish are confined to the higher links of the food chain. In the Potamon community structures

usually contain a very high proportion of predatory species and piscivorous predators are generally

very common. The relative abundance of these elements of the community tends to increase during

the dry season giving the very high predator-forage fish ratio recorded by some authors. For example,

Mago-Leccia (1970) noted that up to 75 percent of the population of some floodplain pools of the

Orinoco consisted of fish eaters such as Hoplias malabaricus. Lowe-McConnell (1964) and Bonetto,

Dioni and Pignalberi (1969) equally commented on the abundance of piscivores and the absence of

small fish in flood pools of the Rupununi and Parana rivers respectively at the end of the dry period. I

have observed the same phenomenon in the Ouémé River and a number of other workers have

commented on it in passing with reference to other African systems. Strangely, prolonged and

stabilized high water in the normally fluctuating Everglade marshes also produced an increase in

predatory species according to Kushlan (1976). This change was due to the migration into the marsh

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of large predatory species which are normally intolerant of the extreme swamp conditions. Similar

shifts may be anticipated where floodplains are impounded to increase the inundation time.

Lowe-McConnell (1975) concluded that there is a linear succession of dominant food sources in

streams and rivers. Fishes in headwater streams depend mostly on allochthonous foods. As the stream

enlarges grazers and generalized predators feeding on benthic invertebrates become more important.

Finally in the lower reaches, the accumulation of detritus and soft mud supports a number of mud-

eating species although piscivorous predators are also abundant. Nikolsky (1937) described this same

succession from the Amu Darya and Syr Darya and correlated with it the increase in length and

complexity of the guts of fish as one proceeds downstream in these rivers. This progression of feeding

types is in accordance with the predictions of the river continuum concept and also agrees with similar

progressions in other organisms.

SPECIALIZATION AND RESOURCE PARTITIONING Specialization

Feeding habits of individual fish species in rivers have been described by many authors. These have

generally been analysed as trophic “statics” which, by approaching diet in a manner limited in space

or time, tends to assign fish to trophic categories or niches. This has led to the assumption that the

considerable specializations of dentition, jaw structure, body form and alimentary tract, corresponds

to real feeding niches particular to the species. Closer examination of this assumption shows the

situation to be considerably more complex. In stable systems such as reservoir rivers it may be

supposed that specialization leads to the species adopting a fixed position in the community whereas

in fluctuating systems typified by flood rivers specializations may be more valuable in one or other of

the phases of the system. There has been some debate as to whether the advantage accrues during

times of abundance (usually high water) when adequate food of diverse nature is present or at times of

scarcity when specialization would sharpen the competitive capacity for those resources which are

available. In support of the latter hypothesis Zaret and Rand (1971) found trophic overlaps in a

Panamanian stream to be minimal during the dry season, when food abundance was at its lowest. For

example, Astyanax, which normally occupies the surface during the nutrient rich wet season, is forced

into the mid waters by the more specialized Gephyrocharax during the dry season. Here access to its

usual diet is prevented by Roeboides and as a consequence switches from its normal concentration on

allochthonous insects to a purely vegetarian diet. In more temperate waters, Angermeier (1982) also

found an increase in the variety of prey selected by nine species of United States stream fish during

times of scarcity whilst the same species all exploited virtually the same food resources in times of

abundance. Greater specialization has been found during the high water period by several other

authors. Matthes (1964) found greater degrees of specialization during the floods of the Congo River.

Lowe-McConnell (1964 and 1967) also found high water to be the period when fishes are most

segregated trophically in the Rupununi. This has also been illustrated for the frugivorous fishes of the

Amazon by Goulding (1980) where the various species concentrate on particular types of fruit and

seeds during the floods but are less selective during the dry season. In direct contrast to Zaret and

Rand's (1971) findings, Power (1984) established that species in a Panamanian stream concentrate on

the major substrates when food was abundant, but diversified their choice as food became scarce.

The habit of switching diet seasonally, noted by most of these authors, has also been commented on

by others. For example Cabrera et al., (1973) found that the diet of Basilichthys bonariensis in the

Plata system could be separated into three main components: constant elements (algae, mud,

vegetable remains); seasonal elements (cladocerans, copepods, diatoms, malacostraca, gasteropods

and fish all of which appeared mainly in the flood); occasional elements (rotifers and ostracods). This

pattern of feeding, where a basic major trophic category is at the same time flexible enough to take

advantage of other food items as or when they are available seems very common. Even such species

as piranha (Serrasalmus spp.) which are noted for their predatorial ferocity can switch to an

alternative diet of vegetable detritus or mud during the dry season (Mago-Leccia, 1970; Goulding,

1980). Some such species are clearly examples of an adaptation to one food type serving equally for

171

an alternative food, as in the case of mud-eating microphages such as Heterotis, Oreochromis

niloticus or Alestes spp which may adopt a planktonophage habit in lentic waters. Many species select

a succession of food types as the flood season progresses. Typical of such are the Brycinus species

studied by Daget (1952) that changed from a diet of insects and seeds or even higher vegetation

during the rising waters to feeding on phytoplankton as the waters begin to contract. Similarly,

Colossoma bidens or C. macropomum concentrate of allochthonous fruits during the flood in the

Amazon switch to autochthonous planktonic Crustacea at low waters (Honda, 1974). In the savannah

floodplain of the Apure River the same species shows a much greater range of diet including fish,

birds and other animal matter in the range of items eaten.

There is also a tendency for food preferences to change as individuals grow older, and the diet of

juvenile fish often differs widely from that of adults of the same species. Young Prochilodus

platensis, for example, feed on planktonic diatoms and crustacea, whereas the adults are uniquely mud

eaters (Vidal, 1967). Most extreme in this respect are the juveniles of the major piscivorous predators,

such as Lates niloticus or Hydrocynus which eat small Crustacea or even phytoplankton. Plankton

appears to play a very important role in the diet of the youngest fish and many of the movements

described under migration appear designed to bring the fry into contact with this food source.

That fish can respond to the concentration of their preferred food is illustrated by Quiros' findings

with iliophagous fishes in the Parana system. Power (1984) also found that concentrations of

Ancistrus spinosus and other loricariid catfishes in Panamanian streams corresponded to the

productivity of the algae upon which they graze. Here there were 6–7 times more fish in sunlit areas

of stream where algae productivities were up to 7 times faster than in shaded areas. Algal growth is

fastest in shallow waters (20 cm) but fish did not enter these waters because of risks of predators.

However, in other streams in Panama, Angermeier and Karr (1983) found that the distribution of

feeding guilds by biomass was not generally correlated with the availability of their major food

resources.

Resource Partitioning

Knoppel (1970), in his study of the nutrient ecology of 49 species from streams and lakes of the

Amazonian terra firma and floodplain was forced to conclude that there were no specialists in these

habitats thus the resource was virtually unpartitioned. Knoppel's study, however, was limited to a

small forested stream where all species concentrated on allochthonous items as the most abundant

food source and was also limited in season. In other systems it is generally both convenient and

possible to classify fish into broad categories or trophic guilds according to their predominant feeding

habits. For example, Matthes (1964) distinguished the following categories in the Congo River basin:

Mud feeders, which eat finely divided silt together with the microorganisms and organic decay

products it contains. In the floodplain pools this niche is filled by Phractolaemus ansorgii.

Detritus feeders, which ingest mainly vegetable debris, leaf litter and the associated animal

communities: e.g., Stomatorhinus humilior, Clarias buthupogon, Clariallabes variabilis, C.

brevibarbis, C. melas, Channallabes apus.

Omnivores, which are widely represented by all families and most genera in the floodplain water

bodies by Stomatorhinus fuliginosus and Ctenopoma fasciolatum.

Herbivores, which can be further separated into:

(i) microherbivores which eat algae and diatoms

(ii) macroherbivores which eat higher plants

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These were unrepresented in the floodplain pools, but in adjacent floating prairies Neolebias gracilis,

Distichodus affinis and Synodontis nummifer ate this type of food.

Plankton feeders, which are rare due to the lack of plankton in the riverine environment, but which

are nevertheless represented by Aplocheilichthys myersi in the floating vegetation.

Carnivores, the most important group which subdivide into:

(i) Meso-predators which feed mostly on insects and crustacea, and which are either

(a) feeders on allochthonous matter or neuston, such as Pantodon buchholzi, Ctenopoma

nigropannosum or Ctenopoma ansorgei in the pools

(b) bottom feeders which eat insects and molluscs from the bottom, such as Polypterus

retropinnis, Stomatorhinus polli, Clarias submarginatus, Kribia nana

(c) carnivorous browsers which inhabit floating vegetation and feed on the small insects and

Crustacea found there, for example Xenomystus nigri, Nannocharax schoutedeni, Hemistichodus

mesmaekersi, Heterochromis multidens or Ctenopoma kingsleyae

(ii) Macro-predators

(a) generalized predators which feed on fish or larger invertebrates such as decapod Crustacea or

insect larvae. In floodplain pools Clarias platycephalus feeds in this manner.

(b) piscivorous predators which feed only on fish. Although Matthes did not record any such

species in the floodplain pools, fish such as Parachanna obscurus, Hydrocynus vittatus or Lates

niloticus have been recorded from such habitats elsewhere.

(c) fin nippers which are representative of specialized predators generally.

Similar tabulations have been presented by other authors, for example Marlier (1967) who

investigated the feeding habits of fish in the Lago Redondo of the Amazonian Varzea (Table 6.1) and

Vaas (1953) who classified the fish fauna of the Kapuas river, West Borneo (Table 6.2)

The trophic relationships of river and floodplain communities can be summarized as a generalized

food web of the type shown in Fig. 6.2. Not all elements of this diagram are necessarily present in all

environments. As we have seen the heavy bias towards allochthonous food in the forest environment

and low order streams favours sequences following from this source of nutrition and diminishes the

importance of phytoplankton.

Table 6.1

Fish fauna of the Lago Redondo classified according to their feeding habits (after

Marlier, 1967)

Unspecialized STENOPHAGES Specialized

Carnivores

Serrasalmus nattereri Piscivores Arapaima gigas

Serrasalmus elongatus Boulangerella cuvieri

Eigenmannia virescens Ageneiosus ucayalensis

Pimelodella cristata Symbranchus marmoratus

Plagioscion squamosissimus Cichla ocellaris

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Geophagus surinamensis Insectivores Triportheus elongatus

Apistogramma taeniatum Oxydoras niger

Colomesus psittacus

(=asellus) Zooplankton-feeders Metynnis hypsauchen

Astyanax fasciatus

Hypophthalmus edentatus

Herbivores

Anodus laticeps Grass seeds Ctenobrycon

hauxvellianus

Cichlasoma bimaculatus Water grasses Metynnis maculatus

Cichlasoma festivum Leporinus maculatus

Fruit Colossoma bidens

Algae and Aufwuchs Poecilobrycon trifasciatus

Poecilobrycon

unifasciatus

Mud-eaters

Curimatus spp.

Prochilodus sp.

Pterygoplichthys multiradiatus

Potamorhina pristigaster

EURYPHAGES

Predominantly carnivores

Osteoglossum bicirrhosum

Serrasalmus rhombeus

Predominantly herbivores

Phytoplankton, zooplankton Anchoviella brevirostris

Grass leaves and seeds, insects Pyrrhulina brevis

Hyphessobrycon rosaceus

Hyphessobrycon callistus

Benthic and epiphytic diatoms and cladocera Hyphessobrycon sp.

Cheirodon piaba

Algae, grass and cladocera Metynnis lippincottianus

Aufwuchs Corydoras sp.

Seeds, molluscs Acarichthys heckeli

Littoral zooplankton and higher plants Pterophyllum scalare

Table 6.2

Fish fauna of the Kapuas river and adjacent lakes and river arms, classified according to their feeding

habits (after Vaas, 1953)

a

b

c

d

e

f

g

h

i

j

k

l

Plankton feeders

Helostona temmincki xx x

Thynnichthys thynnoides xx xx x x

Thynnichthys polylepis xx x x x

Dangila ocellata xx x x x

Dangila festiva xx x x x

Periphyton and vegetable feeders

Amblyrynchichthys truncatus x xx x x

Osteochilus melanopleura x xx x x xx

Osteochilus brevicaudata x xx x x xx

Osteochilus waandersi x xx x x xx

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Osteochilus vittatus x xx x x xx

Vegetable feeders on submerged higher plants, inundated land

plants, fruits and seeds

Puntius waandersi x xx x

Puntius nini x x x xx x x x

Puntius bulu x xx x

Puntius schwanefeldi x xx x

Leptobarbus hoeveni x xx

Leptobarbus melanotaenia x xx

Pristolepis fasciatus x xx

Osphromenus gourami x x xx x

Omnivores feeding mainly on insects and larvae, zooplankton

Balantiocheilus melanopterus x xx x

Cyclochilus repasson x xx

Luciosoma trinema x x x xx

Rasbora argyrotaenia x x x xx

Rasbora vaillanti x x x xx x x

Eaters of insects at surface

Chela oxygastroides x xx x x

Toxotes chatareus x x x xx x

Omnivorous bottom feeders

Barynotus microlepis x xx x

Pangasius pangasius x x xx

Pangasius polyuranodon x x xx

Mastacembelus armatus fayus x x xx

Mastacembelus argus x x xx

Omnivorous predators

Macrones nigriceps x xx xx x

Macrones nemurus x xx xx x

Hemisilurus chaperi x xx

Hemisilurus scleronema x xx

Predators on small fish and small animals, insects, shrimps

Lycothrissa crocodilus x x xx

Kryptopterus cryptopterus x x xx

Kryptooterus schilbeides x x xx

Kryptopterus limpok x x xx

Kryptopterus micronema x x xx

Macrochirichthys macrochirus x xx xx

Setipinna melanochir x xx

Datnioides microlepis xx xx

Hampala bimaculata x x xx

Large predators eating fish of all sizes, shrimps, prawns and

crabs

Ophicephalus striatus x xx

Ophicephalus micropeltes x xx

Ophicephalus pleurophthalmus x xx

Ophicephalus lucius x xx

Notopterus chitala x xx

Wallago leeri x xx

Silurodes hypopthalmus x xx

Note:

x = additional food;

xx = main food

a Phytoplankton b Periphyton c Filamentous algae d Bottom algae

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e Submerged plants inundated land plants, fruite seeds f Small zooplankton g Cladocera, copepods and rotifera h Insects and their larvae i Allochthonous insects j Shrimps k Insect larvae in the bottom, worms l Fish, prawns and crabs

When feeding habits are matched with habitats some complex relationships emerge, as is shown by

Matthes (1964) for Lake Tumba and the adjacent forested floodplains of the Ikela region. Table 6.3

illustrates this for the Lower Ouémé river and floodplain during the dry season, where one of the

major primary feeders on detritus are the decapod crustacea Macrobrachium macrobrachion and

Caridina sp. which, because of their size and abundance, are more closely allied to the fish

ecologically than they are to the other invertebrate fauna.

Fish in rivers, however, appear to be highly facultative in their feeding and with few exceptions may

move within the guild structure according to the species composition of the fish community, the time

of year and shifts in the non-biotic components of the ecosystem. Thus the assignment of species to

particular niches may be inappropriate. Indeed evidence from a variety of systems indicates that i) the

same food resource may be shared by numerous different species and ii) the same species may

successively exploit several different resources during the year. The long term persistence of fish

faunas consisting of many tens of species the generalized feeding patterns of the fish, the temporal

succession of dietry items and the consequent overlaps of trophic habit cause conceptual problems in

that some communities seem to violate the Gaussian principle of competitive exclusion. Alternative

interpretations of the trophic niche have therefore been sought. In stable systems such as reservoir

rivers, it is to be supposed that the range of specialization among the fish community leads to a more

or less defined partitioning of the available resources among the various species present and the

assignment of the species to more or less fixed niches. The available evidence would support this

view, as most observations of systems where more or less stable partitioning of the resource, whether

food or space, occurs, are from smaller streams and rivers with more or less stable flow regimes.

Despite the apparently greater stability of such systems there are still reasons to suppose that

considerable flexibility in feeding and spatial niche selection exists. Detailed studies of the cyprinid

communities of small relatively stable streams of Sri Lanka show that even within one family in

similar systems different levels of partitioning may be found. For example Puntius bimaculatus and P.

titteya co-occur but do not overlap in trophic niches (De Silva et al., 1977). Three species in the same

streams show considerable overlap in the choice of food items but avoid direct competition by

different preferred living spaces (De Silva and Kortmulder, 1977) and a further four species overlap in

diet and living space thereby competing directly but apparently co existing without problems (De

Silva et al., 1980). Where close apparent competition exists, investigations by Werner and Hall (1979)

indicate that a species may switch trophic (and spatial) habitats within a range of acceptable or

accessible food items depending on the relative profitability of the individual item. Thus a preferred

item under one community structure may be selected against if a more efficient competitor alters the

profitability balance. Such switches may occur from season to season as in the case of the Astyanax in

Panama (Zaret and Rand, 1971) or from year to year. Year to year changes in the trophic structure of a

community would then depend on the relative abundance of its component species which in turn is

determined by factors other than food availability. Here Schlosser (1982) considered that changes in

temporal reproductive success were more important than competitive exclusion or predation in

determining community organisation. Grossman et al. (1982) reached the similar conclusion that

“random” factors (e.g. environmental variables) rather than deterministic ones were responsible for

the lack of repeatability of community structures in the same Indiana stream over a twelve year

period. The term “Condominium” first suggested by Wynne-Edwards (1962) to describe associations

of nearly related species of similar habitats that can be united ecologically and are not in competition

for resources within the association could well be extended to include these grouping of river fish

176

species which coexist over long periods showing similar feeding, breeding and spatial distribution

patterns. The flexibility in trophic organization among stream fish communities implies that inter-

specific competition is not a major factor in regulating biomass and community structure. This

conclusion is to some extent supported by Bayley (1983) in his observations on Amazonian fishes.

Figure 6.2 Diagram of trophic relationships in a river-floodplain community. Broken line = influence;

Solid line = feeding interaction

Table 6.3

Main dry season habitats of fish species in the Ouémé River ordered by major trophic categories

Main river channel bottom Floodplain pools and lagoons

Trophic

category Surface Mud Sand

Bank

vegetation Surface Bottom

Vegetated

(swampy)

area

Mud and

detritus

feeders

Heterotis

niloticus

Synodontis

schall

Clarias

ebriensis

Heterobranchu

s longifilis

Clarias

ebriensis

177

Citharinus

latus

Labeo

senegalensis

Heterobranchu

s longifilis

Heterotis

niloticus

Neolebias

unifasciatus

Labeo

ogunensis

Auchenoglanis

occidentalis

Synodontis

schall

Phractolaemus

ansorgii

Citharinus

latus

Synodontis

schall

Herbivores

micro

Labeo

senegalensis

Synodontis

nigrita(Juv)

Oreochromi

s galileus

macro Distichodus

rostratus

Distichodus

rostratus

Tilapia

guineensis

Tilapia

guineensis

Zooplankton Pellonula

afzeliusi

Allochthonou

s and neston

feeders

Brycinus

longipinnis

Brycinus

macrolepidotus

Epiplatys

bifasciatus

Epiplatys

sexfasciatus

Omnivores Brycinus

nurse

Marcusenius

brucii

Clarias lazera Synodontis

nigrita

Clarias

lazera

Chrysichthys

auratus

Protopterus

annectens

Chrysichthys

walkeri

Synodontis

melanopteru

s

Synodontis

nigrita

Micropredator

s

Synodontis

sorex

Synodontis

sorex

Chromidotilapi

a guntheri

Chromidotilapi

a guentheri

Ctenopoma

kingslayae

Physailia

pellucida

Petrocephalu

s bane

Brienomyrus

brachyistius

Thysia ansorgii

Barbus

Hyperopisus

occidentalis

Petrocephalu

s bovei

Hemichromis

bimaculatus

callipterus

Pollimyrus

Mormyrus

rume

Cyphamyrus

psittacus

adspersus

Brienomyrus

niger

Eutropiellus

buffei

Pollimyrus

petricolus

Pollimyrus

adspersus

Generalized

predators

Schilbe

mystus

Chrysichthys

nigrodigitatu

s

Protopterus

annectens

Malapterurus

electricus

Calamoichthy

s calabaricus

Eutropius

niloticus

Calaraoichthys

calabaricus

Piscivores Hydrocynu

s forskahlii

Bagrus

docmac

Hemichromis

fasciatus

Hepsetus

odoe

Parachanna

africanus

Hemichromis

fasciatus

Lates

niloticus

Polypterus

senegalus

Polypterus

senegalus

Hydrocynu

s vittatus

Gymnarchus

niloticus

Gymnarchus

niloticus

Hepsetus odoe Parachanna

obscurus

178

Parachanna

obscurus

SEASONALITY OF FEEDING

In temperate rivers and streams the onset of winter usually marks a drop in overall productivity of

aquatic system and in the production of food organisms. Furthermore, the amount of food eaten by

fishes is closely related to temperature thus a general cessation of feeding occurs in most temperate

and arctic species during the winter months. In tropical waters the effects of temperature are clearly

less pronounced but since Chevey and Le Poulain (1940) remarked on the fact that fish did not feed in

the Mekong system during the dry season it has become generally accepted that feeding by fish in

tropical rivers is likewise highly seasonal all over the world. In flood rivers the feeding cycle is

clearly linked to two factors, firstly the food supply and secondly the population density. During the

flood the rapid increase in food organisms, together with the wide dispersal of fish over an extensive

biotope, favours intensive feeding. At low water, when the aquatic environment is contracted the fish

are concentrated in a few permanent reserves of water and food sources are limited or exhausted,

fasting therefore ensues. In the tropics this contrasts with the more or less continuous feeding of fish

in lakes; although in some species inhabiting rivers closely allied to lakes such as the Lake

Chad/Yaeres system the fish cease feeding at low water despite the adequate supply of food which

would enable them to continue feeding at all times of year. In reaches of Indian rivers having little or

no floodplain the seasonality of feeding may be reversed with more intense food intake during the dry

season. Bhatnagar and Karamchandani (1970) attributed this to the food being washed away by the

high current during the flood in the case of Labeo fimbriatus. Tor tor showed a similar pattern to L.

fimbriatus although in this case Desai (1970) correlated the lessened feeding with breeding. It would

seem that feeding stops just before and during breeding in flood and reservoir rivers alike. There are

nevertheless seasonal differences in the availability of food which depend on the morphology of the

river.

The intensive feeding by fish during the periods of abundance permits them to build up large stores of

fat which are sufficient, not only to tide the animals through the following barren winter or dry

season, but to elaborate gonadial tissue in preparation for breeding. Starvation during the winter or dry

season causes fish to lose condition. Daget (1956) for example traced this for Tilapia zillii in the

Niger. Fig.6.3 shows the variation in weight over the dry season using condition factor K (where K =

weight in gms x 105 over total length in mm³) as an index of change. In the river relatively little

change occurred over the dry season until the start of reproduction when there was a sudden reduction

in weight corresponding to about 10.7 percent for the whole period. In a floodplain pool fish ended

the flood in better condition but lost weight more evenly throughout the dry season: in October and

November - fish were fat and full of food (K = 4.52); in December - feeding was reduced (K = 4.68);

January - stomachs empty (K = 4.67); February - very little food (K = 4.39); March - even less food

(K = 4.48); April - only mud (K = 4.34); May (K = 4.16); June (K = 4.2). The net loss in weight over

five months was 11 percent. In a second, somewhat richer, pool the loss in weight was less rapid

(dashed line). Daget (1952) had previously noted similar seasonal changes in weight with Brycinus. In

the Amazon, Junk (in press) observed seasonal changes in the fat content of 40 species. The majority

of these showed a pronounced seasonality in chemical composition with peaks in fat content during

the falling flood and minimum fat content during the breeding season at the end of low water.

The pattern of abundant feeding during the flood and fasting during low water is, perhaps, not as

simple as it appears. Observations by Willoughby and Tweddle (1978) indicated that peak feeding

takes place at different times in different species (Fig. 6.4). The food consumption of Clarias

gariepinus in the Shire system, for instance, reached its maximum just before the flood peak, whereas

Oreochromis mossambicus fed more intensively as the floodplains were draining. A third species,

Clarias ngamensis, fed at a fairly constant rate for most of the year. In all three species food intake

was minimal at low water. There are also indications that certain categories of feeders continue to

feed throughout the dry season. Microvores such as Heterotis niloticus may feed throughout the year,

179

although only at maintenance level during the dry season (Daget, 1957a). Surface feeders also have a

continuing food source and Brycinus macrolepidotus continues to feed long after other species of

Alestes and Brycinus in the Niger. The change in predator-forage fish ratio throughout the dry season,

and the gradual disappearance of smaller fishes from floodplain pools, as noted by Lowe-McConnell

(1964) would suggest that predators may continue feeding well into the dry season. Goulding (1980)

also found that, whereas the majority of frugivorous Amazonian fish greatly reduced their feeding

during the low water seasons, the predatory characins were less inclined to do so in that roughly equal

proportions of such fish contained food in the dry and wet seasons. However, the nature of the food

changed from primarily plant foods (seeds and fruit) in the wet season to fish scales in the dry.

Neverthless, predators appear to stop feeding in other systems and Mago-Leccia (1970), in noting that

Piranhas turn to mud as a feeding substrate in the dry season, also remarked that small fish are not

eaten by predators at low water. Such continued feeding does appear to be somewhat exceptional and

most observers confirm the dry season fast.

180

Figure 6.3 Changes in condition factor between October and July for adult Tilapia zillii in: (A) the

Niger River, and (B) two floodplain pools (after Daget, 1956). Also shown are (C) changes in

condition factor of Brycinus leuciscus (after Daget, 1957a)

181

Figure 6.4 Seasonal variations in daily food intake by three species of fish from the Shire River.

(After Willoughby and Tweddle, 1978)

182

GROWTH

FACTORS AFFECTING GROWTH

Fish from most river systems show well-defined rings or annuli on their scales, bones or otoliths, a

fact which has been noted from tropical and subtropical systems as well as from temperate ones. The

rings of temperate salmonid and coarse fish species have been widely described. Chevey and Le

Poulain (1940) noted rings on the scales of cyprinid species in the Mekong, and rings have been

described from other cyprinids such as Labeo spp. in both the Gambia (Johnels, 1954) and Indian

rivers (Khan and Jhingran, 1975), or Catla catla from the Ganges (Natarajan and Jhingran, 1963).

Numerous characin species have been studied, Prochilodus scrofa (Godoy, 1975), P. platensis is

(Cabrera and Candia, 1964; Vidal, 1967) in South America, and Brycinus leuciscus (Daget, 1952) and

Alestes baremoze (Durand and Loubens, 1969) in Africa. Cichlids from both Africa (Oreochromis

spp., Dudley, 1972) and Latin American rivers (Cichlasoma bimaculatum, Lowe-McConnell, 1964)

have been recorded with annuli on their scales. Johnels (1954) also mentions rings on the scales of

Notopterus sp. and mormyrids. Other hard parts of the fish also show qrowth rings. Opercular bones

were used by Cordiviola (1971) for ageing Prochilodus platensis. The scaleless siluroids (Candia et

al., 1973) have shown clear rings in the otoliths and pectoral spines of Parapimelodus valenciennesi

and Fenerich et al., (1975) have demonstrated their existence in the otoliths of Pimelodus maculatus.

Pectoral spines and vertebrae were used to age Clarias spp. from the Kafue flats by the University of

Idaho et al. (1971). Other growth studies, such as those by Bayley (1983) for 12 species of the

Amazon system, have been based on length frequency analyses without recourse to structures.

The marks or rings have been correlated with the partial or complete cessation of growth during one

or more periods of the year. In the temperate zone these are clearly associated with the winter

cessation of growth. However, in the tropics more rings have often been recorded than would be

expected if ring formation depended solely on a regular seasonal event. Care therefore has to be taken

in interpreting rings in scales or other hard structures as indicators of age or time series. Nevertheless

at low water feeding either stops completely or is seriously reduced in most species. The fish live on

their fat reserves, sometimes losing condition to the point where resorption occurs at the margin of the

scales. Durand and Loubens (1969) made a useful distinction between growth in weight and growth in

length. The latter is a good indicator of long-term change, but as it depends mainly on skeletal

structures it is not so liable to modification during the growth arrest. Growth in weight is as much

through the addition of soft tissues including fat. These fat stores are liable to be rapidly modified

under adverse conditions, as has been shown for Tilapia zillii and Brycinus leuciscus (Fig. 6.3), and as

Durand and Loubens (1970) showed for Alestes baremoze, where the condition factor (K) fell from

1.30 in April to 1.00 in September. Of course care has to be taken in the interpretation of changes in

weight and condition factor, as the development of gonadial tissue and discharge of eggs or milt is

also reflected in these parameters. However, changes in K are more often slow over the whole dry

period, than abrupt at the time of breeding, as would be the case if the discharge of reproductive

products were the sole cause.

Three main reasons have been advanced for the arrest of growth during several months of the year.

(i) temperature

(ii) effects associated with drawdown; and

(iii) reproduction

In the temperate zone, temperature is clearly the dominant feature and ring formation is correlated

with winter but the growth rate arrest coincides with a drop in temperature. In several tropical river

systems too, for instance, in the Lake Chad basin, winter temperatures drop at least 8°C below the

summer maxima (Durand and Loubens, 1969) and coincide with the minimum growth rate. Similarly

in the Senegal, Reizer (1974) considered the slower growth from December to February to be

correlated with lower temperatures, and in both the lower La Plata system and on the Kafue flats, the

183

minimum rate of growth occurs at the same time as similar drops in temperatures. In most of these

rivers, however, low water coincides with the winter and it is difficult to distinguish the effects of the

two factors. In the southern Okavango swamps the floods arrive during the colder part of the year

(Fox, 1976) which leads to very low growth rates of the fish living there. It is not yet clear when the

growth arrest occurs in these waters, although current work may shed some light on this.

In equatorial rivers growth checks occur regularly where there are only minimal changes in

temperature. In such circumstances Lowe-McConnell (1964) has suggested that crowding and

lessened availability of food, brought about by drawdown conditions, are responsible. That this too

cannot be the whole answer is shown by the Lake Chad fishes which stop growing in the lake even

though there is abundant food. In the Niger river (Daget, 1957) reported that growth arrests can be

distinguished on the scales of carnivores, herbivores, limnivores, insectivores and plankton-eating

species, despite the fact that the predators and limnivores at least have sufficient food. Furthermore,

some fish such as the young of Oreochromis and Tilapia in the Kafue, resume feeding before the

onset of the floods, at a time when conditions are at their most cramped (Dudley, 1974).

The effects of population density on growth rate are somewhat problematic. In the Danube,

Chitravadivelu (1974) was unable to detect changes in the growth rate of Alburnus alburnus and

Rutilus rutilus, despite great differences in biomass and population density from one year to another.

However, Frank (1959) did record increases in growth in Rutilus rutilus and Abramis brama when the

population decreased from 69 124 i/ha to 19 394 i/ha in an Elbe oxbow. This he traced to the greater

availability of planktonic food following the decline in competition for this food source. Clear

relationships between population density and the growth rate and the total production of bullhead

(Cottus gobio) have also been demonstrated by Edwards and Brooker (1982), (Fig. 6.5) from

tributaries of the river Wye. Similar experiments with brown trout show a pronounced drop in

individual growth rate as the number of fish present per unit area increases (Backiel and Le Cren,

1978). Such conflicting results indicate a need for further investigation into the question of the

relationship of food supply to population density, as this influences the amount of fish available to the

fishery.

184

Figure 6.5 Relationships between density and (A) growth, and (B) production in bullhead (Cottus

gobio) in tributaries of the Upper Wye. (From Edwards and Brooker, 1982)

The elaboration of gonadial products during the fasting period probably accelerates the depletion of

fat reserves and exaggerates the low physical condition which is reflected as an annulus. It is,

however, doubtful whether this is the prime reason for growth checks, as maturation is frequently

preceded by a long period of negligible feeding in many species. Furthermore, rings are laid down by

immature fish as well as adults. That there may be deep physiological rhythms which dictate the

seasonal cessation of growth is suggested by an isolated experiment quoted by Johnels (1954). Here

some Barbus gambiensis, which had been transported to Sweden and were being maintained in the

even conditions of an aquarium, still stopped growing and laid down scale rings at precisely those

times when their congeners did so in the Gambia river.

MODELS OF GROWTH

Annuli on scales and other hard parts have been used to calculate growth in many species.

Supplementary information and independent estimates of growth have also been made from the

analysis of length frequency distributions for the progression of individual age groups and also from

the growth of tagged fish. Comparisons between the various methods of age determination show that

they give good agreement at least in sane species (Rao and Rao, 1972; Gupta and Jhingran, 1973).

Several workers have used the Von Bertalanffy model of growth:

185

in which length at time t + 1(Lt+1) is a function of length at time t (Lt) according to the Ford-Walford

equation:

where K is the coefficient of growth and Lo the theoretical asymptotic length achievable by the

species if it grows for an infinite period of time.

Most species seem to conform well to this model in respect of growth in length, or when subject to an

appropriate conversion factor, in respect of growth in weight, as is shown by examples in Table 6.4.

Table 6.4

Representative Von Bertalanffy relationships for growth in length of fish from some tropical river

systems

Species Sex Growth equation Author Alestes baremoze ♂ Lt=237.8[1-e-0.8163(t-0.57)] Durand and Loubens, 1969

♀ Lt=267[1-e)-0.7172(t-0.52)] Durand and Loubens, 1969

Catla catla ♂ + ♀ Lt=1275[1-e)-0.28(t-0.11)] Natarajan and Jhingran,1975

Labeo rohita ♂ + ♀ Lt=1015[1-e)-0.276(t-0.333)] Khan and Jhingran, 1975

Labeo calhasu (Ganga river) ♂ + ♀ Lt=1028[1-e)-0.15(t-0.19)] Gupta and Jhingran, 1973

(Godavari river) ♂ + ♀ Lt=944[1-e)-0.14(t+0.86)] Rao and Rao, 1972

Parapimelodus valenciennesi ♂ + ♀ Lt=333[1-e)-0.14(t-2.4)] Candia et al., 1973

Pimelodus maculatus ♂ Lt=45.4[1-e)-0.2104(t-0.61)] Fenerich et al., 1975

♀ Lt=56.5[1-e)-0.1938(t+0.36)] Fenerich et al., 1975

Prochilodus reticulatus ♂ + ♀ Lt=41.0[1-e-0.20(t-0.35)] Espinosa and Gimenez, 1974

From the table it may be seen that male and female fish of the same species frequently have different

rates of growth and also maximum sizes as indicated by L∞. Growth curves of some representative

species from an African river (Niger) and the Latin American La Plata system shows part of the range

of interspecific variation (Fig. 6.6). Most species grow very rapidly in their first season, a feature

which Dowe-McConnell (1967) regarded as adaptive. Predation is intense in floodplain rivers, so

rapid growth to get to a size too large to be swallowed before the shelter of the floating vegetation on

the floodplain disappears, is a great advantage. Fish probably also need to attain an adequate size to

migrate by the time the floods recede. There seems to be little difference in the size attained by year 1

in fish having a wide range of maximum sizes as shown by Merona (1983) in his examination of over

100 African species.

186

Figure 6.6 Growth in length of representative fishes from rivers: (A) Niger: (i) Heterotis niloticus; (ii)

Lates niloticus; (iii) Mormyrops deliciosus; (iv) Citharinus citharus; (v) Eutropius niloticus (B)

La Plata: (i) Prochilodus platensis (after Cordiviola de Yuan, 1971); (ii) Pimelodus maculatus

(after Fenerich et al., 1975); (iii) Parapimelodus valenciennesi (after Cabrera et al., 1973)

Whereas the Von Bertalanffy growth curve adequately describes year to year progression in length,

growth within any one year does not conform to the model. The long period in which growth either

ceases completely or is considerably restricted, means that most of the year's increase in length occurs

during a comparatively short period. Dudley (1972), for instance, recorded that 75 percent of the

187

expected first year’s growth of Oreochromis andersoni and O. macrochir took place within six weeks

of peak floods in the Kafue river. Growth in weight is even more subject to seasonal variation, often

with temporary losses occurring during the dry season.

Because the within-the-year growth pattern has important implications for the estimation of biological

production, Daget and Ecoutin (1976) have produced a modified growth model applicable to species

with prolonged annual growth arrests. This requires the introduction of two new parameters into the

growth equation. These are q, which represents the duration of the annual growth arrest in months,

and t, which is the duration of the first period of growth also in months. The parameter t is necessary

where reproduction does not coincide with the end of the parent's period of arrested growth. When the

growing period is 12 - q months, the normal Von Bertalanffy curve is expressed as:

Lt = L∞ {1 exp [-g' (t-to)]}

where t and tO are expressed in months and g' = g/12-q; tO is obtained from the equation

The arc of the growth curve thus obtained is thereby compressed into 12-q months and is followed by

a horizontal line q months long. Daget and Ecoutin applied this model to Polypterus senegalus from

the Middle Niger obtaining the mean growth curve shown in Fig. 6.7. A similar modification of Von

Bertalanffy's model was proposed by Cloern and Nichols (1978) to take into account seasonal

variations in growth rate in temperate waters. This model:

where L is length at time (t), L max is maximum body size, L min is body size at time of recruitment

obtains growth predictions with a much smoother transition from growth to arrest phases.

188

Figure 6.7 Mean linear growth of Polypterus senegalus for the first five years of life assuming an

annual growth arrest of six months and a first year's growth period of seven months (T0 = -3).

(After Daget and Ecoutin, 1976)

These models describe situations where growth stops completely during the dry season and have the

advantage of being based on current growth models, but are somewhat inflexible when applied to

situations where growth varies from year to year depending on favourable or unfavourable conditions.

Such conditions require a growth trajectory that is less pre-determined, and Welcomme and Hagborg

(1977) had to adopt a different model for growth within the year to allow for this. Their formula: has

the characteristics of fast initial increase in length followed by a period of slower growth (but not a

complete halt). Values of Lt for successive years can conform to the Von Bertalanffy relationship,

although the form of the curve within one year calculated for successive weeks does not. The

advantage of this relationship in modelling the growth of fish living on floodplains is that the terminal

value of the year’s growth can change according to the intensity of flooding by the operation of an

appropriate coefficient on G.

Lt+t1 = Lt + G exp(t1)

In his studies on fish production in the Kafue river, Kapetsky (1974 and 1974a) was presented with a

similar problem of modelling within year growth patterns. From his own observations as well as those

of Dudley (1972) it was obvious that growth in weight of Sarotherodon and Oreochromis spp. on the

Kafue flats does not stop completely in the dry season, although it is considerably slowed. Kapetsky,

therefore, proposed to rotate the relationship Wt = WO exp (Gt) on its diagonal and then reverse it.

This gives an equation of the form:

Wt = WO + W1 [1 - exp(-gt)]

where W1 = W0 exp G(12). Here W1 = the weight at the end of a year's growth, whereas individual

segments of t and the growth coefficient g are in months.

189

YEAR-TO-YEAR VARIATIONS IN GROWTH

There appear to be few studies on interannual variation in growth rate of fish from the rhithron and

from low order streams of the temperate zone. It may be assumed that, given the relatively stable

conditions of such environments between years, similar rates of growth are obtained. However, there

are indications that density dependant factors may influence the rate of growth in some species,

although the origin of the fluctuations is not clear.

From the studies of growth of fish species inhabiting the potamon it has become obvious that there are

considerable year-to-year variations in growth within the same species. The most detailed

examination of the possible causes of such variations has been carried out for some cichlid species in

the Kafue River. Here, Dudley (1972 and 1974) and Kapetsky (1974) found significant correlations

between some physical variables and the main growth increment. The intensity and duration of

flooding particularly could have accounted for much of the year-to-year variation in the growth of

year class I and II, Tilapia rendalli, Oreochromis andersoni and O. macrochir. Low temperature in

the dry season also appeared to influence some year classes and gave good partial correlations when

entered into the equation after some measure of flood intensity. Typical relationships are shown in

Table 6.5 where TI is an index of temperature, FI is an index of flooding drawn from the area under

the flood curve (Dudley 1972), and HI 2 and HI 3 are indices summarizing the degree of drawdown in

the dry season (Kapetsky, 1974).

Kapetsky's regression equations were successfully used to predict growth increments for certain year

classes, but the consistency of the results is not uniform, possibly due to the short time series upon

which the calculations were based. They are sufficient, however, to indicate the importance of

external physical factors in determining the growth of fish in such systems. This work on the Kafue is

not isolated. As early as 1934 Wimpenny (quoted in Holden, 1963) found that the yield of fish from

the Nile delta, Lake Manzala, was correlated with flood level, high floods being followed by better

than average yields which were due in part to higher growth rates of first year fish. Similarly,

conditions for feeding, and hence growth, of non-anadromous fishes in the Amur river are

considerably improved in years when there is plenty of water (Krykhtin, 1972, quoted by Krykhtin,

1975). The exceptionally poor flood years during the 1968–74 Sahelian drought provided an

opportunity to assess the effects of this on the growth of fish species in the Senegal, Niger and Logone

rivers. In the Senegal, Reizer (1974), discerned great differences in growth of Citharinus citharus

between 1968, a year of particularly poor flood, and other years (Fig. 6.8). The first year class was

missing totally for that year. The second year growth increment for the 1967 year class in the 1968

flood was 3.91 cm, whereas the 1966 year class grew 7.99 cm during the 1967 flood. The third year's

growth showed similar differences; an increment of 2.32 cm for the 1966 year class (1968 flood) and

8.37 cm for the 1967 year class (1969 flood). Differences in growth were also noted from the Niger

where the floods of 1971 and 1972 were particularly bad. Here Dansoko (1975) and Dansoko et al.

(1976) studied two species of Hydrocynus, H. brevis and H. forskahlii, and found that growth,

particularly of the young of the year in both species, was poor during these two years. Hydrocynus

forskahlii, which only inhabits the river, showed this effect less than H. brevis which depends much

on the floodplain for feeding, but nonetheless the differences were still marked. It is also of interest

that year classes with poor first year growth appear to continue to grow badly despite better conditions

in later years. Likewise year classes with good initial growth do not suffer so badly in poor years. In

the Logone Benech and Quensiere (1984) were also able to demonstrate improved growth, as

represented by the mean weight of fish leaving the Yaeres floodplain through the El Beid, as a

correlate of the Logone flood in several species including Hyperopisus bebe, Brachysynodontis

batensoda, Marcusenius cyprinoides, Oreochromis aureus and O. niloticus.

Table 6.5

Parameter estimates for simple and multiple linear regressions of first and second year

190

growth increments on temperature and hydrological indices

Species Year of growth Sex Model r

Oreochromis andersoni 1 M Growth (cm) = 0.02FI+12.87 a 0.92

1 M TL (mm) = 146.51–0.11(HI2) b 0.94

1 F Growth (cm) = 0.014FI+13.4 a 0.78

2 M TL (mm) = -29.47+1.98 (TI) b 0.90

2 F TL (mm) = 38.24–0.30(HI3)+0.83(TI) b 0.93

Oreochromis macrochir 1 M Growth (cm) = 0.2FI+11.02 a 0.9

1 M TL (mm) = 130.39–0.13(HI2) b 0.92

1 F TL (mm) = 130.13–0.32(HI2) b 0.85

2 M TL (mm) = 74.72.0.10(HI3) b 0.58

2 F TL (mm) = 14.69–0.18(HI3) b 0.95

Tilapia rendalli 1 M Growth (cm) = 0.029FI+12.8 a 0.80

a Dudley, 1972 b Kapetsky, 1974

Figure 6.8 Growth of Citharinus citharus in the Senegal River: 1966, 1967, 1969 year classes.

Numbers in parentheses = total length at growth arrest in centimetres. (After Reizer, 1974)

191

REPRODUCTION

SPAWNING SITES AND REPRODUCTIVE ADAPTATIONS

Fish inhabiting rivers show a diversity of reproductive habit which adapts them to the varying

conditions encountered along the length of the river and to the particular difficulties inherent in

breeding in systems with rapidly fluctuating water levels and often extreme conditions of flow or

oxygen deficiency. It seems that physical and behavioural specializations for reproduction are more

varied than those for feeding in these ecosystems. The range of adaptations is indicated by the fact

that nearly all of Balon's reproductive guilds (Balon 1975 and 1981) are represented in the various

rivers of the world. These guilds which form a useful ecological classification of breeding behaviour,

localities and substrates are listed in Table 6.6 together with some representative taxa. Several broad

reproductive strategies have evolved within these guilds which are summarized in Table 6.7.

Table 6.6

Classification of reproductive guilds of fishes with representative taxa from river systems (from

Balon, 1981)

Ethological section - A. Nonguarders

A.1 Open substratum spawners

(Selected key features of early ontogeny)

Pelagic spawners (pelagophils)

Numerous buoyant eggs, none or poorly developed embryonic respiratory organs, little pigment, no

photophobia. Ctenopoma muriei, Lates niloticus.

Rock and gravel spawners with pelagic larvae (lithopelagophils)

Adhesive chorion at first, some eggs soon buoyant, after hatching free embryos pelagic by positive

buoyancy or active movement, no photophobia, limited embryonic respiratory structures.

Prochilodus spp.

Rock and gravel spawners with benthic larvae (lithophils)

Early hatched embryo photophobic, hide under stones, moderately developed embryonic respiratory

structures, pigment appears late. Many cyprinid and characin spp., Barbus and Labeo.

Nonobligatory plant spawners (phytolithophils)

Adhesive eggs on submerged items, late hatching, cement glands in free embryos, photophobic,

moderately developed respiratory structures. Many cyprinids and Rutilus rutilus.

Obligatory plant spawners (phytophils)

Adhesive egg envelope sticks to submerged live or dead plants, late hatching, cement glands, not

photophobic, extremely well developed embryonic respiratory structures. Many cyprinid, characin

and siluroid spp., Puntius gonionotus.

Sand spawners (psammophils)

192

Adhesive eggs in running water on sand or fine roots over sand, free embryos without cement

glands, phototropic, feebly developed respiratory structures, large pectorals, large neuromast rods

(cupulae). Many migration cyprinid and characin spp.

Terrestrial spawners (aerophils)

Small adhesive eggs scattered out of water in damp sod, not photophobic, moderately developed

respiratory structures. Brycon petrosus.

A.2 Brood hiders

Beach spawners (aeropsammophils)

Spawning above the waterline of high tides, zygotes in damp sand hatch upon vibration of waves,

pelagic afterwards. Not represented.

Annual fishes (xerophils)

In cleavage phase blastomeres disperse and rest in first facultative diapause, two more resting

intervals obligate - eggs and embryos capable of survival for many months in dry mud.

Nothobranchius.

Rock and gravel spawners (lithophils)

Zygotes buried in gravel despressions called redds or in rock interstices, large and dense yolk,

extensive respiratory plexuses for exogenous and carotenoids for endogenous respiration, early

hatched free embryos photophobic, large emerging alevins. Many salmonid species.

Cave spawners (speleophils)

A few large adhesive eggs, must hide in crevices, extensive embryonic respiratory structures, large

emerging larvae.

Spawners in live invertebrates (ostracophils)

Zygotes deposited via female's ovipositor in body cavities of mussels, crabs, ascidians or sponges,

large dense yolk, lobes or spines and photophobia to prevent expulsion of free embryos, large

embryonic respiratory plexuses and carotenoids, probable biochemical mechanism for

immunosuppression. Rhodeus sericeus.

Ethological section - B. Guarders

B.1 Substrate choosers

Pelagic spawners (pelagophils)

Nonadhesive, positively buoyant eggs, guarded at the surface of hypoxic waters, extensive

embryonic respiratory structures. Some Ophicephalus and Anabas spp.

Above water spawners (aerophils)

Adhesive eggs, embryos with cement glands, male in water splashes the clutch periodically.

Copeina arnoldi.

193

Rock spawners (lithophils)

Strongly adhesive eggs, oval or cylindrical, attached at one pole by fibres in clusters, most have

pelagic free embryos and larvae. Loricaria parva, L. macrops and some small cichlids.

Plant spawners (phytophils)

Adhesive eggs attach to variety of. aquatic plants, free embryos without coment glands swim

instantly after prolonged embryonic period. Polypterus spp.

B.2 Nest spawners

Froth nesters (aphrophils)

Eggs deposited in a cluster of mucous bubbles, embryos with cement glands and well developed

respiratory structures. Hepsetus odoe, Hoplosternum, some anabantids.

Miscellaneous substrate and material nesters (polyphils)

Adhesive eggs attached singly or in clusters on any available substratum, dense yolk with high

carotenoid contents, embryonic respiratory structures well developed feeding of young on parental

mucus common. Notopterus chitala, Hoplias malabaricus.

Rock and gravel nesters (lithophils)

Eggs in spherical or elliptical envelopes always adhesive, free embryos photophobic or with cement

glands swing tail-up in respiratory motions, moderate to well developed embryonic respiratory

structures, many young feed first on the mucus of parents. Aequidens and other cichlids, some

characins, e.g., Leporinus.

Gluemaking nesters(ariadnophils)

Male guards intensively eggs deposited in next bind together by a viscid thread spinned from a

kidney secretion, eggs and embryos ventilated by male in spite of well developed respiratory

structures. Gasterosteus aculeatus

Plant material nesters (phytophils)

Adhesive eggs attached to plants, free embryos hang on plants by cement glands, respiratory

structure well developed in embryos assisted by fanning parents. Clarias batrachus.

Sand nesters (psammophils)

Thick adhesive chorion with sand grains gradually washed off or bouncing buoyant eggs, free

embryo leans on large pectorals, embryonic respiratory structures feebly developed. Tilapia spp.

Hole nesters (speleophils)

At least two modes prevail in this guild: cavity roof top nesters have moderately developed

embryonic respiratory structures, while bottom burrow nesters have such structures developed

strongly. Several cichlids.

Anemone nesters (actiniariophils)

194

Adhesive eggs in cluster guarded at the base of sea anemone, parent coats the eggs with mucus

against nematocysts, free embryo phototropic, planktonic, early juveniles select host anemone. Not

represented.

Ethological section - C. Bearers

C.1 External bearers

Transfer brooders

Eggs carried for some time before deposition; in cupped pelvic fins, in a cluster hanging from

genital pore, inside the body cavity (earlier ovi ovoviviparous), after deposition most similar to

nonguarding phytophils. Callichthys, Corydoras.

Auxiliary brooders

Adhesive eggs carried in clusters or balls on the spongy skin of ventrum, back, under pectoral fins

or on a hook in the superoccipital region, or encircled within coils of female's body, embryonic

respiratory circulation and pigments well developed. Loricaria spp.

Mouth brooders

Eggs incubated in buccal cavity after internal, external synchronous or asynchronous, or buccal

fertilization assisted by egg dummies, large spherical or oval eggs with dense yolk are rotated

(churning) in the cavity or densely packed when well developed embryonic respiratory structures

had to be assisted by endogenous oxydative metabolism of carotenoids, large young released. Many

cichlids, Osteoglossum.

Gill-chamber brooders

Eggs of North American cave fishes are incubated in gill cavities.

Pouch brooders

Eggs incubated in an external marsupium: an enlarged and everted lower lip, fin pouch, or

membraneous or boy plate covered ventral pouch, well developed embryonic respiratory structures

and pigments, low number of zygotes. Loricaria vetula and L. anus.

C.2 Internal bearers

Facultative internal bearers

Eggs are sometimes fertilized internally by accident via close apposition of gonopores in normally

oviparous fishes, and may be retained within the female's reproductive system to complete some of

the early stages of embryonic development, rarely beyond the cleavage phase; weight decreases

during embryonic development. Rivulus marmoratus, Oryzias latipes, Pantodon buchholzi.

Obligate lecithotrophic livebearers

Eggs fertilized internally, incubate in the reproductive system of female until the end of embryonic

phase or beyond, no maternal-embryonic nutrient transfer; as in oviparous fishes yolk is the sole

source of nourishment and most of the respiratory needs; some specialization for intrauterine

195

respiration, excretion and osmoregulation; decrease in weight during embryonic development.

Poeciliopsis monacha, Poecilia reticulata, Xenopoecilus poptae.

Matrotrophous oophages and adelphophages

Of many eggs released from an ovary only one or at most a few embryos develop into alevins and

juveniles, feeding on other less developed yolked ova present and/or periodically ovulated

(oophagy), and in more specialized forms, preying on less developed sibling embryos

(adelphophagy); specialization for intrauterine respiration, secretion and osmoregulation similar to

the previous guild; large gain in weight during intrauterine development.

Viviparous trophoderms

Internally fertilized eggs develop into embryos, alevins or juveniles whose partial or entire nutrition

and gaseous exchange is supplied by the mother via secretory histotrophes ingested or absorbed by

the fetus via epithelial absorbative structures (placental analogues) or a yolksac placenta; small to

moderate gain in weight during embryonic development. Poeciliopsis turneri, Heterandria formosa,

Anableps dowi.

Table 6.7

Examples of main types of reproductive behaviour in fishes (adapted from Lowe-McConnell, 1975)

Type of fecundity Seasonality Examples Movement and

parental care Big bang Once in a life-time Anguilla Very long

catadromous

migrations, no

parental care

Total spawners (very high

fecundity)

Highly seasonal concentrated

on annual or bi-annual floods

Characins: e.g. Prochilodus,

Salminus, Alestes

Long distance

migrants, open

substratum spawners

Cyprinids: e.g. Labeo, Barbus,

Cirrhinus

Siluroids: e.g. Schilbe

Heteropneustes, Catla catla,

Labeo rohita

Local lateral migrants

open substratum

spawners

Mormyrids

Partial spawners Throughout flood season(s) Some cyprinids, characins and

siluroids: e.g. Clarias, Micro-

alestes acutidens

Mainly lateral

migrants: open

substratum spawners

Grades into Protopterus, Arapaima,

Serrasalmus, Hoplias,

Heterotis

Bottom nest

constructors and

guarders

Ophicephalus, Gymnarchus Floating nest builders

Hepsetus Hoplosternum

Anabantids,

Bubble nest builders

Small brood spawners (low

fecundity)

High water but may start

during low water or may

continue throughout the year

Tilapia, Hypostomus Nest constructors with

various behavioural

patterns

Aspredo, Loricaria sp. Egg carriers

Osteoglossum, Sarotherodon

spp.

Mouth brooders

Potamotrygon, Poeciliids Live bearers

End of rains Some cyprinodonts Annual species with

resting eggs

196

Fish spawning within rhithronic reaches of the main channel usually have to contend with high flow

and turbulence. Consequently they tend either to have adhesive eggs which stick to rocks and plants

(lithophils or phytophils) or to place the eggs in crevices of the rocky substrates of the riffles

(lithophils). Some salmonids for instance cut “redds” into fine gravel which are designed both to

protect and aerate the eggs. Other species spawning in small streams may lay their eggs in very

shallow water or in damp terrestrial environments (aerophils). For example, the group spawning

Brycon petrosus is described by Kramer (1978) as lodging its eggs in the splash zone at the margins

of Panamanian streams. Several other species including Fundulus similis, F. heteroclitus and

Hypomesus pretiosus also have similar behaviour. Other fish from the Amazon, Madeira, Orinoco,

Danube and Amur as well as presumably many other rivers which spawn in the main channel have

semi-buoyant eggs (lithopelagophils) which drift downstream with the current until finding suitable

feeding grounds usually on floodplains or backwaters.

Many species migrate considerable distances upstream in order to spawn in the rhithronic lower order

streams where presumably the good aeration, rich food supply of the riffles and relative freedom from

predation all favour the development of the fry. To compensate for the high risks inherent in these

environments however nearly all species spawning in such regions produce large numbers of eggs and

are total spawners. That upstream migrants do not all seek the same type of headwater is shown by the

observations made by INDERENA (1973) on the Magdalena river. Here Brycon moorei moved into

small short side arms off the main channel, whereas Prochilodus reticulatus stayed within the main

channel, Salminus affinis swam up side streams to areas of high flow and Brycon henni ascended to

the highest accessible reaches of the main river.

In Europe breeding success of some species appears mainly dependent on the presence of suitable

substrates. Even under considerably modified hydrological regimes species such as Rutilus rutilus or

Abramis brama have shown themselves capable of changing from the type of migratory behaviour

they show in the Volga or Lower Danube to their more static behaviour in Western European

modified streams provided the required substrates are available. Other species, such as Chandrostoma

nasus, Barbus barbus or Leuciscus idus have shown themselves much less able to so adapt.

Migratory species may alter their reproductive strategy to adapt to different climatic conditions. For

instance Alosa sapidissima, an anadromous species which enters rivers on the East coast of the United

States to spawn, breeds more frequently, but releases few eggs per breeding in the cooler northern

rivers than in the warmer southern rivers (Legget and Carscadden, 1978). This means that in northern

rivers, more of the available energy is allotted to migration in order to place eggs and fry in the most

favourable conditions. Legget and Carscadden add that available literature suggests that many other

species are equally capable of fine tuning their reproductive strategies to accord with local

environmental circumstances. Such behaviour would mean that populations specific to certain rivers

would develop and that homing capabilities would be needed to ensure that the behavioural patterns

would match the climatic conditions of the spawning environment.

The most diverse breeding strategies are found in those fish spawning on the floodplains, and many

species migrate considerable distances up or downstream to breed so as to place their fry in these

productive habitats. Utilization of the floodplain for spawning is in fact recorded from most tropical

river systems and is also common in such temperate rivers as still have plains. Thus the various

authors writing on larval fish distribution in the Mississippi-Missouri system (e.g. Kallemyn and

Novotny, 1977) state that the floodable marsh areas of the backwaters are vital for the spawning of

many species. Similar observations have also been made in the Danube (e.g., Holcik and Bastl, 1976)

and the Volga (Koblitskaya, 1985).

Migratory blackfishes show a wide range of behaviour. Most are open stratum phytolithophils or

phytophils which attach or scatter their eggs among vegetation or over open bottoms of the floodplain.

Often the localities chosen for egg deposition are in the channels conducting water on to the plain as

in the Alestes species of the Logone river which lay their eggs in the channels feeding the Yaeres

197

floodplain while the adults themselves never penetrate the plain. Alternatively species migrating on to

the floodplains of the Ayuthaya province of Thailand (Chao Phrya river system) illustrate the

selection of substrates for spawning (Tongsanga and Kessunchai, 1966). Crossocheilus reba spawns

in the inundated rice fields as does Wallago attu. Another catfish, Pangasius sutchi scatters its eggs

among submerged weeds and bushes in the shallow inundated margins of the Klongs or canals. The

spring eel Macrognathus aculeatus spawns both in flooded rice fields and in the channels on clear

bottoms or among weeds. Some species, such as Puntius goniotus drop their eggs in the middle of

channels and presumably rely on currents to wash them into suitable nursery sites.

Closely related species and genera also show contrasting behaviour. For example, Hydrocynus brevis

breed on the floodplain of the Niger during the floods whereas the morphologically similar H.

forskahlii breeds in the main river channel during low water. Jackson and Coetzee (1982) have

remarked on the habits of S. African Labeo umbratus which move on to flooded grasslands to lay

their sticky eggs among the temporarily submerged vegetation as contrasted to the local Barbus

johnstoni and B. trapilepidotus which both spawn in gravel areas of swift flowing rivers. Their

conclusion that these behaviour patterns can be enlarged to make a fundamental distinction between

the two genera does not, however, stand up to closer inspection as some Labeo species, e.g. Labeo

parvus of the Niger River which inhabit rapids breed in the riffle zones of the main channel and some

small Barbus equally breed in the flooded grass of the floodplain.

Most blackfish species which at the most are only local migrants have some form of parental care and

in some genera a variety of behavioural patterns are found. This is typified by the genus Loricaria,

where L. parva and L. macrops sit on eggs which are attached to cleared areas of rock. The eggs are

cleaned and fanned by the parents from time to time. Female L. piracicalae develop a special spongy

skin on their ventral surface. The fish roll on the fertilized eggs which adhere to this region. L. vetula

and L. anus have a pouch formed by the enlargement of the lower lip of the males in to which the

eggs are lodged, and in Loricaria sp. (near microdon: Lowe-McConnell, 1964) the lower lip is

extended into a special elongation from which the young are suspended. Other loricariids such as the

Panamanian L. uracantha use hollows and cavities in pieces of wood as nesting sites (Moodie and

Power, 1982).

Nest building is very common among floodplain species. Arapaima gigas scoops out hollows in the

bottom of the flooded savannah and both parents .guard the eggs that are deposited (Sanchez, 1961).

Various species of cichlid make nests. Some of these are simply cleared areas of rock, others holes

under rocks or vegetation, and in yet others, excavated pits sometimes of complex design. Heterotis

niloticus, construct nests in the middle of masses of vegetation at the shallow margins of the plain,

whereas Gymnarchus niloticus make floating flask or raft shape masses of aquatic weeds in which

they lay their eggs (Svensson, 1933). The different architecture of the nests of Protopterus has been

described by Johnels and Svensson (1954) for P. annectens and Greenwood (1958) for P. aethiopicus.

These range from simple sunken areas in a sandy substrate to complex domed structures of papyrus

roots. The characin Hoplias malabaricus constructs a simple nest on the bottom with whatever

material is readily available and some species of Serrasalmus are also reputed to guard egg masses

laid on tree roots, and aquarium observations have shown them to excavate nests in plant masses

(Braker, 1963).

Floating nests are found in some species. Primitive types are made by some ophicephalids.

Ophicephalus micropeltes allows its pelagic eggs to float to the surface in a cluster, and then

surrounds them with a ring of bits bitten off surrounding vegetation (Tongsanga and Kessunchai,

1966). The floating mass is guarded by the male. Hepsetus odoe is unequal amongst the characins in

constructing a nest of foam which is lodged between the stems of weeds or grasses at the margins of

the plain. Froth or foam nests made of mucus secretions from one or both parents are common in

other families. Among the siluroids Callichthys callichthys and Hoplosternum littorale both make a

raft constructed of bubbles and aquatic plants. Floating nests are made by many anabantids including

Ctenopoma damasi in Africa (Berns and Peters, 1969) and Colisa, Betta and Trichogaster in Asia.

198

The building of nests enables the eggs and newly hatched fry to be concentrated in a protected locality

that is easily defended by one or both parents. Floating nests also bring the eggs and young fish into

contact with the better oxygenated upper layers of the water column, a very necessary feature as most

species making this type of nest inhabit highly deoxygenated waters. Juveniles of species which build

their nests on the bottom often have gill-filaments to improve their oxygen uptake, and the parent fish

fan the nest to ensure a flow of aerated water. Furthermore, nests are usually placed at the limits of the

advancing water where the dissolved oxygen levels are still moderately high. Such constructions are,

however, vulnerable to sudden changes in water level which can either leave the nest stranded or

submerge it in too great a depth of anoxic water. To avoid this happening some cichlids move their

eggs up and down by mouth to follow the advancing or receding flood.

Parental care reaches its most extreme among those species which bear their young throughout their

development. Many of the African and some South American cichlids and other species such as

Osteoglossum bicirrhosum incubate their eggs in their mouths, and continue to shelter their fry until

they become independent. The mouth brooding habit enables spawning to take place before the

floods. The young can be conserved in a well oxygenated environment (the parent's mouth)

throughout their development, and they can be deposited on the nursery grounds at the fringes of the

plain which are far distant from the breeding sites. Most mouth brooding species also construct nests

which serve both as territorial markers for their breeding rituals and as clean places upon which the

eggs are deposited prior to being picked up by one or other of the parent. Most cyprinodonts and the

glandulocaudine characins have progressed even further and have developed internal fertilization

which in many cyprinodonts is coupled with live bearing. Nelson (1964) in his study of the

glanducaudines considered that internal fertilization is an adaptation to the floodplain environment

permitting mating to occur when the fish are concentrated in the dry season habitats. Egg laying, or in

the case of the poeciliid cyprinodonts the birth of the young, can thus be delayed until the female can

move into the flooded shallows at the margin of the floodplain.

FECUNDITY AND SPAWNING PATTERNS

The fecundity and spawning pattern of fishes is correlated with the types of breeding behaviour

described above. Two main categories exist: total spawners, in which all the eggs ripen and are shed

within a very short time, and multiple spawners, in which repeated breeding occurs in any one season

with only a small proportion of the total eggs stock becoming ripe at any one spawning.

Long distance migrant species of whitefish, which are usually open substratum spawners showing no

parental care, belong to the first category. Their eggs are usually small and are produced in very large

quantities to compensate for the wastage inherent in this type of spawning. Fish such as Prochilodus

platensis produce between 360 000 and 750 000 eggs for individuals between 40 cm and 65 cm

respectively (Vidal, 1967). Egg counts for P. scrofa (Ihering, 1930) and P. argenteus (Fontenele,

1953) also fall within this range. Salminus maxillosus of between 52 and 100 cm were reported by

Ringuelet et al. (1967) to produce between 1 152 900 – 2 619 000 eggs. The regression for the

fecundity of Hilsa ilisha; F = 0.9550 exp 0.5396, formulated by Pillay and Rosa (1963) predicts

between 250 000 to 1 600 000 eggs for individuals within the normal size range. The fecundity of

Alestes baremoze ranges between 32 000 eggs for a female 24.4 cm long (177 gm) and 111 000 eggs

for a female 31.4 cm long (394 mm) and is described by the relationship: F = 0.345 Wt of female in

gms - 25 (Durand and Loubens, 1970). Really large species such as Lates niloticus can produce

extraordinary quantities of eggs, over 11 million having been recorded from some individuals by

Okedi (1971). Although the above examples indicate the numbers of eggs produced by total spawners,

the listing is far from complete and more details are given by Lowe-McConnell (1975).

Partial spawning is usually associated with some degree of parental care. The eggs are larger as is

reflected in the lesser numbers per gm wet weight of ovary. A total spawner such as Eutropius

niloticus has 2 950 eggs/gm, whereas Oreochronis sp. have about 135 eggs/gm (Willoughby and

Tweddle, 1977). The number of eggs released at any one spawning is much lower than in total

199

spawners. However, partial spawners may breed several times during any one season and it is,

therefore, difficult to estimate the total fecundity of the fish. This is further complicated by the fact

that the number of young reared is more often a function of the parent's capacity to care for a brood,

than of the ovarian egg production. A typical example of this is the mouth brooding cichlid

Oreochromis leucostictus whose ovarian egg production increased approximately as a square of the

standard length (F = 1.091 L2.156), but whose brooding capacity only increased linearly (B = 17.6 L -

78.2) (Welcomme, 1967). The partial spawning habit is adapted to areas with erratic water levels as it

allows for the loss of one or more broods through the unpredictability of the environment. A further

mechanism to cope with this is found in species such as Arapaima gigas where only one third of the

available eggs ripen at any one time, the rest being available should earlier broods be lost due to

sudden changes in water level.

TIMING

Reproduction of fish in rivers tends to be highly seasonal throughout the world. This seasonality

appears correlated primarily with two factors, temperature and flow, which in the temperature zones

are more or less synchronous in that increases in flow result directly from the snow melt and increased

precipitation associated with rising temperatures in spring. Approaching the tropics the influence of

temperatures seemingly diminishes and the flood regime becomes increasingly important as the major

regulator of breeding. Thus throughout the world the onset of reproduction of the majority of fish

species tends to coincide with the earlier parts of the flood. Spawning may occur at low water, rising

water or peak flood but only very rarely during falling floods.

Some species are however known to breed outside the most favoured season. The salmonids with

their low water winter season spawning mode are a noted example in the temperate zone. In the

tropics several of the black fish species with elaborate parental care may also continue breeding well

outside the flood. Species of Oreochromis are well known for round the year breeding in lakes and

apparently continue this behaviour in some rivers, for instance in the Shire River (Willoughby and

Tweddle, 1977). Brooding cichlids generally have been seen to spawn at low water in river elsewhere

(personal observation) although the frequency of breeding is very much reduced at this time. Many

cyprinodont fishes spawn towards the end of the flood, behaviour which is correlated with their

annual habit whereby one year's fish hatch at the beginning of the rains and develop throughout the

life of the temporary waters which they populate. As pools desiccate in the dry season the fish lay

small batches of their dormant eggs in the bottom.

Species of other families have been reported as breeding during the dry season in the Congo river

(Matthes, 1964); Roman (1966) noted that Micralestes acutidens too breeds on the floodplain during

the floods but continues to breed at all times of the year in the main channel of the Volta river, the

only one of the dwarf forms studied by him to do so. Dormitator latifrons also shows a continuation

of breeding into the dry season albeit at a lower rate thus the peak spawning during rising floods in the

Chone River, Equador (Chang and Navas, 1984). Kramer (1978), who investigated the breeding

habits of small characins in a Panamanian stream, found that there were great differences in timing of

breeding. Here Bryconamericus emperador and Piabucina panamensis spawned in temporary

tributaries in the rainy season, Brycon petrosus and Hyphessobrycon panamensis spawned in the dry

season and Gephyrocharax atricaudatus and Roeboides guatamalensis spawned throughout the year.

Kramer concluded that in tropical environments where conditions are only mildly seasonal

ostariophysan fishes may show a greater range of timing in breeding than they do in areas with

marked seasonality. Similarly, observations from the Magdalena River on the number of species with

ripe gonads and the number of juveniles present each month indicate that breeding patterns in this

river may be rather diffuse (Kapetsky et al., 1977). On the basis of 18 species studied, Kapetsky was

able to distinguish three main groups of fish from their spawning behaviour, those species which

apparently breed only once per year, those which have two or more breeding periods and these species

which reproduce almost continuously. More than half the species studied breed during February to

April bridging the period from low water and in the first stages of the rising flood. There was a second

200

peak of breeding activity during the second phase of rising water (October) but very few species bred

during falling water (December - January). This spread in the reproductive season is probably

explained by the sustained period of high water in the Magdalena system.

The considerable flexibility shown by a single species in timing of spawning in different river basins

within a geographic region is illustrated by Brycinus macrolepidotus (Paugy, 1982). This species

spawns once during the peak floods in the Niger and Senegal rivers or only during low water in Lake

Chad. In the Bandama, however, the species breeds throughout the year. These differences appear to

be correlated with different hydrological regimes, as the Niger and Senegal are both flood rivers,

whereas the Bandama is a reservoir river lacking lateral floodplains. Lake Chad is of course

lacustrine.

Despite such exceptions, the observations of most authors indicate the similarity in breeding

periodicity in tropical rivers from Asia, Africa, South America and Northern Australia (Bishop, 1983).

Typical of such are the remarks of Schwassmann (1978), who produced evidence that those fishes

which live in Central Amazonian floodplain lakes and which are total, one-shot spawners, begin

migration to spawn during the rising waters, probably to shallow recently flooded areas. Colossoma

bidens, Prochilodus sp. Anodus spp., Brycon spp. and Potamorhina pristigaster all had fully

developed ovaries in January to February and migrated out of the lakes into feeder channels on the

rising flood in the Janauaca Varzea lakes of the lower Solimoes. The synchronization of the

reproductive cycle with the flood is sometimes so good that, in systems where the flood wave takes a

considerable time to progress down-valley, the breeding of downstream populations is delayed, often

for up to a month relative to the fish upstream. This has been noted from the Mekong by Sao-Leang

and Dom Saveum (1955) and the Parana by Bonetto et al. (1971). It is also evident from the

comparison of Daget's (1954) data for the Middle Niger where fish breed mainly in July-August with

that of FAO/UN (1970) for the Niger in Nigeria, where breeding occurred from August to October.

In some equatorial rivers, such as parts of the Amazon and Congo systems there are two rainy seasons

which produce bi-modal floods. In these areas many species have more than one breeding season per

year (Roberts, 1973). In the Congo most species breed at the start of the September-October floods.

During the April to June high water, there is also a period of reproduction, although Matthes (1964)

considered this to be less important. Biennial breeding takes place in the rivers leading to Lake

Victoria where species ascend the streams during the equinoctial floods to reach the flooded swamps

near the headwaters (Welcomme, 1969). The twin breeding peaks of the Magdalena River have also

been described earlier in this section.

In temperate zones fish species tend to spawn within very well defined ranges of temperature. Whilst

these ranges occur mainly in spring or early summer when the water levels are also rising summer

spawners also breed during falling water in some systems such as the Mississippi (Fig. 6.9). The range

of temperatures can be illustrated by some North American species (Data taken from Scott and

Crossman (1973)) (Table 6.8).

Table 6.8

Characteristic spawning temperature ranges for some North American

species

Family Species Temperature

range°C

Salmonidae Salmo trutta 6.7–8.9

Esocidae Esox lucius 5–12

Cyprinidae Hybognathus nuchalis 13–20

Notropsis cornutus 15–18

Hybopsis storeriana 21

201

Nocomis biguttatus 23.9

Cyprinus carpio 17–26

Catastomidae Catastomus catastomus 5

Catastomus macrocheilus 7.8–8.9

Catastomus platyrhynchus 10.5–18.5

Ictiobus cyprinellus 15.5–18.3

Ictaluridae Ictalurus punctatus 24–30

202

Figure 6.9 Number of fish species spawning in different months of the year compared with air

temperature and river discharge in (A) the Danube River and (B) the Mississippi river

203

Where the highest temperatures of the year occur at the same time as low water some conflict between

the two stimuli may occur. In the Okavango Delta spawning of most species takes place in the warmer

months of the year, (September-March). This period coincides with the flood in the North basin of the

swamp, but in most of the delta the flood occurs in the coldest part of the year and the fish breed

during low water (Fox et al., 1976).

The factors which initiate maturation and stimulate breeding of floodplain fishes are elsewhere less

clear and a number of factors have been implicated including changes in individual physical

parameters such as temperature, conductivity or flow, as well as the assemblage of conditions that

mark the beginning of the flood. It is probable that each species is affected by the various factors in

different ways and that such external releasers are only effective when superimposed on an internal

physiological rhythm of the fish. An illustration of the specificity of those conditions that induce

spawning is Pimelodus maculatus of the Jaguari River. Basile-Martins et al. (1975) considered that

maturation of this species started when temperatures reached 22°C and that a temperature of 25°C

was needed for breeding to occur. However, temperature was not the only regulatory mechanism as a

minimum increase in water level of 1 m over the low water level also seemed necessary to trigger

spawning (Fig. 6.10). Changes in gonadosomatic index (GSI) where:

were also used to define breeding of Alestes baremoze in the Chari delta which coincides both with

rising flood and increase in rainfall (Durand and Loubens, 1970). In this species Paugy (1978)

concluded that the high temperature attained during the dry season was necessary for maturation

whereas the arrival of the flood triggered actual spawning. The relationship between endocrine

activity, spawning, rainfall, temperature and day length have been presented for the Indian catfish

Heteropneustes fossilis (Viswanathan and Sundararaj, 1974). These three examples, together with

other observations show the underlying similarity of timing of maturity and spawning. It is well

known that many of the migratory characins and cyprinids will not spawn unless the flood

materializes. As Bonetto (1975) has noted, individuals of Prochilodus platensis which are isolated in

the floodplain lagoons do not mature when fish in the river channel do. Most Indian investigators

agree that the major carps will not breed if the flooding associated with the monsoon fails

(Parameswaran et al., 1970). Under these conditions, suitable inundated spawning grounds also have

to be available for reproduction to be successfully concluded, although one cyprinid, Cirrhinus reba

differs from this in that it can breed even at low water in the Cauvery and Bhavani Rivers provided

sexual readiness is induced by a short initial spate (Rao et al., 1972). According to Sidthimunka

(1972) Probarbus jullieni shows similar behaviour in the Mekong system and Krykhtin (1975)

reported that, while many food fishes from the river Amur bred only on the floodplain with rises in

water level, some species (e.g., Elopichthys bambusa, Erythroculter sp. also reproduced when the

water level did not rise. Indeed many phytophylic species in European rivers appear to have adapted

to the modified conditions following channelization and flood control by spawning within the main

channel rather than on the fringing flooded areas. However, in these species the main external factor

regulating spawning seems to be temperature rather than flow.

Despite such exceptions it may be concluded that in the majority of total spawning species breeding is

so timed that if floods are delayed or inadequate to trigger migration reproduction may fail in that

year. Partial spawning species appear to be capable of breeding whenever suitable conditions are

present and as has already been noted some such species breed throughout the year. Many others have

longer and more diffuse breeding seasons, starting to breed in the river channel before the floods

arrive and continuing with subsequent broods well into the flood season on the plain.

Maturation tends to be relatively rapid in river species. Most small tropical species are ready to spawn

by the onset of the rainy season following their birth and have very short life cycles (Lowe-

204

McConnell, 1967). Equivalent sized species in the temperate zone mature somewhat later, either

during the first or more usually the second year of life. Larger species in both tropics and temperate

zones often delay maturation until their third or fourth years and extreme examples such as Aspius

aspius or Catastomus catastomus may not begin spawning until their fifth year. Species in unstable

systems may respond to unfavourable circumstances by stunting and accelerating maturation within

the first year of life. Such behaviour is typical of species of the genus Oreochromis (Dudley, 1976).

Hatching times of eggs are in most cases closely related to water temperatures and they differ

considerably in any one species. For instance, eggs of Esox lucius which hatch in 12–14 days at 7–

9°C hatch in 4–5 days at 17–20°C, and in brown trout incubation times of 148 days at 2°C are reduced

to 27 days at 12°C (Table 6.9).

Figure 6.10 Gonadosomatic index (GSI) of Pimelodus maculatus in the River Jaguari compared with

temperature (T°C), water level in river (m) and rainfall (mm³) for 1971–73. (After Basile

Martins et al., 1975)

205

Table 6.9

Average times between fertilization and hatching in brown

trout (Salmo trutta) at different temperatures (from Forst and

Brown, 1967)

Water temperature °C Incubation time days 2.0 148

3.6 118

4.7 97

6.0 77

7.8 60

10.0 41

11.0 35

12.0 27

Because of the cooler and less predictable thermal conditions temperate species have larger and more

variable hatching times usually ranging from 5–10 days. In the tropics, with higher and more constant

temperatures, hatching is rapid and in some fishes may occur within 16–24 hours of the eggs being

laid. Such fast development has been noted particularly from open substratum spawners such as Hilsa

ilisha (Pillay and Rosa, 1963), Labeo rohita (Kahn and Jhingran, 1975) and Labeo victorianus (Fryer

and Whitehead, 1959). The eggs of some nest building species develop equally fast, although in

mouth brooders the eggs take longer to hatch (4–5 days) and the fry often stay with the parent for up

to 2 weeks.

Exceptionally long hatching times are also common in some species. Many of the autumn or winter

spawners of temperate rivers, such as Salmo trutta have development times of up to three months

meaning that hatching usually occurs in early spring. Similarly the drought resistant eggs of

Nothobranchius spp. may arrest development for over a year until favourable conditions prevail.

THE INFLUENCE OF HYDROLOGICAL REGIME ON SPAWNING SUCCESS

Several aspects of the flood regime can have an impact on breeding success or on the survival of fry,

and thereby influence recruitment to the fish stock. That there are considerable differences in the

numbers of young produced in various years is well known. Holden (1963) noted that some species

were only represented by one year class in the Sokoto River. There were also great variations in the

relative abundance of species, indicating year to year differences in breeding success. Non-migratory

species were most abundant in one year (1955) due to a strong 1954 year class correlated with a good

early rise of the river. The same factor obviously did not favour the migratory species. Holden also

mentions the observations of Wimpenny (1934) on the Nile delta lake where heavy floods were

followed by good recruitment giving a high density of fish. LoweMcConnell (1964 and 1967) noted

large fluctuations in abundance of different species in different years, which she attributed to

differential breeding success, short life cycles and rapid maturation. Here strong year classes of

Serrasalmus nattereri and Prochilodus were found in different years.

In the Amur system, the yield of phytophilous species, which breed on the floodplain of the river, is

also directly related to the flood regime. Year classes are usually weak in years of light flooding and

strong when floods are heavy (Nikolsky, 1956). That a similar dependency between recruitment and

flood regime exists in the Danube is indicated by the fluctuations in fish yield as a correlate of water

level analysed by Holcik and Bastl (1977).

From fluctuations in the number of Oreochromis andersoni, O. macrochir, and Tilapia rendalli,

Dudley (1972) deduced that there were seasonal changes in abundance. Furthermore, from differences

in the abundance of Oreochromis juveniles he concluded that breeding success and recruitment to the

206

stock was much better in those years having good floods. Later work on O. andersoni and O.

macrochir indicated that spawning success might have been depressed by high dry season water

levels. The reason advanced for this is that Oreochromis normally stunt under the severer conditions

where drawdown is greater. The stunted fish mature and breed earlier than they would in years when

the high volume of water retained allows fish to grow to their normal maturation size.

The extreme hydrological conditions of the Sahelian rivers during the 1970–1974 period have

provided examples of the effects of failed floods on the recruitment of several fish species. In the

Central Delta of the Niger, Dansoko (1975) and Dansoko et al. (1976) compared the biology of

Hydrocynus brevis and H. forskahlii. The floodplain spawning H. brevis and the main channel

spawning H. forskahlii give a useful contrast in that the effect of variability in the extent of flooding

should be reflected more in one species that the other. This was confirmed by poor recruitment in H.

brevis during the years when the flood failed. A similar succession of events happened in the Senegal

River where recruitment of all freshwater fish was lacking in the poor flood years of 1968 and also in

1971 as exemplified by Citharinus citharus in Fig. 6.8. In the Chari River several species have

disappeared following the sustained Sahelian drought which hindered breeding in the Yaeres

floodplains as well as producing unfavourable conditions in the river channel and in Lake Chad.

Larger species, Distichodus, Citharinus and Labeo and the migratory Alestes dentex and A. baremoze,

declined drastically in abundance and were replaced by other species including Hemisynodontis

membranaceus and Brachysynodontis batensoda. Oreochromis niloticus, because of its multiple

spawning habit could rapidly recolonize the floodplains once more favourable conditions appeared,

whereas in species such as Alestes, which are total spawners where the majority of the new recruits

are produced by the second and third year classes, a failure of two floods in succession so weakened

the stock that it appeared unable to recover especially in the face of continued heavy fishery

(Carmouze et al., 1983).

MORTALITY

CAUSES OF MORTALITY

Any consideration of mortality among river fishes is liable to be complex because of the interplay

among the many causes of death. Two main groups of natural factors may be identified: (i) Density

dependant factors whose rate is a function of the numbers of individuals of the species or of other

species within a given space, and (ii) Density independent factors which usually are non-biotic and

which originate in the changing physical and chemical characteristics of the river system. The two

groups of factors are interrelated especially in rivers with fluctuating water regimes as changes in the

volume of the living space constantly influence the density of the fish themselves. Equally the various

density dependant factors can affect the rate at which others proceed; for instance having mortalities

due to predation at one phase of the flood cycle can so reduce populations of prey species that their

subsequent mortality through competition among themselves is considerably reduced. An indication

of the linkages between some of these factors is shown in Fig. 6.11 for the fry and parr stages of

stream dwelling salmonids.

207

Figure 6.11 Summary of factors which effect changes in numbers of stream dwelling salmonids

during the fry and parr stages (from McFadden, 1969)

To the natural causes of mortality should be added fishing mortality. Attempts at separating natural

and fishing mortalities in river fisheries are rare. In any case, because most techniques employed in

artisanal river fisheries are themselves density dependant, the effects of removal of fish by fishing

may significantly alter the rate at which they die from other causes.

Death due to stranding or isolation of fish in headwater streams, main channel pools or temporary

water bodies on the floodplain is probably one of the major elements of natural mortality. Bonetto et

al. (1969) estimated that some 40 000 t of fish are lost in this way from the Parana system, basing

their estimate on the area of standing water on the plain (2 × 106 ha²) and the standing stocks of 20

kg/ha as estimated from a small sample of them. This figure was greatly superior to the 10 000 t

which were reported to have been caught by the fishery in 1967. There is no doubt that enormous

quantities of fish are lost in this manner in most systems, and this apparent wastefulness has been

widely remarked upon in the literature. Mortality through stranding is obviously closely related to the

flood regime. It is not particularly prominent during the rising flood although temporarily recessions

can leave fish isolated in pools and can destroy young fish in nests at the flood margin. More

extensive flooding can raise the total number or biomass of fish in the system through improved

reproductive success and growth making the whole community more sensitive to later contractions in

the volume of the aquatic environment. The form of the falling flood would appear to be critical, as a

more rapid draining of the plain would seem to give the fish less time to abandon their flood habitats

and would increase the chances of their being isolated in unsuitable places. On the other hand there is

a possibility that there might also be some favourable effects because of the stronger pattern of

currents which may make the fish more responsive to such stimuli. However, the behaviour of fish

during the return movement to the main channel has not been studied in any great detail and needs

considerable work in the future. The duration of the dry season equally affects the proportion of the

standing water that will disappear through evaporation or be rendered unsuitable for habitation by

certain species: Presumably the longer and more severe the dry period, the greater the mortality due to

this cause. The impact of stranding mortality on the dynamics of the community as a whole is difficult

to assess. If all of the fish produced during one wet season were to enter the permanent standing water

at the end of the flood, densities far surpassing the carrying capacity of the water might well result.

This could give rise to a corresponding increase in density dependent mortality with a final survival

not far different from that resulting when a proportion of the individuals are lost by stranding.

Deaths due to unfavourable hydrological conditions during the floods have been described from

several systems. In the Kafue flats (Tait, 1967) and the Parana, these have been traced to the

208

deoxygenation of the whole water column in lagoons when the lowering of the temperature and high

winds cause the breakdown of stratification and sudden overturn. Similar cold spells have been the

source of recorded mortalities in the Nile and in the Amazon, where they are known as “friagems”,

Brinkmann and Santos, 1973). In the Amur River, winter kill conditions arise in years of poor low

water flow when the main channel separates into a number of pools. These become deoxygenated

causing heavy mortalities of adult fishes (Krykhtin, 1975). Heavy fish kills in pools within the

channels of small stream or even major rivers produced by high temperature or deoxygenated

conditions are also common especially in the tropics or in temperate zones where eutrophicated of

polluted conditions prevail. Fish kills due to such causes are often spectacular in that large numbers of

dead fish appear on the surface, but they generally seem to be highly localized in space and time.

Reduced survival of stream living species and of the young of upstream spawners can be anticipated

in years of exceptionally heavy flooding. In such an eventuality the semi-pelagic eggs, juveniles and

even the fish themselves could be swept downstream past the most favourable sites for their

development. Similarly, fish may be lost to the community by the downstream drift of sudd islands in

which they have taken refuge. These islands may eventually reach saline or other unsuitable waters

where the fish die. In estuarine systems too, mortalities of freshwater fish have been observed when

they have been carried out to sea by excessive currents.

Many of the species of fish inhabiting rivers are of small size and have life spans of one or at the most

two years. Death in such fishes is possibly associated with stresses arising from spawning or from

senescence. The mechanisms defining the life span of any species are, however, not well understood

and many fish live for considerably longer periods in captivity than they would seem to do in their

natural habitats.

Intraspecific or interspecific competition is obviously one of the main potential causes of density

dependant mortality among species. Competition may be for food or, perhaps more commonly, for

some other limiting resource such as shade, refuge sites or breeding sites. In some species,

particularly redd cutting species such as salmonids, the phenomenon of overcutting, where one fish

destroys the eggs of a previous spawner, increases at high population densities. Also demonstrated for

salmonids is the exclusion of juveniles from feeding territories by incumbent fish at densities much

above 5 fry/m² (Le Cren, 1965) resulting in a plateaux in population at about this level. Le Cren was

able to obtain experimentally a linear relationship between the coefficient of mortality (%) and the

logarithm of the numbers of Salmo trutta present in a stream. Such relationships indicate the extent to

which direct intraspecific competition may affect populations as density increases.

Interspecific competition may also produce local differences in mortality rates, but in view of the

continual co-survival of multi-speciate fish communities over long periods of time it obviously does

not represent a major cause of mortality. Perhaps the most widespread interspecific contribution to

mortality is that of predation. As has already been noted, the role of piscivorous predators in the

community increases with stream order and predators form a particularly large proportion of the

biomass in the potamon. Here mortality of small species and of the juveniles of larger species due to

predation by fish is probably maximal during the draining of the floodplain after the flood peak has

passed. Evidence presented in the section on feeding indicates that in many tropical rivers, predators

do not feed extensively during the dry season. There is unfortunately no data on the intensity of

predation during the rising floods. At this time cover for prey is maximal and the fish are probably

dispersed widely over the plain. Nevertheless, the condition of the predators improves throughout this

period indicating that they are feeding to a certain extent. The juveniles of most predatory species use

alternative food resources during the earlier stages of the flood, but are already turning to a

piscivorous habit as the flood nears its end. During the run-off phase the stomachs of most predators

are full, and personal observations on the Ouémé and Niger showed them to be typically packed with

the juveniles of a wide range of species. Prey may include the young of the predators themselves

whose non-specific feeding habits may lead them to cannibalism. Selection by length of prey is,

209

however, much more widespread, and there would appear to be a relatively narrow size range at

which different species are vulnerable to predation.

Predation by animals other than fish also appears to be at its highest during the period of the draining

of the plain. Williams (1971) and Lowe-McConnell (1964), together with several other workers, have

remarked particularly on the number of water birds preying on the fish stranded in floodplain

depression or leaving the area by way of the channels.

Another possible contributor to total natural mortality is disease. This may become extremely

important during the phase of high fish concentrations at the end of the dry season. A range of

diseases infect salmonid and cyprinid species in temperate rivers. These are mostly associated with

the high densities of fish attained in intensive fish culture but easily spread to the wild. There is little

knowledge of the epidemiology of tropical fishes in their natural habitats, although Khalil (1971)

recorded 215 genera of helminths, and Awachie et al. (1977) listed a range of bacteria, protozoa and

Crustacea, all of which are parasitic on fish in African inland waters. According to Awachie et al.

(1977) the lotic conditions in rivers are not favourable to heavy infestations with such parasites.

However, large mortalities have been recorded from the various eleotrid species from the Sepik River

in Papua New Guinea. These were traced to the bacterium Aeromonas hydrophila, the causative agent

of dermal necrosis, and occurred not only among species inhabiting the floodplain but also those of

the headwater streams (Coates, 1984). A similar disease has been reported as spread throughout S.E.

Asian rivers in the early 1980s. That reservoirs of infection are present among natural populations in

rivers is also indicated by the rapidity with which diseases appear and spread through populations of

ornamental fish after their capture. Much would appear to depend on the concentration of fish in the

habitat and the swiftness of the current. In sheltered lentic environments the abundance of parasitic

organisms increases. Such conditions are present in floodplain pools or isolated main stream pools in

the dry season where the lack of current, warm temperatures and high population densities of fish

favour the spread of infection. Likewise, parasitic infestations are readily discernible on nursery

beaches where young Oreochromis and Tilapia congregate. Under crowded conditions too, there is a

build up of toxic excretory products, mainly NH3 and N02 in the water. These lead to gill or skin

damage, which in conjunction with heavy parasite infestations can cause death.

SEASONALITY OF MORTALITY

The various factors contributing to total mortality tend to follow a similar pattern in fluctuating

systems. Apart from deaths due to unfavourable conditions during rising floods, and the sweeping

away of juvenile or small fish also during this time, most causes of death intensify as the flood cycle

passes from wet to dry phase. Due to the fluctuating volume of the aquatic components of the river

there is a range of ichthyomass that can be supported by it. Available evidence suggests that the

maximum amount of fish is present during the flood and the minimum just before the onset of the

next flood. This means that there is an overproduction of ichthyomass during the flood which persists

throughout the period of falling water and which must be lost through total mortality during the dry

season. It also implies that the amount of water remaining in the system during the dry season relative

to the amount of water present during the wet is one of the major factors determining mortality.

Because there is an overproduction of ichthyomass during the wet season the biotic system can

respond to differences in the amount of water during the dry season so as to assure maximum survival

into the next year. What proportion of total mortality can be taken as fishing mortality without

affecting this flexibility of response is one of the key questions in the dynamics of river fish

communities and in the management of their stocks for fisheries.

ESTIMATES OF MORTALITY RATES

Total mortality rates (Z) are derived from the formula

210

and represents loss from a habitat or component of the ecosystem by death from all sources including

fishing, and the difference between immigration into emigration from the area under study.

Obviously, the larger the area sampled to establish Nt and Nt0 relative to the size of the whole system,

the more errors arising from immigration and emigration are liable to be reduced and the more Z

tends to represent death rate. Only very few estimates have been made for species of fish inhabiting

tropical rivers and these are given in Table 6.10. Values of Z are generally quite high in the early

years of life (Z 4) but tend to drop during the later years (Z 1). This may be because the fish become

less susceptible to predation as their size increases, and large species such as Colossoma have low

mortality rates even when young although heavy fishing pressure continues on most species

throughout their lives. In the extreme case of fishes whose life spans are very short, for instance the

one to two years of many species of Barbus, cyprinodont or small mormyrid, values of Z, can be very

high.

Comparison with mortality rates of fish from temperate rivers shows that, although survival is better

initially in the temperate species (low mortality rates), their mortality rates tend to increase with age.

An accurate knowledge of mortality rates is essential to the calculation of production an animal

communities, the lack of precise knowledge of this parameter for tropical riverine fish is a severe

handicap in understanding the dynamics of such populations, however indications of mortality rate

may be taken from Pauly's (1980) demonstration of the inverse relationship between mortality and

asymptotic length in fishes.

MODELS OF MORTALITY

The simple exponential model of mortality Nt = Noe-zt has been widely used to describe year to year

mortality of fishes. If used to describe mortality patterns during one year it assumes stable

environmental conditions such as these in reservoir rivers or canals and predicts maximum loss of

numbers early in the period under consideration. Such a model, however, does not adequately

describe mortalities within any one year in rivers with fluctuating hydrological regimes. To more

accurately represent mortality patterns under such regimes Kapetsky (1974) suggested that the form of

the mortality curve might be represented by rotating the curve for exponential decrease in number

around its diagonal axis. This gives a curve of the form:

Nt = N0 - N0 exp - zT[(exp zt)-l]

where z is a weekly mortality coefficient, to is equivalent to the time of recruitment, t = time in weeks

and T = 52 weeks. Such a curve gives a mortality rate which increases steadily throughout the year

(Fig. 6.12). While not wholly satisfactory this model is a useful generalization for use in broader

productivity models. The seasonal patterns of mortality described above indicate very little mortality

during the wet season and increased mortality as the year progresses. The majority of causes of death:

predation, fishing, disease, hostile environments, etc., would seem to be density linked, and for this

reason Welcomme and Hagborg (1977) reduced the number of fish in a simulated floodplain

population by inserting a density dependent mortality parameter in the simple exponential model:

Nt+1 = Nt exp - (exp zM)

Table 6.10

Annual mortality coefficients (Z) for certain fish species from some tropical rivers as compared with

211

fish species from temperate rivers

Species River Year Author 1 2 3 4 5 6 7 8 9

TROPICAL

Polypterus senegalus Chari 0.54 0.53 Daget and Ecoutin, 1976

Brycinus leuciscus Niger 1.20 Daget and Ecoutin, 1976

Hydrocynus brevis Niger 3.08 0.98 Dansoko, 1975

Hydrocynus forskahlii Niger 2.49 2.67 Dansoki, 1975

Oreochromis andersoni Kafue 2.47 0.65 0.65 0.65 0.65 1.70 0.58 0.58 Kapetsky,1974a

Oreochromis macrochir Kafue 3.98 0.70 0.70 0.70 0.70 0.70 Kapetsky, 1974a

Tilapia rendalli Kafue 4.61 1.40 1.40 1.40 1.40 Kapetsky, 1974a

Serranochromis

angusticeps Kafue 3.12 2.20 Kapetsky, 1974a

Hepsetus odoe Kafue 2.74 1.84 1.84 1.84 Kapetsky, 1974a

Total community* Bandama 2.67 Daget et al., 1973

Colossoma macropomum Amazon 0.45 0.45 Petrere, 1983

TEMPERATE

Salmo trutta Bere stream 2.10 1.34 1.81 Mann, 1971

Cottus gobio Bere stream 1.66 1.33 Mann, 1971

Rutilus rutilus Thames 0.42 0.42 0.68 0.70 0.92 1.00 1.26 1.86 Mann, 1971

Rutilus rutilus Danube 0.69 0.98 0.65 0.70 0.56 0.61 0.68 0.75 Chitravadivelu, 1972

Alburnus alburnus Thames 1.29 1.29 1.29 3.90 4.80 4.10 Mann, 1971

Danube 0.88 0.86 0.82 0.84 0.73 0.63 Chitravadivelu, 1972

Leuciscus leuciscus Thames 0.60 0.60 0.60 0.60 0.56 1.15 1.28 1.46 Mann, 1971

Perca fluviatilis Thames 0.98 0.98 0.98 1.61 1.49 1.39 1.45 1.47 Mann, 1971

Gobio gobio Thames 0.88 0.88 0.88 0.88 2.52 4.24 Mann, 1971

* Dominant species Labeo coubie and Alestes rutilus

212

Figure 6.12 Changes in number of fish in one age group over 52 weeks as predicted by a simple

exponential model (—) and Kapetsky's (1974) floodplain model (-----): z = 1.5

In this simulation z was obtained from a mortality coefficient characteristic of selected floodplain fish

species; M was calculated from a formula of the form M = aBx where B is the weight per unit volume

of the fish in the system. M was adjusted to unity at a preselected biomass typical of a floodplain

system, producing slower mortality rates when densities fell below the biomass and increased

mortality rates at greater densities. The results of such a simulation are shown in Fig. 6.13.

213

Figure 6.13 Changes in the number of fish in one age group over 52 weeks assuming recruitment at

the start of week 1 when generated by the simulation of Welcomme and Hagborg (1977). Also

shown are the water levels used in the simulation.

STANDING STOCK AND PRODUCTION

STANDING STOCK

Estimates of ichthyomass or standing stock are available from many rivers and sufficient information

exists to permit a preliminary comparison between temperate and tropical waters. Standing stock

estimates are usually made by one or more of three basic techniques: multiple fishing to the

exhaustion of the stock, mark and recapture experiments, and poisoning a sample area with piscicides.

The assessment of the total fish population by these methods is not wholly reliable due to sampling

errors of various kinds, but the combination of several methods and the taking of samples over large

areas improves the reliability of the estimate.

The density of the population present in rivers with fluctuating water regimes depends much on the

amount of water present at the time of sampling. Population densities (ichthyomass per unit area) are

generally higher during low water when the fish are concentrated together than at high water when

they are dispersed, even though the total weight of fish in the system may be larger during the floods.

Furthermore, the degree of exploitation affects the standing stock with probable reduction in mean

214

ichthyomass as fishing pressure increases. As many water courses are very heavily fished, particularly

in the tropics, the values of ichthyomass might be expected to be lower than they would be in a virgin

or unexploited stock. Comparison between different systems is therefore difficult and can only

legitimately be made on the basis of samples taken during the same phase of the flood cycle and

taking into account the state of exploitation of the stock.

Main River Channel

Differences in standing stock in the main channel may be correlated with ecological parameters such

as slope. In general there is a progressive increase in ichthyomass as one moves downstream, as for

instance in the Luanza River, Congo where Malaisse (1969 and 1976) found 1.3 kg/km, 26.1 kg/km

and 31.7 kg/km in successive downstream reaches (see also Table 6.11). Much of this increase may be

traced to the widening of the river channel downstream; hence standing stock (Ichthyomass/unit area)

may not increase at such a rate. However, where the fall line of the river does increase lower

ichthomasses are present, as in the case of the Kaloma River where a reach with greater slope had an

ichthyomass of 7 kg/km and was intermediate between an upstream reach with 21 kg/km and a

downstream reach with 91 kg/km (Balon and Coche, 1974). This implies that there may be a trend to

increase in overall productivity along the river channel with higher standing stocks, downstream

although much more work is needed to clarify the ecology of small tropical streams.

In most rivers, flows are too swift for accurate samples to be taken during the floods in the main

channel. Consequently most estimates have to be made during low water. It was assumed by the

University of Michigan et al. (1971) that during the floods the ichthyomass in the Kafue river would

not differ significantly from that of the open water areas of the floodplain where, on the basis of

repeated fishing with seine nets and on rotenone samples in the open waters, an ichthyomass of 337

kg/ha was established. Samples taken in the channel at low water showed the standing stock to be

less, 204 kg/ha. In the light of work by Kapetsky (1974) this dry season estimate may be somewhat

low as he found the standing stock of the five main commercial species to be very different for three

separate reaches of the Kafue river, at 106.5 +29.21; 576.7 ± 129.2 and 386.6 ± 63.9 kg/ha

respectively. The differences in standing stocks were attributed to differences in fishing intensity in

the various reaches. The discrepancy between the mean 348.2 ± 59.5 kg/ha and the 204 kg/ha found

by the University of Michigan could be explained in part by the reduction in water volume by a factor

of 0.7 between the two sets of samples, but because the University estimate is based on all fish

species, the actual difference between the two estimates is most probably greater than it appears.

In the Chari river, Loubens (1969) carried out an estimate of standing stock in a 360 m² pool left in

the main channel at low water obtaining 15 138 fish/ha weighing 861 kg/ha. More comprehensive

results were quoted by Daget et al. (1973), whose rotenone sampling at two sites in the Bandama

River enabled the evaluation of changes occurring in the dry season. These were reduced into the

formulae: for the reduction in number (dj) with time in days (j). At one site, numbers were reduced

from 3 417 fish/ha on 31 January to 1 411 fish/ha on 30 May, and biomass fell from 125 kg/ha to 50

kg/ha in the same period. At the other site, 2 271 fish/ha on 31 January decreased to 996 fish/ha on 2

August, a loss of biomass of 144 kg/ha from 257 kg/ha to 113 kg/ha. The lower estimates of standing

stock obtained from the Mekong by Sidthimunka (1970) of 6.0 and 7.4 kg/ha were attributed to

difficulties in sampling and one sample from the Mong tributary of the river gave the much higher

value of 137.4 km/ha. Other small rivers in Thailand have been recorded with very similar levels of

standing stock, 118, 186 and 81 kg/ha for the Bori Pat, Lami Pi and Muak Lek rivers respectively

(Geisler et al., 1979). Bishop (1973) also calculated an ichthyomass of 179 kg/ha in the small black

water Gombak river in Malaysia although that estimate was arrived at by assuming only 20%

efficiency with electro fishing gear applied to catches equivalent to about 38 kg/ha.

Log dj = 1.63292 - 0.003202j and Log dj = 1.67286 - 0.003166j

215

Unpublished fishery studies relating to the lower Mekong basin, prepared under the auspices of the

Mekong Committee present values of 60 kg/ha for the upstream Mun river tributary to the Mekong,

and 91.9 ± 51.7 kg/ha for 11 other tributaries. The value of 60 kg/ha for the Mun river is a mean of

several estimates which fall from 120 kg/ha at low water to 5 kg/ha during the rising flood. The

progressive drop of ichthyomass per unit area is attributed to the dispersal of the stock with increasing

volume of water. Downstream in the main inundation zone the riverine standing crop was estimated at

135 kg/ha, but as this and the other values quoted are derived from capture fisheries the authors

consider them to be low and they probably exclude the first year fish which, as has been shown,

contribute the major part of the ichthyomass. The basic productivity of the water may also affect the

standing stock, especially in exploited populations. In black water rivers relatively low values of

ichthyomass may be anticipated, such as the 88.7 ± 75.7 kg/ha from the Baram River (Watson and

Balon, 1983), although no correlation between nutrient status and standing stock was found by Geisler

et al. (1979) in Amazonian rivers and analysis of standing stock against conductivity yields no

significant results. Considerably more data is needed from black waters in order to more fully

understand their ecology.

Much work has been done on the fish populations of temperate rivers to determine their ichthyomass

(Table 6.11). For example, Mann (1965) found a total ichthyomass of 659 kg/ha in the river Thames,

a value which he considered high for temperate rivers, although Backiel (1971) estimated that total

ichthyomass in the Vistula river may be between 200 and 1 100 kg/ha. In Belgian rivers of various

sizes, Timmermans (1961) and Huet and Timmermans (1963) found between 130 and 300 kg of

fish/ha by electrofishing. Philippart (1978) found that the total ichthyomass in the river Ourthe

corresponded to 315 kg/ha, or approximately 900 kg/km. Within this mean biomass fluctuated

following various factors such as temperature or depth of water but the main correlate was with

channel width according to the formula:

Ichthyomass = 6.9569 x 1.8456 (r = 0.919)

Detailed analysis of 40 Belgian rivers shows the main biomass to be 197.8 ± 137.8 kg/ha although

potamon reaches such as the Ourthe or Semois Rivers range between 250–350 kg/ha. An inverse

relationship was established by Timmermans and by Cuinat (1971) for French rivers:

posit that small rivers with high slope have a larger biomass per unit area, at least in Europe.

Philippart (1978a) proposed a modified model:

B = -295 + 0.19A + 5.72T+17.58S +16.8W

where A = alkalinity; T = mean temperature; S = mean slope and W = mean width based on the set of

Belgian rivers. Similar values have been obtained from temperate rivers other than those of Europe.

For example, the Horokiwi stream in New Zealand supported up to 311 kg/ha of fish (Allen, 1951)

and ichthyomass in trout streams in North America may reach 471 kg/ha (McFadden and Cooper,

1962). Swingle (1954) recorded 143 kg/ha from deep parts of the Coosa river and 154 kg/ha for

shallow areas. Ichthyomass in deep parts of other large Alabama rivers varied from 51.2 to 1 730

kg/ha, this latter being attained in the Tenson river which had a particularly wide floodplain. Further

to the North, Mahon et al. (1979) found 11 126-74 765 individuals/ha, equivalent to 32.4 to 190 kg/ha

for the Speed river Ontario.

This brief survey of standing stocks from streams all over the world indicates a wide range of values

at all latitudes. It is perhaps surprising, however, to find that there is little basis for any supposed

216

higher carrying capacity in the tropics. In fact values of between 100 and 600 kg/ha are common in all

continental areas.

Backwaters

Secondary channels and blind river arms, which have many of the characteristics of floodplain

standing waters but remain connected to the main channel for most of the year, are richer habitats than

the main river.

Perhaps the most detailed studies of fish populations of river backwaters come from the Danube. Bastl

et al. (1969), Holcik (1972), Holcik and Bastl (1973 and 1976) and Chitravadivelu (1974) have

investigated the Biskupicke, Zofin and Vojka branches using mark-recapture and repeated catching

methods. Their work led to the generalization that ichthyomass varies between 300 and 500 kg/ha.

About 20 species were present although species number varied with the area of the arm. Population

densities were high in the summer when fish took refuge in the side arm from the current in the main

channel. In the autumn the ichthyomass dropped as larger fish disappeared from the shallower waters.

Very low values, often below 100 kg/ha were associated with unusually strong flooding which

dispersed the population, or with pollution which was especially common in same side arms of the

Danube. A similar ichthyomass has been recorded from the Poltruba arm of the Elbe, where Oliva

(1960) found a population density of Rutilus rutilus and Alburnus alburnus of 222.3 kg/ha using

rotenone. As this was obtained from only two species the total ichthyomass was presumably

somewhat higher. As many as 22 350 fish/ha were found in a secondary arm of the Chari by Loubens

(1969) although 98 percent of these were small fish of less than 10 cm length. By contrast, 96 percent

of the 2 150 kg/ha were contributed by the few fish larger than this length. The same backwater had 5

616 kg of fish/ha at the end of one flood which had fallen to 1 600 kg/ha two months later. A fourth

sample from the same area a year later, only yielded 369 kg/ha, indicating the year to year variability

that can be expected in the same body of water. Another backwater of the Chari gave an estimate of 2

166 kg/ha from an area of 6 000 m². In the Bandama, Daget et al. (1973) showed that population

densities can increase in such backwaters. Contrary to the trends shown in the main channel, the

number of fish in a minor river arm increased from 1 408 individuals/ha to 3 311 individuals/ha in

June. This corresponded to an increase in weight from 149 kg/ha in March to 350 kg/ha in June.

These figures, coupled with those for the main river, indicated that there is some movement during the

dry season from the open pools of the main river into the more shaded habitats of the backwaters.

Sidthimunka's (1970) figure of 219.8 kg/ha for a backwater to the main stream of the Mekong was

also considerably higher than the estimates from the main river itself, although the Mekong studies

figure of 125 kg/ha for a non-flowing backwater is similar to that in the main river. In the large

Alabama Tombigbee river the ichthyomass in a backwater sampled by Swingle (1954) was much

higher at 2 084 kg/ha, than either deep or shallow areas of the main stream with 457 and 570 kg/ha

respectively.

Standing Waters of Floodplains

Populations of fish in the floodplain lakes of the Danube are of similar densities to those of the

backwaters. Balon (1967) in his sample of 13 water bodies, found between 335 to 318 632 fish/ha.

The composition of the population varied considerably from one sample to another. The ichthyomass

also had a wide range of variability; 230.4± 277.6 kg/ha. It was lowest during high water (136 ± 127

kg/ha in July) and highest during low water (480 ± 334 kg/ha). Other observations from seasonal

water bodies of the Danube floodplain by Holcik et al. (1981) gave a mean of 366.7 ± 293.6 kg/ha. In

the Russian Nyamunas River the floodplain is inundated by the spring floods which bring large

quantities of fish into the oxbows and artificial channels. These are isolated during the summer and

Gaygalas and Blatneve (1971) estimated that about 200 kg/ha of fish remained in the autumn. As

about twice this number were caught by anglers the total spring biomass was probably closer to 500

kg/ha. The floodplain pools of the Canadian Irvine Creek System described by Halek and Balon

(1983) supported standing stocks of 132.6 ± 57.7 kg/ha. Much of the variation between pools was

217

attributed to the duration of the connection between the pools and the main river. Pools in which this

connection persisted for some time had greater density of young of the year fish and hence higher

mean biomass.

In tropical rivers standing stocks of floodplain pools tend to be somewhat higher although on some

floodplains, such as the Magdalena and Mekong, there is a considerable movement out of the

depressions toward the main river channel which may account for the low values obtained from such

system. Table 6.11 summarizes the dry season ichthyomass from the standing waters of a sample of

tropical rivers. Ichthyomass apparently varies according to a number of factors. As with other parts of

the aquatic system, population densities are normally greater during the dry season. Thus when

Kapetsky et al. (1976) estimated a minimum biomass of between 0.23 and 251 kg/ha (mean 55.7

kg/ha) as being present in the open water and between 20 and 232 kg/ha (mean 79.8 kg/ha) in the bay

areas of the cienagas of the Magdalena floodplain he also found a good negative correlation between

the population density and the water level in the cienagas. Strangely enough, whilst the University of

Michigan et al. (1971) also found population densities to be higher in the open waters of the lagoons

of the Kafue river during the dry season (426 kg/ha) than during the floods (337 kg/ha), vegetated

areas showed the opposite tendency and very high concentrations of fish (up to 2 682 kg/ha) were

present in such areas at high water. High densities of fish under floating vegetation seem to be

comparably rare, however, and permanent swamps, particularly those under papyrus, are notorious for

the poverty of their fish communities. This situation may change temporarily during the flood when

currents can oxygenate the water column under the plants. The University of Michigan study also

indicated the loss of ichthyomass during the dry season when in one pool an initial standing stock of 2

693 kg/ha diminished by 75% to 684 kg/ha in three months. In a second pool a similar decline from 3

306 to 501 kg/ha occurred in 10 weeks, although in this case the pool was connected to the river for

part of the time and emigration might have occurred. Bonetto (1980a) studied a population of

Prochilodus platensis which colonized the Laguna Gonzalez when it was flooded by the Riachuelo

River in 1971/72. The lagoon subsequently remained isolated from the river and he was thus able to

trace a single year class through a number of years by extrapolation from mark-recapture experiments.

The isolation of the population from other year classes of Prochilodus undoubtedly gives a rather

distorted picture of what happens in more normal lagoons. At the time of separation from the river the

fish were probably about 1 year old and, perhaps because of the limitation or food availability, the fish

grew to a relatively small terminal length of 36 cm, as opposed to 60 cm in lagoons connected to the

river. The standing stock increased from 232 kg/ha in the first year to 351 kg/ha in the second and

decreased thereafter to 199 kg/ha in 1976 when observations ceased.

The form of the lagoon and the nature of the bottom may also influence ichthyomass. In the Senegal

river, long narrow pools formed from isolated drainage channels supported a much higher standing

stock than round depression pools, 205 ± 155 kg/ha as against 13 ± 6 kg/ha (Reizer, 1974). In the

Sokoto, Holden's (1963) analysis shows that a greater proportion of fish preferred intermediate

sand/mud bottoms (1 012 kg/ha) as opposed to sand (785 kg/ha) or mud (233 kg/ha).

Differences in standing stock have been related to the degree of organic fertilization of the water body

by Fox (1976). His estimates for small unenriched pools in the southern Okavango delta showed

between 100–200 kg/ha to be usual, whereas a highly enriched peripheral lagoon had the highest

estimate of 700 kg/ha. While this evidence is far from conclusive due to the small sample size it

would appear reasonable that eutrophicated waters should support higher densities of fish relative to

those less rich in nutrients. Nutrient poor black waters on the other hand, appear to support lower

standing stocks. For example, eight floodplain pools from the Rio Pacaya and Rio Saniria tributaries

of the Peruvian Amazon gave 89.6 ± 37.1 kg/ha (Montreuil pers.com.). However, there is still

insufficient data available to attempt drawing some relationship between nutrient content and standing

stock of fish in rivers.

Table 6.11

218

Estimates of dry season ichthyomass in pools and lagoons of the floodplains of

some tropical rivers

River Size of

sample Estimated

ichthyomass kg/ha Author

Apure 1 982 Mago-Leccia, 1970

Candaba - 500–700 Delmendo, 1969

Chari 2 701–2 166 Loubens, 1969

Kafue 8

444 University of Michigan et al.,

1971

Magdalena 28 122 Kapetsky et al., 1977

Mekong 1 63 Sidthimunka, 1970

390

Mekong Fish. Studies

(pers.comm.)

Mogi Guassu 1 313 Gomez and Montiero, 1955

Nile Sudd 2 306–433 Mefit-Babtie, 1983

Okavango 2 200–700 Fox, 1976

Oueme 68 1 835±825 FAO/UN, 1971

Parana

Temporary

lakes

18 959±1 512 Bonetto, 1976

Permanent

lakes

4 766±348

Sabaki 1 786 Whitehead, 1960

Senegal 8 110±144 Reizer, 1974

Sokoto 25 661±557 Holden, 1963

Standing stocks may also be influenced by the preceding flood history of the river, a topic treated in

more detail under “Catch”. An example of such variability comes from the Atchafalaya distributary of

the lower Mississippi where Bryan and Sabins (1979) recorded a lowering of standing crop of 795 kg

ha in 1977 from 904 kg ha in 1975–76. The higher estimate comes from years preceded by good

spring floods (1973–75) whereas the lower estimate comes from years preceded by poor floods

(1976–77).

Total System

Studies analysing the contribution of the various components of a riverine system to the total standing

stock are rare. However, one such study was carried out by the University of Michigan et al., (1971)

who extrapolated rather widely from a limited number of samples of four different habitats of the

Kafue River and floodplain. Their results are shown in Table 6.12.

A similar analysis was carried out by Holcik et al. (1981) for the Middle reaches of the Danube River

with results as shown in Table 6.13.

Table 6.12

Summary of high water and low water estimates of ichthyomass from the Kafue

River and floodplain system

Area (ha²) Ichthyomass

(kg/ha)

Total

ichthyomass

(t)

High water

Open water lagoon 126 000 337 42 462

Vegetated lagoon 16 000 2 682 42 912

219

Grass marsh 136 000 64 8 704

River channel 5 300 337 1 786

Total 283 300 338.4 95 864

Low water

River channel 4 800 204 959

Open water lagoon 113 000 426 48 138

Vegetated lagoon 14 000 592 8 288

Total 131 800 435.6 57 405

Table 6.13

Present status of ichthyomass in the Danube between the mouth of the Morava

and Ipel rivers (from Holcik et al., 1981)

Area (ha²) Ichthyomass (kg/ha) Total ichthyomass (t)

Main channel 7 936.9 35.0 277.8

Backwaters

Czechoslovak bank 1 777.9 350.0 622.3

Hungarian bank 1 336.0 400.0 534.4

Both banks 3 113.9 371.5 1 156.5

Total 11 050.8 129.76 1 434.3

A general estimate of ichthyomass has also been made for Atchafalaya River Louisiana from samples

both in the main channel and some floodplain lakes. Here a mean standing crop equivalent to 860

kg/ha was calculated for the lower part of the basin and 555 kg/ha for the upper part (Bryan and

Sabins, 1979).

PRODUCTION

ESTIMATES OF PRODUCTION

Production can be defined as the total elaboration of fish tissue during any specified period, including

gonadial products and material formed by individuals that do not survive to the end of the period

(Ivlev, 1966). Standing stock and production estimates for a number of rivers are given in Table 6.14.

Estimates of production range from a maximum of 2 000 kg/ha/yr in the Thames to as little as 16.2

kg/ha/yr in the Speed River but have a mean of 218.6±335.6 kg/ha/yr. Several factors may influence

production among which the latitude, the basic productivity of the waters and the age structure of the

population are perhaps the most important.

Table 6.14

A selection of data on total biomass and production of fish in rivers

River Country Biomass

kg/ha Production

kg/ha/yr P:B Authority

Amazon Manaus Brazil 1600.0 2800.0 1.75 Bayley (1983)

Thames

(0+fish) UK 1315.0 2000.0 1.52 Mann (1972)

(1+fish) UK 659.0 426.0 0.65 Mann (1965)

Hinaki N.

Zealand

880.8 735.4 0.82

Hopkins (1971)

Kafue Zambia 520.0 618.0 1.19 Kapetsky (1974)

220

Ellis Canada 376.0 375.4 1.0 Mahon (1981)

Warkosc Poland 307.5 161.4 0.5 Mahon (1981)

Carrol Canada 274.8 280.3 1.0 Mahon (1981)

Hopewell Canada 228.4 263.5 1.0 Mahon (1981)

Hinau II N.

Zealand

217.7 241.7 1.11

Hopkins (1971)

Lemki USA 212.0 136.0 0.64

Goodnight and Bjornn

(1971)

Jaruma Spain 178–221 221–583 1.2–2.6 Lobon-Cervia and Penczak

(1984)

Tarrant UK 198.0 596.0 3.01 Mann (1971)

Kejin Borneo 173.1 261.5 1.50 Watson and Balon (1984)

Bere UK 161.7 270.0 1.69 Mann (1971)

Irvine Canada 149.9 226.4 1.50 Mahon (1981)

Hall UK 129.0 52.0 0.40 LeCren (1969)

Swann Canada 124.4 174.5 1.40 Mahon (1981)

Struga Poland 111.2 110.3 1.0 Mahon (1981)

Deer USA 84.7 160.0 1.89 Chapman (1965)

Utrata Poland 84.5 289.3 3.4 Penczak (1981)

Big Springs USA 84.2 118.0 1.4

Goodnight and Bjornn

(1971)

Mesta Bulgaria 80+82 80.0 1.0 Penczak et al. (1984)

Dockens UK 75.0 140.0 1.87 Mann (1971)

Clemons Fork

(3rd 0) USA 71.5 77.2 1.08 Lotrich (1973)

Kejin Borneo 71.0 98.3 1.4 Watson and Balon (1984)

Clemons Fork

(2nd 0) USA 63.6 105.5 1.66 Lotrich (1973)

Appletreeworth UK 62.0 30.0 0.48 LeCren (1969)

Black Brows UK 59.0 100.0 1.69 LeCren (1969)

Clemons Fork

(1st 0) USA 54.9 83.5 1.52 Lotrich (1973)

Bobrza Poland 50.2 83.4 1.7 Mahon (1981)

Needle Branch USA 45.9 90.0 1.96 Chapman (1965)

Zalewka Poland 43.9 54.0 1.23 Penczak (1981)

Kaka Borneo 38.5 47.5 1.2 Watson and Balon (1984)

Wolborka Poland 37.4 35.3 0.94 Penczak et al. (1982)

Speed (3rd Order) Canada 32.3 54.0 1.67 Mahon et al. (1979)

Lava Borneo 30.5 39.3 1.3 Watson and Balon (1984)

Payau Borneo 27.1 31.2 1.2 Watson and Balon (1984)

Lawa Borneo 21.3 33.3 1.6 Watson and Balon (1984)

Bulu Borneo 21.5 26.0 1.2 Watson and Balon (1984)

Hinau I N.

Zealand

19.6 42.8 2.18

Hopkins (1971)

Speed (4th Order) Canada 7.0 16.2 2.31 Mahon et al. (1979)

It is commonly held that tropical waters are more productive than their temperate counterparts, and in

lakes this undoubtedly appears to be the case. Unfortunately few observations exist on the

productivity of tropical waters but those that do exist give little support to this contention. In fact the

range, mean and standard deviation of standing stocks of 17 tropical rivers, 21.4 (154.4±198.7) 861

kg/ha/yr falls entirely within those of standing stocks for 26 temperate rivers, 7.0 (205.0±300.3) 1315

kg/ha/yr.

221

There is as yet equally little evidence for the lower productivity of rivers with lower ionic content,

partly because so many other factors appear to influence the characteristics of the stock. However, in

the tropics Watson and Balon's (1984) figure for black water streams from Borneo (conductivity >50

μmhos) is decidedly lower at 179.9 kg/ha/yr than Kapetsky's (1974) estimate of 678 kg/ha/yr for the

richer Kafue (conductivity 130 – 320 μ mhos).

The influence of the proportion of juveniles in the stock is much clearer and Table 6.15 summarizes

the percentage of total productivity contributed by various year classes in some fish species.

Table 6.15

Percentage contribution to total production by year class in some species of river

fish

Species Percentage products by year

class Author

0+ I II III IV+

Cottus cognatus 65.1 20.0 12.2 2.5 0.2 Petrovsky and Waters, 1975

Salvelinus fontinalis 40.8 41.0 14.7 3.1 0.4 Dept. of Natural Resources,

1974

Alburnus alburnus 46.2 10.1 20.7 17.0 5.9 Matthews, 1971

Cottus gobio 32.0 18.0 27.0 20.0 3.0 Matthews, 1971

Oreochromis

andersoni 61.2 22.8 12.2 2.2 1.6 Kapetsky, 1974

Oreochromis

macrochir 46.4 26.3 12.7 7.6 7.0 Kapetsky, 1974

Theoretically there should be a smooth relationship such as that shown by S. fontinalis or C. cognatus

where data was averaged over several years, but differences in year class strength due to differential

success in recruitment, such as is shown by A. alburnus or R. rutilus may cause variations in overall

productivity from year to year. Because of the great significance of juvenile fish to the total

productivity, the efficiency of sampling for the younger year classes is of great importance and

differences in population estimates may arise according to the degree to which the smaller fish are

included in the sample. That year to year variations do occur in some systems is a matter of record but

others show a remarkable constancy in productivity despite considerable changes in biomass. For

example, the production of Salvelinus fontinalis varied by less than 20% around a mean of 117 kg/ha

over 11 years of sampling in Lawrence Creek, Wisconsin. During the same period the biomass

underwent almost threefold fluctuations from 13.5 to 99 kg/ha. This means that the efficiency of

production was in this case strongly density dependent and a regression of P/B against biomass gave a

significantly high correlation coefficient (Dept. of Natural Resources, 1974). From Table 6.14 it may

be seen that, whereas standing stocks tend to be lower in lower order streams than in those of higher

order, the productivity tends to increase resulting in higher P/B ratios. This high productivity of the

rhithron, especially in the riffles, may be accounted for in part by the larger numbers of juvenile fish

in such areas (Schlosser, 1982). Production/Biomass ratios are generally useful indicators of the

rapidity of turnover of biomass and, while productivity is strongly correlated to standing stock:

variations do occur for a variety of reasons. As mentioned above the population density may influence

the rate of production as also might the proportion of juvenile fish in the stock and a further factor that

has been implicated is species composition. This latter may operate in two ways, firstly by the

longevity of the species and secondly by its trophic status. The contention that short lived species

have greater productivities (Leveque et al., 1977) cannot be verified from the present data set,

although it is to be anticipated that communities composed of small short lived species will have

greater productions than communities of larger species inhabiting the same body of water. For similar

reasons it may be supposed that predatory species, partly because of their generally large size and

partly because of their position in the food chain, would have relatively lower productivities. In this

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case the mean value for P/B for 13 observations on 7 predatory species was much lower at 0.54±0.16

than the value of 1.2±0.5 for 95 observations on 37 species of all trophic categories and from both

tropical and temperate waters. The difficulties inherent in this type of analysis are illustrated by the

very wide range of P/B values obtained for some species; S. fontinalis, for instance, have been

recorded as having ratios as low as 0.4 (LeCren, 1969) and as high as 3.0 (Mann, 1971).

P = 3.3 + 1.2 B (r² = 0.85 for the data listed in Table 6.14)

MODELS OF STANDING STOCK AND FISH PRODUCTION IN RIVERS

Several approaches have been used to study and estimate standing stocks in rivers. These vary

according to the type of river in question, mainly because of difference in the ease with which various

kinds of waters can be sampled. In general the difficulties of sampling large streams, usually defined

as any water deeper than wading depth, and the large numbers of watercourses dictate the use of some

type of predictive model, usually an index. Obviously such models have to be based on a certain

number of estimates from the field and have to be verified from time to time in actual rivers. A second

approach is through simulations based on theoretical concepts of the ecology of the fish community.

Simple models based upon environmental characteristics

Several attempts have been made at relating some characteristic of the fish community to the

environment in which it lives. Usually such models or indices have been derived for practical

purposes of fisheries management and it is not always clear exactly what is being predicted. For

example, models will often claim to predict production when it is in fact catch that is meant and there

is therefore some overlap between models dealt with in this section and those described in the section

on appraisal of catch.

During their studies on the fishery resources of the rivers of Belgium and Northern France Leger, and

later Huet, were able to formulate some generalizations as to the zonification of the running waters of

temperate Europe. Arising from this Huet (1949 and 1964) proposed a simple model:

P = BLK

to assess the approximate ichthyomass available for exploitation in temperate rivers. In this model P =

annual productivity (or harvest) of the water in kg/km of river, L = average width of the river, B =

'biogenic capacity' and K = coefficient of productivity. Values between 1 and 10 are assigned to B

according to the amount of fish food (benthos) available. The coefficient K is the sum of K1, K2 and

K3 where the value of K1 is derived from mean annual temperature, K2 depends on the acidity of the

water and K3 summarizes the type of fish population.

This original method was modified by Lassleben (1977) for the assessment of fish catch from the

German stretch of the Danube (Kolbing, 1978). In this case fish catch (C) for a section of river is

determined as:

C = BK 10 kg/ha/yr

where B and the various components of K are as above except for K3 whose value may be derived

from a simple equation establishing the proportion of each type of species present. This method has

given good results in north temperate streams and were extended to larger rivers such as the Danube

by Holcik (1979) using the simple percentage of rheophilic and limnophilic species. The biogenic

capacity of the river may be assessed using the biomass of benthic invertebrates instead of the

quantity of aquatic vegetation as used in the other formulae. According to Albrecht (1953 and 1959)

streams with a biomass of less than 60 kg/ha can be considered as poor, those with a benthic biomass

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of 60 to 300 kg/ha as medium and streams with 300 to 700 kg/ha good. Binns and Eisermann (1979)

followed a similar approach in developing a Habitat Quality Index to predict trout standing crops in

streams in Wyoming by deriving two models which were moderately easy to estimate. Their most

successful model was the expression:

logY+1 = [(-0.903) + 0.807 log(X1+1) + 0.877 log(X2+1) + 1.233 log(X3+1) + 0.631 log(F+1) + 0.182

log(S+1)] [1.12085]

where

Y = predicted trout standing crop

X1 = late summer stream flow

X2 = annual stream flow variation

X3 = maximum summer stream temperature

F = food index consisting of the product of;

X4 = nitrate nitrogen

X9 = substrate type

X10 = water velocity

S = shelter index consisting of;

X7 = cover

X8 = eroding stream banks

X11 = stream width

The various parameters were rated on a scale of 1 to 5 according to a simple table. This model, which

works well for the trout streams for which it was developed, illustrates the complexity of models of

this type. It also indicates the number of factors which may influence standing stocks and

productivity, as well as the difficulties of forming reliable estimates on the basis of any one factor.

This approach has been further elaborated by other workers in the United States using Habitat

Suitability Indices based upon a wide range of parameters. These are usually species specific (Rabern,

1984) and are based on a thorough understanding of the ecological requirements of the species

concerned as well as detailed information on a large number of parameters.

Models of dynamics of fish populations in floodplain rivers

Models of population dynamics and production derived for lakes may well be applied to rivers which

have stable hydrological regimes, such as reservoir rivers or those whose flow has been modified by

man. The situation is, however, very different in rivers having a seasonal fluctuation in water level

where the generally accepted simple exponential models of mortality and growth are generally

inconsistent with what is known of the biology of floodplain fishes. They are thus inadequate to

derive estimates of within the year changes in biomass and production. The catch that can be expected

from flood rivers is most probably linked to the excess of ichthyomass produced in the flood over that

which can be supported in the dry season. A more detailed knowledge of within the year changes in

these parameters is necessary for the management of both the fish stock and the hydrological regime.

Alternative models based on the assumptions of rapid growth and low mortality during the flood, and

low growth and high mortality during the dry season, have been proposed. Kapetsky (1974)

contrasted the production derived from standard models of growth and mortality with linear and

floodplain models of these parameters, and found that production estimates for age III fish onwards

gave very different results according to which model was used. The floodplain model combines

number:

Nt = No - Noexp - zT[(exp Zt) - 1]

and mean weight

(Wt = W0 + W1 exp gT [(-1-(exp gt)]

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Bt = NtWte.

It gives estimates for production of up to 4.4 times that of the simple exponential models and twice

that of the linear model. The reason for this is shown in the differences between the theoretical

biomass-time curves for age group 0 fish which are plotted in Fig. 6.14 from the simple exponential

(a) and floodplain (b) models for the two values of z and a growth rate (G) = 5.3.

Figure 6.14 Theoretical biomass-time curves for age group 0 fish during one year when calculated

from (a) simple exponential expressions for growth and mortality, and (b) “floodplain”

expressions

Daget and Ecoutin (1976) also derived a model of biomass and production based on their growth

equation for fish which have an annual growth arrest. Biomass is calculated from Bt=NtWt where:

Wt = 0.68 10-5 Lt³

Lt is derived from the formulae discussed in the section on growth and Nt is derived from the simple

exponential mortality equation:

Wt = N0e-zt

The model was applied to Polypterus senegalus, giving good comparability with observed results.

Estimates of production ranged from 528.5 kg/ha/yr (P/B=0.56) to 281.6 kg/ha/yr (P/B=1.12) for

mortality coefficients z=0.04 and z=0.10 respectively. This illustrates the importance of correctly

estimating mortality rate in deriving production and biomass values. The use of a constant mortality

factor in this model is somewhat limiting and the abrupt termination of growth produces a rather

sharply peaked annual biomass curve (Fig. 6.15) which does not appear to reflect the natural situation.

A more detailed simulation of the dynamics of a floodplain fish community was proposed by

Welcome and Hagborg (1977) based on a combination of their growth and mortality models. By

introducing density dependent mortality rates, and growth and recruitment which were dependent on

the intensity of the simulated flood model enabled the exploration of the effects on a theoretical fish

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community of changes in high and low water components of the hydrological regime. The simulation

gave a series of curves for total ichthyomass derived from hydrological regimes in which either the

flood intensity or the amount of water remaining in the system in the dry season could be varied

independently of one another (Fig. 6.16). It indicated that differences in the flood regimes produce

great differences in within the year ichthyomass whereas the magnitude of the population passing

through to the following year largely depends on the amount of water remaining in the system at low

water. They also indicated that the greater amount of water remaining in the system at low water the

more the differences induced by the high water regime are transmitted to the following years. The

curves in Fig. 6.16 were combined into a three dimensional plot of mean ichthyomass for different

high and low water regimes (Fig. 6.17). Values for fish production per mean flooded area derived

from this simulation agree well with the general findings on flood rivers ranging from 241 kg/ha/yr to

564 kg/ha/yr depending on flood regime with P/B values of between 1.35 and 1.77. Production and

Biomass are both maximal in the higher floods but the ratio between them increases as the maximum

area flooded decreases. This model, appropriately modified to the growth and mortality of local

Oreochromis stocks, has been applied to Lakes Alaotra and Ihotry in Madagascar (Moreau, 1980,

1982). Here the area under water in the dry season was 22 000 ha, whereas in the wet season the water

flooded 57 000 ha. Apart from some differences in the numbers of fish the predictions of the model

agreed closely with results obtained from experimental studies.

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Figure 6.15 (A) Variations in ichthyomass of one cohort as a function of age for three different

mortality coefficients; (B) Ichthyomass of population for three different mortality coefficients,

assuming the same recruitment each year and the accumulation of cohorts. (After Daget and

Ecoutin, 1976)

227

Figure 6.16 Computer generated curves showing changes in total ichthyomass with time for different

flood regimes where (A) the low water regime is constant and the high water regime varies;

and (B) the high water regime is constant and the low water regime varies. Also shown is a

typical water regime (----). (After Welcomme and Hagborg, 1977)

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These three models indicate a similar evolution of biomass throughout the year with a convex

ichthyomass-time curve. According to this there is an initial rapid increase in ichthyomass which

attains a maximum at about bankfull on the declining flood and which would seem to represent the

natural state fairly accurately. The models equally emphasise the great importance of the 0+ age group

which contribute up to 80% of the total ichthyomass depending on the mortality rate. Both the shape

of the biomass-time curve and the great preponderance of juvenile fish have important implications

for the management of the fish community for fisheries. The variations in production and ichthyomass

corresponding to differences in flood regimes predicted by the simulation suggest that equivalent year

to -year variations occur in the fish populations themselves. From records in many systems it is

known that the catch does indeed fluctuate in this manner, and so presumably do production and

ichthyomass.

229

Figure 6.17 Computer generated curves showing changes in population density (kg/ha) with time for

different flood regimes where: (A) the low water regime is constant and the high water regime

varies; and (B) the high water regime is constant and the low water regime varies. Also shown

is a typical water regime (----). (After Welcomme and Hagborg, 1977)

Curves plotted from simulation for ichthyomass per unit area indicate that the population is very

dispersed during the flood but concentrates rapidly as the water drains from the floodplain until

bankfull (Fig. 6.17). This phase is followed by a steady attrition which reflects the real situation

observed by Daget et al. (1973) in the Bandama and the University of Michigan et al. (1971) in the

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Kafue River. Changes in the ichthyomass per unit area are also evident from the seasonality of the

fisheries which are at their most intense during those periods when the fish are concentrated in the late

falling flood and the dry season.

CHAPTER 7

THE FISHERY

THE FISHERMEN

All fisheries are to a great extent shaped by the nature of the environment and the characteristics of

the fish stocks they exploit. The factors influencing fisheries in rivers are very different from those

shaping lacustrine and marine fisheries. Three factors in particular give riverine fisheries their

character, diffuseness in space, seasonality and diversity. Because these are common to the vast

majority of the world's rivers there is a remarkable parallelism in the general form of riverine fisheries

and the communities that exploit them. This is subject, of course, to certain modifications according

to the general state of development of the countries in which fishery is practiced.

Rivers are lineaform and of limited width so the total area of water that can be reached from any point

on the bank is limited by the capacity to move up- or downstream. In most temperate rivers

commercial fisheries are usually motorized and the developed road networks make access relatively

easy. In many of the tropical rivers movement is by hand propelled canoe, although outboard motors

are also used. Whatever the mode of propulsion it is rare for there to be an expanse of water available

to one landing sufficiently large to support a capture and marketing operation of any size. In many

regions the problems posed by the linear form of rivers are aggravated by the swampy and changeable

nature of the terrain, which, together with the periodic submergence of any flanking floodplains,

hinders the installation and maintenance of permanent roads and other forms of communication. In

most areas of the world spatial dispersion and inaccessibility have combined to make fisheries labour

intensive, artisanal operations which are located in a series of small settlements along the river

channels or spread over the floodplain on islands of higher elevation. There are some obvious

exceptions to this general principle in estuarine 10/12/0waters, large floodplains or lakes and

reservoirs associated with rivers where the available area of water is greater.

In flood rivers the seasonal expansion and contraction of the water area can separate settlements from

the main course of the river by many kilometres in the dry season even though they are at the water’s

edge during the flood. Coupled to the hydrological cycle are the migrations of fish within the system

which causes the stock to undergo changes in abundance density and location. In response to the

fluctuations in the ecosystem fishermen either have to alternate with other occupations or they

themselves have to migrate. Because of this it is very difficult to define accurately who is, or is not, a

fisherman. In some systems the situation is fairly clear, as fishing is the task of one particular ethnic

group or tribe, and practically the whole of the active population of such a group may be interpreted

as being involved in the fishery in some way. In such cases the fishing group often does not own land

and coexists with other groups equally specialized in agriculture or pastoralism. In other rivers, the

riparian population remains relatively unspecialized as a whole, although sub-groups may concentrate

on one or more of the specialized activities including fishing. Within the fishing community there is

usually a well defined division of labour. The men fish, construct and maintain the gear and build the

231

boats, whilst the women collect, treat and market the produce. In such populations most able bodied

male, and sometimes female, individuals fish at some time or other. Furthermore, the role of small

pre-adolescent boys in many artisanal fisheries should not be underestimated as they act as fisheries

aids. In this capacity they paddle canoes, bait and control long lines, lift traps and generally keep the

fishery going when the adult members of the community may be occupied elsewhere.

For the purposes of convenience fishermen may be classified into three categories on the basis of the

time they spend fishing (FAO/UN, 1962). These are:

(i) occasional fishermen;

(ii) part-time fishermen;

(iii) full-time fishermen.

Such classification refers mainly to food fisheries in the tropics and sub-tropics and excludes such

special categories as temperate zone recreational fishermen.

OCCASIONAL FISHERMEN

In contrast to lakes, where a boat is usually essential to reach fishing grounds and to operate gear,

many river channels and floodplain waters can be reached on foot during part of the year at least and

fished by simple apparatus from the bank or by wading. This relative ease of access for the inhabitants

of floodplains coupled with a certain amount of free time between the sowing and the harvest of

floodplain crops means that casual fishing for subsistence is popular in floodplain communities.

Furthermore, in many areas of Africa and Asia, certain depression lakes or sectors of the river and

floodplain are traditionally reserved for the inhabitants of particular villages. These are fished during

festivals or fish drives which take on all the aspects of a holiday and in which all members of the

community participate. Most of the fish caught enters directly into the diet of the fishing community.

The individual time spent is low and for the most part the gear used is simple and relatively

unproductive. On the other hand the numbers participating are often very high. It is therefore difficult

to assess the contribution of such efforts to the total catch of any particular system.

PART-TIME FISHERMEN

Many sedentary peoples living on floodplains fish during part of the year. This is an activity that is

co-equal to or inferior to the alternative activities of such populations. The flood cycle, the biological

cycle of the fish, and the seasonal needs of agriculture impose a cyclicity on such communities.

During the floods there is very little activity in either domain, but as the waters drain from the plain,

fishing increases. As the floodplain dries the preparation of the soil and sowing of the seeds take

priority, to be followed with a second burst of fishing at low water. Harvesting of the crops follows

and the cycle repeats itself year by year. It is perhaps not surprising that, while part-time fishermen

use most of the types of gear used by professional fishermen, they also have a tendency to practice

various types of extensive aquaculture techniques. For example, the development of drain-in fish

ponds is associated with a certain agricultural type of land and water tenure in countries as widely

separated as Cambodia (Chevey and Le Poulain, 1940) and Benin (Hurault, 1965). Similarly, the

association of drain in ponds with paddy fields is a feature of many rice growing communities (Tang

Cheng Eng et al., 1973).

PROFESSIONAL FISHERMEN

In most aquatic systems there are groups of individuals that live entirely by fishing. The need for year

round employment and the movements of the fish stocks often force such groups to be nomadic.

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Migratory fishermen have been noted from many systems and are particularly a feature of river

fisheries. For example, Bhuiyan (1959) described the migrations of the Hilsa fishermen of the Indus

who followed the movements of that species up and down the river. The fishermen of the Kafue

similarly move around depending on the water level and abundance of the fish stocks (Everett, 1974).

In the Magdalena fishery the fishermen move from the cienagas where they fish in the dry season, to

intercept the “Subienda” migration in the river (Bazigos et al., 1977). Some of these migrations by

fishermen can be of very large proportions. The historical upstream movements of the Haoussa in the

Niger River (Daget in FAO, 1962) might reasonably be compared to the operations of a factory

trawler in oceanic fisheries when the size of the investment relative to the per capita income of the

community is taken into account. This movement, which may be taken as fairly typical, involved

whole families of 20–30 persons who moved upstream in September and October to fish the northerly

portions of the Central Delta on the falling flood. The distance moved was in excess of 1 000 km. The

main vessels, which were often up to 15 m long and were equipped to support whole families on the

journey, were accompanied by a flotilla of small craft which were used in the actual fishing. On the

return journey, which was made on the next flood, the main vessels were loaded with many tons of

fish for sale in the Nigerian markets. These northern parts of the Central Delta are far from the main

centres of population, and have been exploited by wandering fishermen of the Haoussa and Bozo

tribes for many centuries. The incidence of nomadism therefore increases from the southern parts of

the basin where only two percent of the population are mobile to 52 percent in the heart of the lake

district (Raimondo, 1975).

Not all nomadic fishermen undertake such long journeys, but most have to leave their native villages

during at least part of the annual cycle. Many fishermen construct temporary 10/12/01fishing camps

on high ground within the floodplains which they occupy during the fishing season and which they

move following changes in the river level. To encourage this, it has been proposed that artificial

islands be constructed on some floodplains, especially the Kafue and the Sudan Sudd, to enable the

fishermen to exploit areas that have so far remained undeveloped for lack of suitable living space. In

some flooded areas, such as the Mesopotamia area of the Tigris-Euphrates system or the Ganges delta

in Bangladesh, the rural community including fishermen and their families already live on islands

made of soil and domestic refuse accumulated over the years. As an alternative many fishing

communities have developed houses on stilts which remain above all but the highest floods. These are

typical of the lowland rivers of the West African coast and of Asia. In Asia, too, some fishermen live,

at least temporarily, on boats or rafts which are associated with traps, stownets or lift nets in the river

channel (see Fig. 7.1).

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Figure 7.1 Weighted average monthly landings related to mean water level of the Yamuna River at

Agra during 1968–69 (After Wishard, 1978)

Professional fishermen use a vast range of fishing gear, but in recent years have tended to concentrate

on one or two of the fishing methods based on modern materials such as seine nets, gillnets or cast

nets. Such fishermen, too, use powered craft to a great extent, if not for fishing then for the transport

of fish from the fishing grounds to the markets.

In recent years long distance nomadism has been discouraged in many parts of the world as this

practice has involved the movements of the nationals of one country into the waters of another. With

increasing awareness of the potentialities of their inland fisheries most countries have felt the need to

retain these benefits for their own nations and have trained local people accordingly. Many of the

traditional movements, such as that described for the Niger, have therefore broken down and in their

place shorter local migrations have intensified.

Most artisanal fishing communities have developed elaborate traditions, legends and even religious

systems which enable them to integrate culturally with the general ecology of the river on which they

live. Parallel institutions are found, not only among tropical communities, but also in many early

European and North American temperate peoples. Such studies as those of Smith (1981) or the many

papers included in Gunda (1984) indicate the rich folklore associated with such communities, many

elements of which serve genuinely conservationist ends. Such traditional mechanisms for regulating

the fishery include the definition of closed seasons, the delimitation of refuge areas or limitation on

access to the fishery. Such traditional institutions are fragile and disappear under pressure from more

modern political and socio-economic systems. For example, fishing is often seen as a kind of relief

occupation from the chronic unemployment and land hunger of much of the tropical world. The influx

of new and inexperienced fishermen into the traditional fishing communities results in their

disruption. Unfortunately no new institutions arise to replace the traditional management of the

fishery and the resulting anarchy may harm not only the fishing communities but also the fishery

resources on which they depend.

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FISHING GEAR

SEASONALITY

In stable water bodies, including reservoir rivers and the larger permanent water bodies associated

with river systems, fisheries are pursued around the year. In cold temperate zones fishing may be

intensified in such rivers during the otherwise unfavourable cold season because of the greater activity

of salmonids at such times and in some areas fishing even continues under the ice cover. In flood

rivers, on the other hand, fish are vulnerable to the fishery for only part of the year when their

accessibility depends much on the ichthyomass per unit area as represented in Fig. 6.17 or during

temporary concentrations of migrating fish. During high water fish are dispersed over the floodplain

and concealed by mats of vegetation. The bulk of the population are juveniles, often too small to be

captured by any practical gear. Physical conditions are also unfavourable to the fishery as strong

currents sweep down the main channel and out over the plain, carrying with them logs and floating

masses of vegetation that carry away any fixed engines. As a consequence there is a slackening or

complete absence of fishing during the flood season in most river systems, although some gears such

as the ‘Atalla’ lift net of the lower Niger river, are operated specifically at such times to catch small

species which have taken refuge in vegetation fringing the main channel or juvenile fish which are

concentrated in the shallow margins of the lagoons (Awachie and Walson, 1978). Fishermen may also

continue to be active in some of the larger floodplain lakes where conditions are more stable or in the

flooded forest where specific fisheries for frugivorous fishes take place (Goulding, 1981). In older and

more tradition bound fisheries this cessation of activity is often reinforced by laws or taboos which

close the fishery to some or all gears. Many such traditional bans seem to have arisen from the

recognition of the flood season as a period of breeding essential for the continuation of the stock.

Fish become more available for capture as they congregate in the channels and pools of the floodplain

as the water begins draining off the main channel. At this time there is a very heavy and concentrated

fishery. Intensive fishing continues throughout the dry season both in the standing waters of the

floodplain and in the main river channel. Later as the water begins to rise again fairly specialized

fisheries concentrate on the adult fish migrating to their breeding sites. In most river systems this

sequence gives a pattern of capture whereby the efficiency of capture is inversely proportional to the

strength of the flood or is related to water level by more complex relationships such as those described

by Wishard (1976) for the fisheries of the Yamuna river at Agra (Fig. 7.1).

CAPTURE METHODS

The types of fishing method chosen for use in rivers are conditioned by three factors; first, the nature

of the fish stock, second, the form of the river and third the degree of development of the fishing

community. As has been described the fish communities of tropical and sub-tropical rivers are

particularly diverse containing a large number of species, most of which differ to some degree in their

selection of habitat, diet, migration pattern and ease of capture. They are also represented by an age

structure which is more than usually biased toward juvenile fish. The fishermen have, therefore, to

establish priorities for their fishery with respect to which species and age classes they attempt to

capture. Preference is often imposed by local food taboos or customs, although there are a few

fisheries which have been founded on the export of species which are not locally accepted items of

diet. In temperate climates the diversity of the community is reduced but, even in complex fisheries

such as that of the Mississippi, a few species only are selected as forming the basis for capture. In

some tropical fisheries too, particularly those at an early stage of development, only the larger species

are captured and in such restricted fisheries the range of gear in use is quite limited. In many other

fisheries, however, the fishery exploits all species that are catchable and so a great number of fishing

methods have evolved. Until recently, when cotton and later nylon twine and nets were introduced to

river fisheries, gear was constructed uniquely from local materials. Roots, vines, plant fibres, leaves,

stems etc. have all been, and still are, used for much of the gear encountered in rivers and floodplains.

The similarity in comportment of the different elements of the fish communities and the raw materials

235

of which the gear is made, have led to a considerable parallelism in most methods, even though they

have clearly been developed independently.

There are a number of local variations, and these are described in the many catalogues of fishing

methods in various countries such as Chevey and Le Poulain (1940) for the Mekong, Ahmad (1956)

for the waters of Bangladesh or Blache and Miton (1962) for the Chari/Lake Chad basin. A detailed

description of all gear used in river fisheries is inappropriate here so this chapter will examine the

general principles of the capture methods used in as far as they are relevant to the broad ecology of

these systems. Clearer descriptions of artisanal and other fishing gears, classified by type are available

elsewhere, for instance in Andreev's (1966) “Handbook of fishing gear and its rigging” or in Brandt's

(1984) book on “Fish catching methods of the world”.

While general types of gear used are determined by the above there is also a seasonal selection

depending on the flow regime. In flood rivers this tends to fluctuate during the year leading to the

successive use of a variety of gears. Typical examples of this are the fisheries of the Oueme R. in

Africa (Fig. 7.2) and the Mekong in Asia (Fig. 7.3). Similar examples of differential use of gears

according to flood stage are given by Goulding (1981) and Smith (1981) for rivers of the Amazon

system. In rivers with more constant flows the methods used are more restricted and generally

correspond to the gear types used during the low water phase in flood rivers.

Figure 7.2 Fishing timetables for the Mekong fishery. Thin line = fishing possible; thick line = peak

effort with method. Subsistence methods are not included in the tabulation. (Adapted from Fily

and d'Aubenton, 1966)

236

Figure 7.3 Fish gear utilization in the lower Oueme River during the course of one year

Certain types of fishing gear relying on modern industrially manufactured twines are in widespread

use. These are the seine net, the gillnet and the cast net. In many systems these methods are replacing

much of the traditional gear, and they are especially favoured by professional (full-time) fishermen as

their individual catching power is superior. Their care and maintenance requires an expertise often

lacking amongst the more casual elements of the fishing community. The cast net is in fact one of the

mainstays of these fisheries. Conditions in reservoir rivers and in flood rivers in the floodplain lakes

and the main stream at low water are well suited to its use. The water is fairly shallow, the bottom

unencumbered and the fish are sufficiently concentrated to give a good chance of capture. The mesh

size can be varied according to the species and size of fish sought. Fishermen using cast nets may fish

either individually or in combination with others whose manoeuvres serve to concentrate the fish still

further. The seine net requires relatively large teams to operate it and is very expensive. It is,

therefore, the gear of the professional “par excellence”. Its use is limited by current and especially by

the availability of bottoms which are sufficiently free of obstructions which would otherwise cause

the net to snag. The tenure of suitable seining beaches is often hotly disputed in the fishing

community. As a gear it has several precursors among traditional methods and communal fish drives

often have a seine like approach with lines of fishermen wielding baskets or clap nets. Alternatively a

barrage fence, which is one of the basic items of equipment for the floodplain fisherman, is set in a

large semi-circle and moved inshore in the same way as the seine. The gillnet is also common

although it is sensitive to floating vegetation and will not operate in strong flows. On the other hand, it

does come into its own in the larger floodplain lagoons and lakes where the depth, the large stretches

of open water and lack of current make it one of the most effective gears.

These gears either singly or together dominate certain fisheries. In the Magdalena River the cast net in

the main method, although seines, gillnets, traps and spears are also used to some extent. In recent

years the seine net has become the principal gear in the Senegal River, to the exclusion of many

traditional methods which are still current in the neighbouring Niger River (Reizer, 1974). The Kafue

river fishery also relies almost entirely on the combination of gillnets and seine nets (Everett, 1974).

Catches by these methods vary considerably according to the construction of the gear, the habits of

the fishermen and the time of year.

237

Low Water

During the low water period the majority of fish tend to remain relatively static. Riverine migrant

species which ascend the main course of the river and its tributaries during low water are an exception

to the more general behaviour pattern and form the basis of such fisheries as those of the “Subienda”

and “Piracema” in Latin America, the Alestes fishery of the Chari river, the Khone Falls fishery as

described by Chanthepha (1972), and the “Roak” fishery of the Yamuna river (Wishard, 1976).

Fishing methods tend to be active although some static gear is also used during low water. This is

often baited, for fish are still attracted to baited gear despite the general lowering of feeding intensity.

At this time the use of gillnets, seine nets and cast nets is maximal both in the river and in the lakes

and lagoons, but these gears on their own cannot reach all habitats or catch all species, consequently

many other fishing methods are in use. Hook lines of various types are common in the main river

channel. Longlines, baited with starch paste, offal, small fish, etc., are stretched across the river and

catch mainly the larger predatory species which are often not vulnerable to other gears. Unbaited

snagging and entangling lines are laid in the deeper portions of channels where big fish are

accustomed to rest, and small boys are usually to be seen in most places with a rod or leger line with

which they capture a seemingly endless succession of small fish.

Barrages are common throughout river systems, sometimes taking complex labyrinth-like forms, but

more often simply dividing the main river channel or the smaller drainage canals into sections which

limit fish movement and facilitate their capture by other means. Many types of trap, (Fig. 7.4), are

associated with the barrages where they capture the fish that are milling about trying to pass the

barrier. Traps are also set by themselves amongst vegetation where they attract fish seeking refuge

there. The traps may be unbaited, but they may also be baited to select for certain species. In West

African rivers for instance the same type of cylindric trap catches greater proportions of Clarias when

baited with oil palm fruits, Tilapia/Oreochromis when baited with maize meal and Macrobrachium

when rotting meat is used.

The problem of extracting fish from under the vegetation which fringes lagoons and river channels is

tackled in a number of ways. Special robust hand nets, which may be made out of netting or basket

work (Fig. 7.5), are scraped along the under-surface of the vegetation, or the vegetation mass might be

surrounded by a fish fence. In the latter case the plants are cut out piece by piece and the fence

advanced inwards so as to enclose the fish within a small space from which they can be captured by

hand nets or baskets. In the Oueme River such vegetation masses may be planted deliberately at the

end of the flood, either attached to the bank or recessed into it at the mouth of the channels which

drain the plain. They are left to collect fish for about two months after which they are fished and

replaced to be emptied again toward the end of the dry season. Harvests from such “refuge traps” or

“fish parks” can be quite high and in the Oueme river 15 installations of this type of mean area 440m²

gave a mean harvest equivalent to 1.88 t/ha (of park) per fishing or 3.88 t/ha/yr between 1958 and

1968.

A variety of gears have been developed to fish the main channel, especially the deeper portions where

the larger fish come to rest. Many of these are drawn or propelled by boats. A fairly widespread

device is the frame trawl drawn by one or two boats but the most popular is probably the Vee-shaped

net (Fig. 7.5) which can be mounted in a variety of ways and has great operational flexibility. Armed

with small mesh netting, it can, for instance, skim the surface to catch the small pelagic clupeid,

cyprinid and characin species found there, or be scraped along the bottom to capture many small

bottom-living species. With larger mesh it can also be plunged near the bottom where it drags for

bigger bottom-living species. With larger mesh it can also be plunged near the bottom where it drags

for bigger bottom-living siluroids. Nets similar to this may also be mounted on the bank where they

take migrating fish as they follow the shoreline or move inshore away from the current (Fig 7.6). In

Asian rivers lift nets perform a similar function. They may be mounted on the bank, but are

sometimes operated from rafts on which the fishermen live with their family. Such an apparatus is the

“Sadung” of the middle Mekong, whose operation was described by Fraser (1972).

238

The standing waters of the floodplain are often the property of a particular village or group, which

exploits them communally using clap nets or plunge baskets to virtually empty the water of any living

thing. But many waters are exploited by individuals.

Figure 7.4 Various types of fish trap from tropical rivers: (A) cylindrical drum trap (worldwide); (B)

vertical slit trap (Asia, Bangladesh and Mekong River); (C) folded woven trap (Niger River);

(D) funnel trap (worldwide); (E) spring trap (Africa, Niger, Chari and Congo rivers)

Larger lakes, of course, are fished in much the same manner as the main channel with a full variety of

gear. However, smaller floodplain lakes are usually either poisoned or fished out with fences in much

the same manner as the brush parks. There are many plant toxins which are used to capture fish both

in the lagoons and in pools of the main river channel. Table 7.1 gives some indication of the number

of bio-active plants in the Benue river system alone; although many of the shrubs listed are also found

in other parts of the tropical world, and Gunda (1984) describes an equally large range of plants used

for poisoning fish in European rivers. In recent years, even more powerful poisons have become

available in the form of insecticides such as dieldrin or endrin, which are used with apparent abandon.

Fishing with poisons is one of the main methods used by occasional fishermen and has the

unfortunate side-effect that it selects against the young fish which are particularly susceptible to these

substances. Its effect on the fish stock is of course minimal where temporary pools are fished, for

there the fish would die anyway before they could rejoin the stock but in permanent standing waters

the annihilation of the population is more serious, as here the fish take refuge during the dry season to

form a reconstituted stock in the floods. Poisons are also used to eradicate unwanted species, for

example, the piranhas (Serrasalmus spp.) in Brazil (Braga, 1976).

239

Figure 7.5 (A) Basket dip-net (Ouémé river); (B) “Vee” shaped dip-net (worldwide); (C) “Vee”

shaped net mounted on a canoe (Chari river) (adapted from Blache, Miton and Stauch, 1962)

240

Figure 7.6 (A) Bank mounted lift-net (widespread, example from Niger River); (B) handheld life-net

(widespread, example from Bangladesh); (C) raft mounted lift-net “Sadung” (middle Mekong)

Table 7.1

Ichthyotoxic plants used in fishing in Benue River (after Stauch, 1966)

Plant Active part Effect

Balanites aegyptica Bark which is crushed Kills fish within a few hours

Tephrosia vogelii Leaves and young shoots crushed Fish appear on surface very quickly and die

soon after

Momordica charantia Dried leaves and fruits: usually mixed

with Balanites

Effect very slow

241

Unidentified plant: (local

name: Horesoungsoungko)

Whole plant used after crush- ing in a

mortar. The shallow lagoon to be

fished is stirred up and the poison

mixes with the mud

Especially effective for catch- ing bottom

living mud-eating species such as Clarias

Crinum sp. Bulbs crushed in a mortar and put in a

sack which is drawn through the water

Very effective, kills all fish in a short time

Indigofera pilosa Ripe seeds Useful in waters of little volume but is

more often mixed with other products

Parkia filicoidea Pre-ripe seeds pulped Slow in action and ineffective against

siluroids

Syzygium guineense Bark which is crushed Rapid effect against all species

Euphorbia kamerunica Latex Renders fish inedible in large quantities:

small amounts are used to intensify the

effect of other poisons

Prosopsis africana Seed pods, dried and crushed Very slow (three days to produce death) but

intensifies effect of other poisons

Sarcocephalus esculentis Bark which is rubbed between two

stones

Irritant which fish avoid by taking refuge in

traps which are placed in the pool to be

fished

Adenium obaesum Fresh wood cut in discs and sun dried

for 2 days

Kills fish within three hours

Moringa pterygosperma Bark crushed between two stones Kills fish within three hours

Acacia ataxacantha Flowers dried in the sun Never used alone but acts as an intensifier

to Balanites or Momordica

Ximenia americana Bark which is pounded in water Particularly effective against cyprinids and

characins which appear belly up on the

surface within an hour. Frequently mixed

with Balanites

Ziziphus mucronata Flowers which are poun- ded in a

mortar

Kills all fish within two hours even

siluroids

As has been shown in the section on standing stocks, many lagoons contain a very high ichthyomass,

most of which is removed during intensive fishing of these waters. This fact has apparently been

recognized and, as we have seen in some areas, fishermen attempt to retain the maximum volume of

water in the depressions by damming the outlet channel. In more developed floodplains, the shape of

the lagoons may be regularized and eventually drain-in fish ponds may be dug into the surface of the

plain.

Fishing during the Rising and Receding Floods

As the floods start to rise there is a burst of activity by the fish as the adults move preparatory to

breeding. Locally high concentrations are present giving rise to very heavy but localized fisheries.

When the floods recede from the plain the water becomes confined increasingly into depressions and

channels. The fish follow these flow patterns to reach either the main channel or what will become the

standing waters of the plain itself. Fishing methods take advantage of these movements and are

mainly aimed at either directing the fish into places where they are more easily captured, or to

retaining the fish in floodplain depressions from which they may be more easily removed later. Such

gear may be based on bamboo or palm frond fish fences which are installed across the plain or the

channels through which the water enters or leaves. They can reach a considerable length and can be

arranged in complex forms giving a labyrinth-like effect. Capture is either in trap shaped chambers

(Fig. 7.7) or in special cylindrical traps or nets which are placed in openings in the barrage. Durand

(1970), for instance, has described a cross river barrage from El Beid River which drains the Yaeres

floodplains of the Logone River. Here the fences are arranged in a series of Vee's at the apexes of

242

which are held large hand nets. Both upstream and downstream migrants may be caught in the same

type of trap.

Figure 7.7 Barrage trap from coastal floodplain of Benin; note heart-shaped capture chamber

Thus in the Nzoia river the ‘kek’ type of barrier trap (Fig. 7.8) intercepts breeding fish moving

upstream, as well as adult fish returning from the spawning grounds (Whitehead, 1959a). Such fences

can contribute a large proportion of the total annual catch in some systems. In the Lubuk Lampam

(Indonesia) guide fences accounted for about 50 percent of the fish caught in 1975 (Arifin and Arifin,

1976) and on the African Barotse plain the “maalelo” fishery produced about 25 percent (631 t) of the

total catch in 1969. The “maalelo” of the Barotse floodplain is perhaps typical of the more open plain

type of barrier fishery. Here the fish guides are usually earth bunds some 75 cm high and between 3 to

40 m long, which deflect fish into traps placed at intervals along their length, although reed, wire

mesh or brushwood fences may also be used. Alternatively the bunds may join two areas of high

ground so as to retain a pool behind them. When eventually the dam thus formed is breached, the fish

are caught in traps and baskets at the outflow giving a yield equivalent to about 33.6 kg/ha of area

impounded (Bell-Cross, 1971). A few “maalelo” are fished during rising flood. Weiss (FAO/UN,

1970a), estimated that there were 10 000 of these weirs operating each year on the Barotse plain.

243

244

Figure 7.8 Fishing basket used in Kenya (A) and arrangement in a “kek” barrage (B). Fish enter

through the vertical funnel. (After Whitehead, 1958)

Complete blocking of small channels leading water out of depressions is common in other systems; it

has been noted from the Gambia River by Svensson (1933) and by Chevey and Le Poulain (1940)

from the Mekong. The pool thus created is fished, often considerably later in the dry season, either by

breaching the dam and catching the fish in traps, baskets or nets or by bailing the water out until the

enclosed section is dry. This principle has been suggested as a means for improving the fishery

productivity of floodplains as an extensive form of aquaculture.

In large channels which are not easily blocked with dams, stownets and wing traps form an alternative

to the barrage trap. The stownet is a conical fixed gear which operates rather as a static trawl, the

water passing through the net rather than vice versa. In the River Rhine these are operated using a

single anchored otter board, in the Oueme such nets are slung from poles securely stuck into the

bottom, but in the Tonle Sap, which drains the Grand Lac into the Mekong, they reach complex

proportions in the form of the “day” (Fig. 7.9). Such gears give very high yields during the main

migrations into and out of the major flood depression and in the main channel. Fily and D'Aubenton

(1966) recorded a mean catch of 33 t per unit for two lunar catching periods for 11 installations in

1962–63. Earlier results by Chevey and Le Poulain for 1938-39 gave about twice this figure (64.5 t).

In the rivers and canals of the Chao Phrya and Mekong deltas, stationary wing traps of the type

illustrated in Fig. 7.10, also take large quantities of migrating fish as they moved up and down the

channels during the dry season. According to Tongsanga and Kessunchai (1966) catch rates ranged

between 1.2 and 3.8 t/day in various canals.

245

Figure 7.9 Contribution of a “day” stow-net from the Tonle Sap: (A) cylindrical trap (“day”); (B)

disposition of individual traps in stow-nets mounted in a barrage; (C) method of use (i) with

gear lowered, and (ii) with gear raised (after Chevey and LePoulain, 1940)

Fishing at Peak Flood

Although in most rivers fishing is highly seasonal with only minimum effort being expended at high

water, some fisheries are pursued during the floods particularly in forested rivers such as the Amazon.

246

Such fisheries concentrate on frugivorous species which congregate in areas rich in fruit bearing trees

and shrubs. Here fish are captured mostly by hook lines, although bows and harpoons are also

employed for larger species (Goulding, 1981; Smith, 1981). In savannah rivers fishing continues in

some systems using traps, lines or gill nets set in areas of slack water or in the deeper parts of

floodplain lakes. However, the risk of loss of gear, the low return on effort and the difficulty of

working on a vegetation encumbered floodplain tend to restrict the practice.

Figure 7.10 stationary wing-traps from the Chao-Phrya delta. (After Tongsanga and Kessunchai,

1966)

BOATS

A detailed description of the various kinds of fishing craft is not appropriate here, but the availability

of suitable means of water transport is crucial in a fishery which depends much on the mobility of the

fishermen. Furthermore, in flood rivers the plains are inundated for several months of the year, and

the communities inhabiting them are forced to adopt a semi-aquatic way of life. During the floods

water-born transport is the only means of communication and even the markets are conducted from

canoes. As a simplification, two main types of boats are used in tropical and sub-tropical fisheries.

The first are the fishing craft themselves which are usually dugout canoes or planked craft between 4–

6 m long. Motorization of the smaller fishing boats is comparatively rare, especially among part-time

fishermen, and is in many cases of no great advantage. The second class of boats are longer, often up

to 10 m long and are used for transport of fish from the landing to the market. These are more

frequently motorized. In the main equatorial rivers, the construction of boats presents few problems as

wood is plentiful and dug-outs are easily made. In the savannah rivers, however, the lack of good

wood is limiting and canoes are often scarce and expensive as they have to be imported from

elsewhere. The development of suitable substitutes for traditional types of craft, which are cheap

enough for the fishermen, is one of the main preoccupations of fisheries administrators in such areas.

247

PRESERVATION OF FISH

TYPES OF PRODUCTS

A certain amount of the fish caught in river systems is consumed fresh by the fishermen themselves

and by communities with a limited radius of the fish landings, but in the more important fisheries, a

surplus to local requirements is produced which is sold for transport elsewhere. To improve the

quality of their product, fishermen as far apart as the Mekong River and the Magdalena River, keep

their catch in live chambers and even transport it in special boats with wet holds. Some species

survive and keep better than others in the fresh state after landing, and air-breathing fish particularly

are sought after because of the time they can be kept alive after capture. In India and part of Africa,

murrels, cat fish and some anabantids are transported for considerable distances in baskets lined with

damp weeds or moss. However, because of the dispersion and inaccessibility of fishing sites in the

river-floodplain system, and the rapidity with which fish deteriorate under tropical conditions, most

fish have to be preserved by one means or another for it to arrive in the markets in an acceptable

condition. Furthermore, the seasonal nature of the fishery means that a period of excess fish

production is followed by one of scarcity. Preservation techniques are thus needed to prevent fish that

are in excess of demand being lost and to even the supply of the year. Several types of treatment are

used, depending on local conditions and preferences.

Chilled Fish

The preservation of fish by icing or freezing is a comparative innovation to the river fisheries. Even

now its use is limited to areas where sufficient fish is caught to justify the expense of ice-making

plants, and where communications are sufficiently easy for the fish to be collected, iced and removed

rapidly. This method of keeping fish has grown up on major fisheries located in the main river

channel such as that for Hilsa ilisha in the Indus (Husain, 1973) where the fish are collected at certain

landings for icing prior to transport by rail to the major towns. Icing is especially popular in Latin

America, where the comparatively recent development of the river fisheries has left little time for

traditional methods of preservation to have arisen. On the Magdalena River and the Amazon, for

instance, fish traders come to the fishing sites in motorized boats equipped with ice to preserve the

fish. In recent years, preservation in ice has tended to replace other methods of treatment. In the

Mekong, Fraser (1972) recorded the addition of ice preserved fish to the customary range of products,

and in the Kafue, Williams (1960) commented on the growing proportion of fish which is carried in

this form.

Dried Fish

Sun drying of fish without salting is not practical in many of the world's river systems because of the

high atmospheric humidity. But in desertic or Sahelian savannah rivers the practice is common,

especially for smaller species. In the Senegal basin, for example, simple drying is the usual form of

treating the fish after they have been eviscerated, scaled and, in the case of larger species, cut into

strips. On the Niger, only the smaller species, such as Alestes, are sun-dried and in the Chad basin one

of the traditional fish products, “Salanga”, consists of Alestes dentex and A. baremoze which are split

open ventrally and laid on mats to dry in the sun. In the Mekong, and other Asian rivers, sun-drying is

also common, although this is sometimes combined with salting.

There is a noticeable loss of weight as fish dry. This varies much with the species of fish, for instance,

Clarias with their massive head bones tend to lose less than do characins or cyprinids. It is generally

agreed that the ratio wet fish: dry fish is about 3:1.

Salting

248

Preserving fish with salt is not common in inland waters, largely because of the high cost of the salt,

and secondly because in the more humid areas the deliquescence of the salt shortens the life of the

product rather than increasing it. In the Indus valley, salting is used only when there is a very heavy

catch of Hilsa which exceeds the capacity of the ice plants (Husain, 1973). Certain species are

prepared in this way in the Parana River (Vidal, 1969), where Lycengraulis olidus and more recently

Hoplias malabaricus and Pseudoplatystoma spp. are treated with salt prior to sun-drying.

Smoking

Smoke-drying is perhaps the most widespread way of preserving fish in Africa and is practised in

nearly all river systems. Techniques vary somewhat and the type of oven used changes from place to

place. In Malawi, fish are smoked in a special thatched smoke house, whereas in other regions ovens

constructed of clay are open to the air (Fig. 7.11). Small fish are usually smoked whole. Medium sized

fish are scaled, eviscerated, and sometimes split open or slashed down the sides. The largest fish are

cut into pieces before treatment. As they are smoked fish lose weight in about the same proportion (3

kg fresh fish = 1 kg smoked fish) as when they are dried. One treatment is rarely sufficient, especially

in more humid areas, consequently the fish have to be retreated at intervals of between a week and ten

days to keep them in acceptable condition. The smoke-dried product is often stockpiled over a

considerable period of time, especially in those temporary camps furthest from major centres of

communication.

249

Figure 7.11 Different types of smoking oven: (A) circular oven (Haussa); (B) rectangular oven made

of baked earth; (C) pit oven; (D) sections of rectangular and pit ovens (adapted from Blache,

Miton and Stauch, 1962)

In some areas the lack of wood for smoking is causing problems for the fishery. In the Sahel, for

instance, the availability of domestic firewood is becoming one of the factors limiting human

occupation. Studies carried out in Mali by the Operation Peche, indicate that about 1 kg of wood is

necessary to produce 1 kg of smoked fish, using traditional ovens. An improved oven is being

introduced in the Central Delta of the Niger, which will reduce the demand to 0.5 kg of wood for

every kg of smoked fish produced (Operation Peche, 1976). A similar type of oven is being used in

the development of the Elephant marsh fishery in Malawi where it is also intended to plant copses of

trees to supply them with wood; a copse of about 1 hectare is thought adequate to serve one oven

(Tweddle et al., 1977).

Fish Meal

250

The only river fishery whose catch is used for making fish meal is the Prochilodus fishery of the

Parana. According to Vidal (1967) these “Sabelerias” accounted for nearly 70 percent of the total

inland catch of Argentina, although these have declined considerably from about 1965 onwards.

Fish Oil

Several species of fish are particularly rich in fats and are traditionally used for extracting oil. Oil is,

for instance, a by-product of the “Sabelerias” fish meal factories. Brycinus leuciscus is exploited early

in the fishing season in the Niger for its oil, and several species of the Mekong, principally Cirrhinus

and Dangila are taken for the same purpose. The extraction process involves boiling for a certain time

to release the oil which floats to the surface and may be skimmed off. Dangila spp. can produce up to

15 percent of their weight in oil (Chevey and Le Poulain, 1940).

Fermented Products

Fermentation of the cleaned and gutted fish in water for 12 hours is a common preliminary to salting

in the Mekong, or to sun-drying in the Senegal River. In Asia a number of fermented products

including fish pastes and sauces with a high salt content are also produced.

PROTECTION AGAINST INSECT INFESTATION

One of the major curses of the fishing industry in many parts of the world is the infestation of fish

with insects. Moist fish is susceptible to be damaged by blowflies and their larvae in particular. Blow-

fly larvae consume fish flesh so it is only when the fish is sufficiently dried that it becomes

unattractive to the adult fly for egg laying.

Dried or smoked fish are subject to be attacked by beetles, mainly Dermestes spp. The level of losses

due to Dermestes spp is directly related to the length of storage of the fish.

Prevention measures include the re-heating or re-smoking of the infested fish in a temperature above

50°. The use of insecticides is not advisable as there is no safe insecticide officially recognized yet

that can be used on cured fish.

However, keeping the fish off the ground and improving the hygienic conditions of the processing and

storage areas as well as their surroundings can significantly reduce the problem of insect infestation.

In West Africa early attempts to control Dermestes infestations included scorching, whereby during

the hot smoking process the outer skin of the flesh was blackened and hardened to lessen the

successful penetration of the flesh by the beetle. Other techniques have included packing the fish in

bales wrapped with matting as soon as it is smoked, but such attempts have largely failed. The

Operation Pêche in Mali has claimed a certain success by treating the fish with natural insecticides

(Bioresmetrine) after drying or smoking but improved hygiene and heat treatment using a polythene

tent and solar energy and simply keeping fish off the ground probably provide more satisfactory

alternatives.

SPECIES CAUGHT

Because of the large number of species involved in river fisheries, detailed species lists serve little

purpose. Analysis of catch data from most gears shows them to conform to the canonical distribution

of species abundance already described. Distributions of this type predict that only a few species will

be dominant in the catch of any gear, and knowledge of these is essential, both for the management of

the stocks and for establishing priorities for research.

251

LATIN AMERICA

The river fisheries of Latin America have concentrated mainly on the low water “Piracema” or

“Subienda” migrations. The fisheries are still relatively undeveloped and have selected for species of

large size and consumer appeal. Consequently catches are composed of the larger characins or

siluroids. In the Magdalena River well over half the catch consists of Prochilodus reticulatus

(Granados, 1975) although Pseudoplatystoma fasciatum, Pimelodus clarias, P. grosskopfii, Brycon

moorei, Sorubim lima, Plagioscion surinamensis and Ageneiosus caucanus also contribute

significantly. The preoccupation with the subienda species in Columbia led to a neglect of other

potential food fishes and Bazigos et al. (1977) drew attention to an unexploited stock of

Hemiancistrus and Pterygoplichthys in the cienagas which could substantially increase the yield from

this system. Fish of the genus Prochilodus are the mainstay of other South American fisheries. P.

reticulatus forms the basis of a heavy fishery in the southwestern portion of Lake Maracaibo and the

inflowing floodplain river Catatumbo (Espinosa Giminez, 1974). In the Apure and Upper Orinoco

systems, the fishery concentrates on the larger Pimelodidae: Pseudoplatyptoma fasciatum, P.

tigrinum, Brachyplatystoma filamentosum and B. vaillantii (Canestri, 1972) but a greater diversity of

genera including Prochilodontids are found in the Lower Orinoco and Delta (Novoa, 1982) (Table

7.2).

Table 7.2

Main Commercial Fish Species from the Lower Orinoco (from Novoa,

1982)

Prochilodus mariae Cynoscion spp.

Pseudoplatystoma fasciatum Colossoma brachypomum

Semaprochilodus laticeps Colossoma macropomum

Mylossoma duriventris Brachyplatystoma spp.

Phractocephalus hemiliopterus Brycon spp.

Hydrolicus scombroides Sorubimichthys spp.

Arius parkeri Pellona flavipinnis

Hypophthalmus edentatus Pterodoras spp.

Pinirampus pinirampus Schizodon isognathus

In the Parana, Vidal (1969) listed 18 species as being the principal ones of commercial value (Table

7.3). Further upriver, in the Mogi Guassu, P. scrofa makes up 0–60 percent of the catch which also

contains 16 other species of characin (Godoy, 1975). Although the Amazon basin contains some 2

000 species, only a small proportion of these are captured by the fishery. Meschkat (1975) listed the

major commercial species, based on the catch statistics for Amazonas state (Table 7.4).

Table 7.3

Species of major commercial value in the fishery zone of Rosario, R. Parana (not

listed in order of importance) (from data in Vidal, 1969)

Lycengraulis olidus Colossoma mitrei Pimelodus albicans

Brycon orbygnianus Ageneiosus brevifilis Zungaro zungaro

Salminus maxillosus A. valenciennesi Luciopimelodus pati

Prochilodus platensis Oxydoras kneri Pseudoplatystoma fasciatum

Leporinus spp. Rhinodoras d'orbignyi P. coruscans

Hoplias malabaricus Pimelodus clarias Basilichthys bonariensis

252

The heavy and selective fishing pressure on the largest species A. gigas and C. bidens, had already led

to a sharp decline in the populations of these fishes by 1975 and Bayley (1981) posit that the catch of

all large species cannot be maintained at present fishing pressure. In exchange the share of the catch

contributed by species of such genera as Semaprochilodus, Prochilodus and Triportheus has, and will,

increase until they dominate the fishery.

Table 7.4

Species of commercial value in the Amazon (listed in order of

importance) (from data in Meschkat, 1975)

Arapaima gigas Leporinus and Schizodon spp.

Colossoma bidens Brachyplatystoma flavicans

Prochilodus insignis Brycon hilarii

Plagioscion surinamensis Oxydoras kneri

Plecostomus spp. Osteoglossum bicirrhosum

Brycon nattereri Cichla ocellaris

Colossoma spp. Hypophthalmus edentatus

Rhinosardinia spp. Pseudoplatystoma fasciatum

Prochilodus corimbata Astronotus ocellatus

AFRICA

In Africa, the river fisheries are exploited at a much greater intensity and a very broad spectrum of

species are caught, especially in the basins of the western side of the continent. In the Niger River gill

net and cast net catches can contain over 50 species and it is difficult to identify the major elements of

the catch. However, Raimondo (1975) listed the nine most important species of the Upper Niger

(Table 7.5).

Table 7.5

Predominant species in the catch from the Central Delta of the Niger

from data in Raimondo, 1975)

Alestes dentex Lates niloticus

Brachysynodontis batensoda Bagrus bayad

Hydrocynus forskhalii Mormyrus rume

Oreochromis niloticus Citharinus latus

Labeo senegalensis

Other species commonly represented are Auchenoglanis occidentalis, Clarias anguillaris, and

particularly Brycinus leuciscus which forms the basis of a specialized fishery for fish oil production.

Catches from other West African rivers have a similar combination of species. In the Senegal River,

Reizer (1974) investigated the number of species captured as a function of mesh in gillnets (Fig. 8.7)

and showed that the number of species increased as mesh size decreased. On the basis of these

experimental fishings it appeared that the ten most important species to the fishery were Schilbe

mystus, Lates niloticus, Alestes dentex and A. baremoze, Hydrocynus brevis, Labeo senegalensis,

Eutropius niloticus, Citharinus citharus, Heterotis niloticus and Hepsetus odoe. In some systems,

such is the diversity of gear that it is almost impossible to establish the true weighting of the various

species in the total catch. In these instances, studies of the abundance of fish in the markets adjacent

to the fishery give some idea. For instance, in the Oueme system the order of abundance of the

various species was as shown in Table 7.6 although as many as 40 species figured in the fishery as a

253

whole. This abundance is of course, biased by the food preferences of the fishermen themselves as

some of the species caught rarely reach the market.

Table 7.6

Order of abundance of major species appearing in the markets of the

Oueme valley

Clarias ebriensis Synodontis melanopterus

Clarias lazera Synodontis schall

Parachanna obscurus Schilbe mystus

Heterotis niloticus Distichodus rostratus

Mormyrids Ctenopoma kingsleyae

In the Chari-Logone system the elements of the catch are very difficult to separate from those

produced in the lake. However, Blache and Miton (1962) listed the principal elements of the catch

from a number of fishing methods in the Chari and Logone rivers. From these it appeared that the

migratory Alestes dentex and A. baremoze were by far the most important species to the fishery.

Several larger species were also of major importance including Citharinus citharus and C. latus,

Distichodus rostratus and D. brevipinnis, Labeo senegalensis, Hydrocynus brevis and H. forskahlii,

and Lates niloticus. Some smaller species were also important including Oreochromis galilaeus and

O. niloticus, Schilbe mystus, Brycinus nurse and divers Synodontis spp. In the swamps Clarias lazera

and C. anguillaris were particularly abundant. The drying of the Lake Chad basin induced by the

Sahelian drought has, however, radically changed this composition and the catch is now dominated by

tilapiine cichlids.

In contrast to the specific richness of West African fisheries, the rivers of East Africa produce only a

restricted variety. The Shire river fishery was found to have five species, Clarias gariepinus, Clarias

ngamensis, Oreochromis mossambicus, Marcusenius macrolepidotus and Eutropius depressirostris

which made up about 90 percent of the catch (Willoughby and Tweddle, 1977) despite the fact that

there are 39 species in the system. The Kafue fishery takes a greater number of species (18) but of

these, 6 contributed about 90 percent of the catch (Everett, 1974). These were: Oreochromis

andersoni, O. macrochir, T. rendalli, Serranochromis angusticeps, Schilbe mystus and Clarias

gariepinus.

ASIA

In the rivers of the Indian sub-continent there is one migratory species, Hilsa ilisha, which, in the

Indus, Ganges and Godavari systems, is the subject of specialized fisheries. Apart from this the

fisheries are based largely on a series of large cyprinids known as the major carps, as well as some

siluroids, ophicephalids and notopterids. The major Gangetic carps are Labeo rohita, L. calbasu,

Catla catla and Cirrhinus mrigala, and the principal siluroids Mystus aor, M. seenghala and Wallago

attu. In addition to these, Jhingran (1975) listed a further 12 species which contribute significantly to

the catch. A similar species complex occurred in the Brahmaputra river, with the addition of Labeo

gonius, Puntius sarana and Notopterus notopterus. In the Cauvery river a somewhat different group

of species dominated the fishery: Acrossocheilus hexagonolepis, Tor putitora, Barbus carnaticus,

Labeo kontius, Cirrhinus cirrhosa and Osteocheilus brevidorsalis amongst the cyprinids,

Glyptothorax madraspatanus, Mystus aor, M. seenghala, Pangasius pangasius, Wallago attu and

Silonia silondia amongst the siluroids, together with Channa (= Ophicephalus) marulius a murrel and

Notopterus notopterus.

Husain (1973) listed the 13 principal species of the lower Indus fishery as shown in Table 7.7

although these were drawn from a pool of 66 species. From the list it will be seen that there are many

254

elements in common with the Indian rivers, although O. mossambicus has been introduced from

Africa.

Fishing in the Chao Phrya river is largely done by stationary wing traps which block most of the main

channel (Tongsanga and Kessunchai, 1966). Of the 77 species commonly captured, Crossocheilus

reba made up about 60 percent of the catch. Other important species were Wallago attu,

Macrognathus aculeatus, Ophicephalus micropeltes, Ophicephalus striatus, Puntius gonionotus,

Pangasius sutchi and Cirrhinus microlepis.

Table 7.7

Main food fishes of the River Indus (after Husain, 1973)

Hilsa ilisha Rita rita

Notopterus chitala Mystus spp.

Catla catla Oreochromis mossambicus

Cirrhinus mrigala Channa (=Ophicephalus) marulius

Labeo calbasu Channa (=Ophicephalus) striatus

Labeo rohita Channa (=Ophicephalus) punctatus

Wallago attu

More than 150 of the total 800 species that inhabit the Mekong make up the bulk of the catch in that

system. Of these a few may be singled out as being particularly conspicuous or sought after. These

vary with the region of the river fished, and separate authors have identified different dominant

components. Pangasianodon gigas particularly is distinguished by its size but has diminished in

importance due to overfishing. Adopting the summary of Fily and D'Aubenton (1966) the species

listed in Table 7.8 made up over one percent of the catch in Cambodian waters.

Table 7.8

Species comprising over 1 percent of total catch in the Cambodian

water of the Mekong in order of importance (after Fily and

D'Aubenton, 1966)

Pseudosciaena soldado Ambassis wolffii

Cirrhinus jullieni Puntius orphoides

Cirrhinus auratus Puntius altus

Ophicephalus micropeltes Notopterus notopterus

Thynnichthys thynnoides Hampala macrolepidota

Kryptopterus apogon Puntius bramoides

Macrones nemurus Pangasius larnaudi

Cyclocheilichthys enoplus Wallago attu

Labeo chrysophekadion Clupea thibaudeani

EUROPE AND NORTH AMERICA

Catches in North American and European highland streams are both dominated by salmonid species

but richer faunas are to be found in the lowland reaches. In Europe these are based mainly on

cyprinids and of the 20 species listed from the middle of Danube by Liepolt (1972) and Holcik et al.

(1981) over half belong to this family (Table 7.9). In the lower Danube, as with other Black Sea and

Caspian rivers, migratory species, such as Huso huso, Acipenser ruthensis, A. stellatus, A.

guldenstaedtii and Alosa pontica make up a considerable part of the catch.

255

The Vistula in Poland contains many less species although the fishery does have some of the elements

present in the Danube. According to Backiel (1983) the following species appeared in order of

importance: Abramis brama, Vimba vimba, Anquilla anquilla, Esox lucius, Stizostedion lucioperca,

Chondrostomus nasus, Rutilus rutilus and Blicca bjoerkna. This relative poverty in species is

exaggerated by environmental degradation which has already eliminated some of the previously most

important anadromous fishes from the river.

The range of families appearing in the catches of North American lowland rivers is greater than in

Europe and the diversity of the catch is equally high as is illustrated by the commercial and

recreational catches of the Middle Mississippi (Rasmussen, 1979) (Table 7.10).

Table 7.9

Main commercial fish special fish species from the Middle Danube

Rutilus rutilus Alburnus alburnus

Aspius aspius Leuciscus cephalus

Vimba vimba Chondrostoma nasus

Abramis brama Leuciscus idus

Stizostedion lucioperca Cyprinus carpio

Esox lucius Barbus barbus

Silurus glanis Tinca tinca

Acipencer ruthensis Salmo trutta

Ctenopharyngodon idella Hucho hucho

Anguilla anguilla

Table 7.10

Species composition of Upper Mississippi Fisheries (a) commercial

and (b) recreational

(a)

Cyprinus carpio Ictiobus spp.

Ictalurus spp. Aplidonotus grunniens

Polyodon spathula Catostomid spp.

Acipenser spp. Lepisosteus spp.

Amia calva Hiodon tergisus

Esox lucius Anguilla rostrata

Ctenopharyngodon idella Pomoxis spp.

(b)

Stizostedion canadense Aplidonotus grunniens

Stizostedion vitreum Morone chrysops

Pomoxis spp. Lepomis macrochirus

Ictalurus punctatus Esox lucius

Cyprinus carpio Micropterus salmoides

Perca flavescens Pylodictus olivaris

Ictalurus spp. Micropterus dolomieu

Lepomis cyanellus Catastomid spp.

Ambloplites rupestris Centrarchid spp.

Dorosoma cepedianum Lepidosteus spp.

Amia calva Acipencer fulvescens

Ictalurus furcatus Hiodon tergisus

Polyodon spathula Anguilla rostratus

256

Scaphyrhnchus platorhynchus Lepomis gulosus

Morone mississippiensis

These examples show that even fisheries in rivers from the temperate zone can be based on a large

number of species.

FISHERIES FOR JUVENILE FISH

The very heavy exploitation of juvenile fish, in the form of fish of the year moving to the dry season

habitats at the end of the flood, is a particular feature of floodplain fisheries. In the Oueme, as in many

African and Asian fisheries, small mesh nets of various types are used intensively in the canals

draining the plain. Cross channel dams and barrages, such as those of the El Beid or the Barotse plain

are also designed for the capture of young fish. Durand (1970) estimated that up to 90% of the catch

by number and weight of the El Beid river was made up of juvenile fish moving from the Yaeres

floodplain towards Lake Chad. The “maalelo” fishery of the Barotse removed about 3.7 percent of the

juveniles of the 15 most important species each year (Bell-Cross, 1971). Regression analysis also

indicates that considerable proportions of the catch from African rivers (ranging from 90% in the

Shire and drought affected Niger to 50% in the Niger under more normal regimes) are repeatedly

drawn from 0+ fish. Many millions of fingerlings and fry of the major carps are withdrawn annually

from Indian rivers to stock reservoirs in their basins, and the capture of fry is common throughout

Asia for the stocking of floodplain depressions, rice paddies and culture ponds.

It is common prejudice that the removal of large quantities of juvenile fish will prove harmful to the

stock. The persistence of many of the fisheries indicates that there is little danger so long as the

practice is kept within reasonable limits. Both Reed et al. (1967) in his defence of the “atalla” fishery

for juveniles on the Niger river, and Bell-Cross 1971) in his analysis of the “maalelo” fishery, made

the point that, with the high mortality rates current among river fishes, the loss of a proportion of year

class 0+ fish is hardly liable to affect the final population at all. A theoretical analysis of such a

fishery (Welcomme and Hagborg, 1977) indicated that a high proportion of the juveniles can be

removed during the period of drain-off without damaging the fishery and in simulated fisheries, where

juveniles were exploited at the same time as adults, the combined catch exceeded the maximum catch

of either juvenile or adult fisheries on their own. Careful control of these fisheries is, however,

essential and further studies are needed on actual situations where the juveniles are heavily exploited.

CATCH

ANALYSIS OF CATCH IN DIFFERENT RIVERS

Catch as a Function of River Form

Catch statistics from rivers are often of low quality because of the difficulties inherent in collecting

data from fisheries which operate from many landings dispersed along a system that may traverse

several countries. As in most fields of fisheries, there is a need to improve the quality and quantity of

the catch statistics and the collection of biological and other data as a prerequisite for the proper

management of the riverine stocks. However, because of the lack of concise information on many of

the individual systems, attempts have been made to extrapolate general principles from the small

group of water bodies about which something is known. Some approaches to this have already been

discussed in the section which describes methods used to establish either standing stock, production or

yield in temperate rivers from a wide variety of ecological parameters. The information needed for

such exercises in tropical rivers and an alternative means of analysing catch patterns has been sought.

Despite the inadequacies of the data, an analysis of fish yield patterns from African rivers has given a

fairly coherent picture of the factors involved in determining the catch that can be expected from any

particular system (Welcomme, 1976). In the rivers used in this analysis, see Table 7.11 all of which

were moderately to heavily fished, there was a good correlation between the drainage basin area of the

257

river system in km² (A) and the catch in tons obtained from it (C). Excluding catches from

exceptionally large flooded areas, the sample conformed to the relationship:

C = 0.03A0.97 (r = 0.91)

Because the basin areas and the total length of the longest channel of the river are also simply related.

(Main channel length = 4.95 basin area0.45, in African rivers) this equation transformed into a

relationship for yield in t as a function of the main channel length in km (L):

C = 0.0032L1.98 (r = 0.90)

or approximately one three hundredth of the square of the length of the stream (L²/300).

Table 7.11

Main channel length, basin area and catch from African rivers

River Channel length

(km) Basin area (km²) Catch (t)

Nile 6 669 3 000 000 40 840

Congo 4 700 4 014 500 82 000

Ubangi 1 060 772 800 4 670

Kasai 1 735 342 116 7 750

Niger 4 183 1 125 000 30 000

Benue 1 400 219 964 12 570

Zambezi 2 574 1 300 000 21 000

Senegal 1 641 335 000 16 000

Gambia 1 120 77 000 3 000

Volta B. 650 45 324 1 560

Volta R. 260 6 871 370

Volta W. 255 6 602 70

Pendjari 330 11 226 140

Oueme 700 40 150 646

Mono 360 22 000 533

Tana 600 38 000 500

Bandama 950 97 000 3 408

Sassandra 650 75 000 1 518

Comoe 1 160 78 000 2 142

Rufigi/Ruaha 750 17 700 3 600

The catch of any reach of river of length x km at distance y from its source can be calculated from xCy

= Cy+x - Cy, where values of Cy can be obtained from the preceding equation. In its most extreme

form, where x = 1 km, this yields a theoretical equation for the catch that might be expected for any

kilometre or river at different distances from the source (i.e., catch at kmy = 0.0064y0.95). When catches

from rivers with extensive floodplains (those whose flood area exceeds 2% of the total basin) are also

included in the analysis, a second relationship emerges whereby C = 0.44A0.90. Values predicted by

these relationships are of course averages over a number of systems from which actual values from

individual systems deviate quite widely. Nevertheless these formulae have been used to predict catch

in a number of systems lying outside the original set. The Mekong, Danube and Magdalena rivers,

with floodplain areas of between 3 and 8 percent of their respective basins, are distributed around the

258

extensive floodplain line. Rivers such as the Indus, Mogi Guassu and Pongolo with no extraordinary

development of their floodplains fit the ‘normal’ relationship nearly exactly. Furthermore some

independent estimates of potential, such as that obtained by Paiva (1973) from the Parnaiba river,

Brazil, of 7370 t/yr for a basin area of 362 000 km², agree completely with the predictions of the

formulae. However, caution should be exercised in their application of rivers outside Africa owing to

the different relationships between basin area and channel length for the various continents.

Deviations from the mean regression line may arise for two main natural reasons. First, there is a

possibility that edaphic factors may influence the basic productivity of a system in such a manner as

to determine yield. There is so far no strong evidence for this in the data set available on savannah

rivers, although there is more than a strong suspicion that certain types of poor river, such as the

tropical black waters, are indeed less capable of supporting sustained fishing than is normal. Second,

morphological factors have been shown to determine the yield characteristics of river systems. The

very fact that it has proved necessary to derive two different relationships for yield against basin area

indicates the predominant role of floodplain area in determining the productivity of a system. This

factor alone accounts for 70% of the differences between actual and predicted catch in different

reaches of the same river. From the data in Tables 7.12 and 7.13 mean catch levels of 17 fully

exploited African floodplains were 54.7 ± 36.5 kg/ha/yr. Six Asian floodplains had mean catches of

44.02 ± 17.9 kg/ha/yr and, given the means and standard deviations of the two populations they can

be considered one set which can be merged. When data from the Orinoco and Magdalena rivers,

which also lie within the limits of confidence, are added, the mean yield for the 25 rivers is 51.55 ±

32.1 kg/ha/yr. With the present data it is not possible to detect any differences between the tropical

continents but at 18.7 kg/ha catches from the temperate Danube are slightly lower. However, the

catches from 1481 km of the Upper Mississippi river (Rasmussen, 1979) show that the 171 136 ha of

pooled river, produced 6499 t of fish (mean of 25 yrs) equivalent to 37.8 kg/ha. In this section of the

river there is a succession of 26 pools in which the lower ends have lacustrine characteristics and the

upstream portions have retained their floodplain. Catches obtained from the three blackwater rivers

for which data are available indicate catch levels of between 11 and 19 kg/ha/yr which are

considerably less than the mean for savannah rivers. When catch is plotted against floodplain area for

the 25 tropical floodplains which are exploited at a reasonably intense level, a relationship:

Table 7.12

Maximum inundated area and catch from some floodplains additional to the

information in Table 7.11

River Area (km²) Catch (t) CPUA

(kg/ha)

AFRICA

Niger Benin 242 1 200 49.59

Niger C.D. Mali 20 000 90 000 45.00

Massilli 150 475 31.67

Pongolo 104 400 38.46

Yaeres 7 000 17 500 25.00

Kamulondo (Blackwater) 6 639 7 355 11.08

SOUTH AMERICA

Orinoco 228 1 000 43.80

Amazon (Peru) 9 960 13 700 13.80

ASIA

Mahaweli 121 413 34.13

Bangladesh 93 000 727 000 78.17

Lubuk Lampan 12 29 24.17

Lower Mekong 54 000 220 000 40.74

Ganges 1958–61 296 1 538 51.96

259

Ganges 1962–69 296 1 430 48.31

EUROPE

Danube 26 450 49 400 18.68

C = 4.23A1.005

is obtained. Although the best fit is a power curve the exponent is sufficiently close to 1 as to make

the relationship almost linear. It does, however, indicate that large floodplains may be marginally

more productive per unit area than smaller ones, and values range from 42.8 kg/ha for a 1 hectare

plain to 44.6 kg/ha for a 5 million hectare plain. These figures all confirm the earlier estimates of

between 40 and 60 kg/ha/yr that may be expected from tropical floodplains.

One question that frequently arises is the degree to which the catch of reservoir rivers or modified

rivers which have no annual flood, compare to the levels of production of flood rivers. The only two

fully fished rivers of this type for which data are available, the Mississippi and the Nile, lie within the

same range of values appropriate to their level of exploitation as do flood rivers. More data are

needed, however, to clarify this as the dynamic processes underlying the fisheries of such rivers are

likely to differ considerably from those in rivers having a pronounced seasonal flood.

Catch as a Function of Fishing Intensity

Standard models relating catch and effort, such as those of Schaefer and Fox, have been used

successfully in some fisheries of small rivers or in the analysis of fisheries based on single species, for

example those for Colossoma (Petrere, 1983) or Plagioscion spp. (Annibal, 1983) both of the Amazon

basin. Examination of catch and effort data in large rivers, however, generally illustrates the

difficulties of applying such models to complex fisheries which are pursued with a variety of methods

on the whole fish community in any river or river reach. Because of the scarcity and poor quality of

most catch and effort data from river fisheries any analysis of the factors regulating catch must be

regarded as somewhat speculative. Two approaches can be adopted for the analysis of the data

available. First, data from individual rivers can be studied to identify any relationships arising from

changes in temporal fishing patterns. Second, data from a number of rivers can be grouped for

treatment as one set with a range of differing regimes. Data sets from individual rivers are rare and

only two sets, one from the Mississippi and one from the Nile, have so far been traced. Similar power

curves fit the data for catch per boat for 30 years from the Mississippi:

(CPUE = 29731E-1.24: data from Rasmussen, 1979)

and for catch per boat for 10 years from the Nile:

(CPUE = 11117E-1.03: data from Borhan, 1981) (Fig. 7.12).

Curves of this type have been described previously for individual fisheries, for instance by Beverton

(1959) who used one to describe the response of Tilapia to increased fishing in Lake Victoria. When

22 observations from 16 of the worlds rivers are compared (Fig. 7.13, Table 7.13) the single best fit

regression line is also an inverse power curve of the form CPUE = a E-b although the value of the

relationship is limited by a lack of points in the middle range (5 – 20 fishermen/ha). The inclusion of

data from various continents and climatic zones appears justified at this time as the points for the

Magdalena River, Ganges and Misissippi all lie well within the confidence limits of the African data.

The relationship between effort and catch per unit effort is perhaps better described by a curve within

the log-log plot which may be interpreted as two different regression lines calculated from all points

between 0.05 and 3 fishermen/km2, where CPUE = 2.72E-0.47, and between 3 and 30 fishermen/km2,

where CPUE = 10.2E-1.09. Separate plots of total catch against catch per unit effort extrapolated from

the two regression lines cross over at about 8 fishermen/km2.

260

The form of these relationships may be explained in three ways:

(a) That there is an interference effect between fishermen as densities increase, whereby gears

compete with one another for an increasingly limited resource. This would mean that fishing mortality

is no longer linearly related to nominal effort.

(b) That the measures of effort customarily adopted for ease of recording, i.e., numbers of fishermen

or numbers of canoes is unsatisfactory. This may be particularly significant in artisanal and

subsistence river fisheries where fishermen have alternative occupations. They may then allocate the

time spent on fishing according to its rewards relative to such occupations, in which case fishing

mortality is again no longer linearly dependent on the variable selected to represent effort. Here some

other measure, for instance fisherman/days, might be a more appropriate index. Such measures

demand an increased effort in surveying the fishery which exceeds the financial possibilities of most

developing countries. In many river systems the switch between farming and fishing is in any case

dictated by the exigencies of the agricultural cycle, which may leave unstructured time which can

only be filled by fishing. Further verification of these questions rests upon detailed socio-economic

studies in such communities but the possibility signals the need for caution in adopting such measures

of effort for management.

Table 7.13

Number of fishermen, catch and maximum flooded area of some floodplains

River No.

fishermen Catch (t)

Area

(km²) No. fisherman

/km²

Catch

fisherman

/year (t)

CPUA (kg/ha/

year)

Seasonally inundated

AFRICA

Shire 1970 2 445 9 545 665 3.68 3.90 143.53

Shire 1975 3 324 7 890 665 5.00 2.37 118.65

Kafue 1963 1 112 8 554 4 340 0.26 7.69 19.71

Kafue 1970 670 6 747 4 340 0.15 10.07 15.55

Oueme 1957 29 800 6 500 1 000 29.80 0.22 65.00

Senegal 10 400 30 000 5 490 1.89 2.88 54.65

Niger Mali 1971 54 112 90 000 20 000 2.71 1.66 45.00

Niger Niger 1965 1 314 4 700 630 2.09 3.58 74.60

Niger Niger 1982 3 200 3 200 600 5.33 1.00 53.33

Niger Nigeria 4 600 14 340 4 800 0.96 3.12 29.88

Benue 5 140 9 570 3 100 1.66 1.86 30.87

Pendjari 65 140 40 1.63 2.15 35.00

Barotse 912 3 500 5 120 0.18 3.84 6.84

Rufigi 3 000 3 589 1 450 2.07 1.20 24.75

Kilombero 341 4 536 6 700 0.05 13.30 6.77

Logomatia 70 300 600 0.12 4.29 5.00

SOUTH AMERICA

Magdalena 1978 30 000 65 000 20 000 1.50 2.17 32.50

Orinoco 200 1 000 228 0.88 5.00 43.80

Amazon (Peru) 3 360 13 700 9 960 0.34 4.08 13.80

ASIA

Ganges 1 600 1 480 296 5.41 0.93 50.00

Mahakam 8 000 14 500 7 178 1.11 1.81 20.20

Stabilized

NORTH AMERICA

Mississippi 1950 3 036 4 037 1 711 1.77 1.33 23.59

Mississippi 1960 1 977 5 337 1 711 1.16 2.70 31.19

Mississippi 1970 2 232 5 256 1 711 1.30 2.35 30.72

AFRICA

261

Nile 3 725 8 410 800 4.66 2.26 105.13

(c) There is a real effect at the fish community level, whereby the yield curve concept as applied to

individual species is not applicable. Some of the changes occurring in exploited fish communities

described below indicate that such an effect does occur.

Figure 7.12 Catch per unit effort as a function of effort in (A) the Nile and (B) the Mississippi

262

Figure 7.13 Catch per unit effort as a function of effort for 17 rivers: Solid line = regression for all

points; dashed lines = best fit lines for catches from 0.05–3 fishermen/km² and 3–30

fishermen/km²

The form of these relationships also implies that the relationship between effort and total catch shows

an initial rise followed by a somewhat flat line during which there is little change in catch over a

considerable range of effort (Fig. 7.14). The Nile downstream of Aswan, for instance, produced 8 410

± 542 t for ten years despite a threefold increase in effort. Plainly such a plateau can be extended

indefinitely neither to the right nor the left of the relationship, and it is suggested that curves of catch

and catch per unit effort have three phases as set out in Fig. 7.15.

263

Figure 7.14 Plots of total catch (---•---) against effort for the Nile. Also shown is catch per unit effort

against effort (---o---)

264

Figure 7.15 theoretical diagrams of changes in catch and catch per unit effort as a function of effort,

letters explained in text

(1) Initiation phase: Limitations on catch per unit effort, and hence total catch, are set independently

off the relationship by the physical capacity of the fisherman to handle his catch. Observations from

all over the world on both lacustrine and riverine fisheries indicate that this capacity rarely exceeds 1

– 4 t per year per fishing unit although exceptionally up to 13 t/yr may be handled. The attainment of

maximal levels at low fisherman densities depends much on the state of development of the society in

which the fishery is pursued. Thus, whereas in advanced economies sufficient support exists to permit

the fisherman to land and market large catches, in developing economies individual efficiency may be

poor for lack of efficient gear and the absence of infrastructure for landing, marketing and

transportation of the catch. Fishing therefore tends to remain a subsistence activity. As the numbers of

fishermen increase the creation of markets will give rise to a rapid expansion of the fishery.

(2) Sustained exploitation phase: During this phase, catch levels are maintained in the face of

increasing effort. At the same time changes occur in the composition and abundance of the exploited

fish community which lead to a progressive shift in catch composition from large to small species.

(3) Collapse phase: Few cases have been documented where the fishery has been intensified to the

point of total collapse of the fish community. It is to be supposed that the fishery becomes

uneconomic long before and most reported overfishing has occurred where larger and highly favoured

species have declined while the rest of the fish community remains intact or underexploited.

It is conceivable that real collapse at the community level can occur under exceptionally heavy

exploitation. This may be impelled by socio-economic factors, as in the case of the Oueme, where the

disappearance of nearly all the larger species, the very small size at capture and the loss of catch over

the decade 1957–68 would suggest a community at the limits of its tolerance. The exceptionally high

fisherman densities of 30/km2 in this river result from a populous ethnic group which is confined to a

floodplain area by demographic pressures. Other drastic declines in fisheries have been recorded in

the African Sahel where the failure of the floods over a number of years has resulted in a decrease in

natural productivity in rivers such as the Niger, Senegal and Chari/Logone. The fish stocks, thus

diminished, have been called upon to support an increased fishing pressure as other food sources have

disappeared.

CHANGES IN COMMUNITY STRUCTURE WITH INCREASING FISHING

PRESSURE

The form of the relationship between catch and effort, whereby there is an initial rise followed by a

more or less prolonged plateau where catch remains stable in the face of increasing effort, masks a

series of adjustments in the fish community. Unfortunately systematic monitoring is still not carried

out in most river fisheries. As a consequence theories on the impacts of fisheries in rivers with

complex multi-species stocks must be derived from a number of river and lake fisheries. Experience

has shown that a similar succession of events, which has come to be termed the fishing up process,

occurs as fishing pressure is applied and increased. It is a common observation that there is a

progressive disappearance of the larger species as pressure is applied. Similarly, increases in fishing

intensity and total catch were correlated with a reduction both in mean size and as a proportion of

catch of the large pimelodid catfishes of the Orinoco river (Novoa, 1982), whereas the proportion of

smaller Semaprochilodus in the catch increased to more than compensate for the deficit. In the

Amazon at Manaus larger species such as Arapaima gigas and Colossoma bidens have progressively

disappeared from the catch and have been replaced by increased quantities of smaller prochilodontids

and clupeids as well as new species such as the pimelodid catfishes. Lates niloticus, Heterobranchus

longifilis and Bagrus docmac have completely disappeared from the heavily fished lower reaches of

the Oueme River, and other large species such as Citherinus latus, Distochodus rostratus, Labeo

senegalensis and Heterotis niloticus diminished in abundance to be replaced by small catfishes,

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characins and mormyrids. Sritingsook and Yoovetwatana (1977) remarked on the decline of the larger

migratory species such as Probarbus jullieni from the Mekong River and its tributaries.

It is useful to examine the history of the Oueme fishery in more detail as one of the best examples of

community overfishing in rivers. The total catch of the Oueme floodplain was estimated at 10 400 t in

1955–57 (CTFT, 1957). By 1968–69 this had fallen to an estimated 6 484 t (FAO/UN, 1971). The fall

was accompanied not only by the shifts in species composition noted above, but by a fall in catch per

unit effort in most gears as summarized in Table 7.14. There was some increase in total effort during

the period of study (about 19%) but this was insufficient to account for the drop in CPUE. Larger

mesh fishing gears were eliminated from the fishery by 1968 and the mean weight of the species

caught decreased in most of the gears that remained in use. For example, the medium-mesh cast net

caught fish of 124.9 g in 1955–57 but the mean weight had dropped to 26.5 g by 1968–69. Similarly

the small-mesh net caught fish of 25.2 g in 1955–57 but only 9.8 g in 1968–69. The only gear in

which catches increased was a trap for Macrobrachium, a detritus eating freshwater prawn that was

previously heavily preyed upon by the fishes.

Table 7.14

Changes in catch per unit effort in gear from the Oueme river between

1955–57 and 1968–99

Catch per unit effort Percentage

Type of gear 1955–57 (g/h) 1968–69 (g/h) changes

Cast nets

large-mesh 1 186.0 249.8 -79

medium-mesh 694.4 372.0 -46

small-mesh 589.5 322.6 -45

Longline 286.0 193.0 -33

(g/d) (g/d)

Traps

Clarias trap 74.0 43.2 -42

Macrobrachium trap 115.0 309.0 +269

Other changes have been noted from a number of systems. In Lake Tanganyika, for instance,

increased fishing led to the elimination of the major predator (Lates) with a considerable increase in

total catch from the small pelagic clupeids and smaller predator (Luciolates) which form the basis of

the fishery (Coulter, 1970). In Lake Victoria the fishery was being pushed from a dominance of large

tilapiine cichlids towards a preponderance of small haplochromine cichlids and even smaller

Rastrineobola before the spread of introduced Nile perch reversed the trend. Further changes occurred

within the haplochromine population under fishing pressure from trawls with the disappearance of

larger piscivorous and molluscivorous species and a progressive reduction in size in the remaining

planktivores and limnivores (Witte and Goudswaard, 1984). The establishment of a trawl fishery in

Lake Malawi also resulted in the successive displacement of the species caught to smaller and smaller

lengths (Turner, 1981). Certain types of environmental pressure would appear to mimic or to reinforce

the effects of increased fishing pressure, which makes it difficult to fully interpret the cause of

observed changes. Thus in the Caspian Sea there has been a long standing tendency toward smaller

species in the fishery in the face of sustained effort and environmental stress although the level of

catch has remained the same (Carre, 1978). Similarly, Regier and Loftus (1972) were able to trace a

succession of alterations in the composition of the fish communities of the N. American Great Lakes

arising from a combination of fishing, ecological and environmental pressures.

266

From this evidence, it may be supposed that individual species within the community pass through a

typical production curve of catch against effort. The succession of such curves agrees with the

observations that exploitation eliminates first the larger individuals of the larger species then the

larger species themselves (a and b in Fig. 7.16). This selective disappearance tends to be due to a

preference, both by fishermen and consumers, for larger individuals, as well as the lower capacity of

the long-lived larger species to support high levels of fishing mortality. In an effort to sustain catch

the fishing methods tend to be replaced by those which capture smaller fish. As the few largest

species are removed from the fishery larger numbers of intermediate-size species (c-j in Fig. 7.16). In

other words, the community is pushed successively from K dominance to r dominance. A feature of r

selected species with short life spans, high fecundity and high productivity is that they fluctuate

considerably in abundance. It is, therefore, reasonable to suppose that as a fishery becomes more and

more dependent on only a few such species it may become destabilized with large fluctuations in

catch with time. Indeed, trajectories of this sort are characteristic of fisheries for pelagic clupeids

throughout the world.

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Figure 7.16 Theoretical changes in a fish community when subjected to increasing fishing pressure:

(A) of certain population and fishery parameters; (B) of total catch showing schematic

evolution of individual species ‘a’ through ‘o’

Effects on species dominance may also be attributable to environmental disturbances outside the

fishery, or to other changes within the fish community, such as the introduction of a new element to

the fauna. In some cases, native or introduced species which have been held at a low level in the

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community are able to expand to occupy a dominant position when indigenous species are overfished

or when the habitat is changed in their favour by human action.

In river communities where there is as paucity of species at lower trophic levels, as shown by high

Predator/Forage fish ratios, the capacity of the system to benefit by such changes is limited. Similar

limitations have been observed in nutrient poor systems such as tropical blackwaters where the

amount of nutrients accumulated within the fish community represents a significant proportion of the

total nutrient pool. Here removal of the larger predators, whether fish or crocodiles, has even been

held accountable for the declining long term productivity of the environment (Fittkau, 1973). In such

systems rapid declines in catch may follow the onset of exploitation as happened in the Kamulondo

depression, where catch fell from 9 063 to 4 810 t in the space of four years after the introduction of

an intensive fishery based on nylon gillnets (Poll and Renson, 1948). More recently similar rapid

declines on individual fishing grounds have been observed in the blackwater tributaries of the

Amazon. Eventually even in the most productive of systems the capacity of the fish community to

absorb further increases in exploitation pressure will be exceeded, recruitment will fail and numbers

and biomass will drop sharply.

During the period when yield changes relatively little with increasing exploitation, the ichthyomass

will tend to remain relatively constant or to decrease as the species composition favours those with

higher turnover rates. At the same time biological fish production will increase. The tendency for

ichthyomass to remain constant may be reinforced in flood rivers where only a certain proportion of it

is able to survive through the dry season anyway. The number of species which make up the catch

will initially increase, although in some regions market preferences may be restricted so that only a

few stocks may be fished. When these are reduced, it may take some time before the market can be

reoriented to replacement species. Until this happens catches may fall and fishing be reduced, as

happened in the Parana River with the decline of the ‘sabalo’ (Prochilodus platensis) fisheries and in

temperate rivers where salmonid stocks were reduced by eutrophication. Where the market demand is

highly diverse, as it is in most African and Asian countries, the number of species in the catch will

first rise rapidly and then diminish as the more susceptible forms succumb to fishing and other

pressures.

These changes are most noticeable in fisheries using mesh selective gears, such as gill nets, cast nets

or seine nets, where changes in the size and species composition of the stock are accompanied by the

classic pattern of the successive replacement of large mesh gear by smaller and smaller mesh sizes.

Such changes are more difficult to detect in traditional fisheries which from the outset exploit a broad

spectrum of species and sizes with a variety of gear. The available evidence from rivers such as the

Oueme indicates that the use of gears designed to capture larger species and individuals progressively

declines as the fishery becomes heavily exploited.

FLUCTUATION IN CATCH BETWEEN YEARS

The available evidence indicates that the year-to-year variations in fish stock abundance are directly

linked to the degree of variability of the hydrological regime. Thus in stable systems such as reservoir

rivers or those with flood control the magnitude of the standing stock varies little from year to year,

whereas it is apparent that the catches from flood systems fluctuate in a manner that is in some way

dependent on changes in the flood cycle. Such changes in yield were noted early in the study of river

fisheries when Antipa (1910) concluded that the fisheries production of the Danube delta was directly

proportional to the extent and duration of the floods. This sentiment was echoed by subsequent

workers on that river (Botnariuc, 1968; Holcik and Bastl, 1977). That a similar state of affairs existed

in tropical rivers was suspected by many workers including Wimpenny (1943) in the Nile prior to its

complete control.

Models of production as a function of water regime would suggest that year-to-year variation in catch

arises from two main sources. First, the amount of water in the system at low water can affect catches

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of the same year by altering the ease with which fish are captured or can influence catches in

succeeding years through dry season mortality. Second, the intensity of the flood determines the

magnitude of the stock in future years through differences in recruitment, survival and growth. In

certain rivers it can also alter the ease of access between the channel and the floodplain water bodies

thus altering catch within the same year. In combination these effects render analysis difficult and

obscure trends over time but sufficient examples of each exist to illustrate these various cases.

Improved catches during exceptionally low waters are well known from most fisheries as the

shallower waters favour the capture of fish. Usually such comments take the form of qualitative

statements such as those of Vidy (1983) when describing the Logomatia fishery of the Yaeres

floodplain. The relationship has been quantified in the Magdalena River as:

Catch y(t) = 171779.36 - 23706.17 Water Level y

for the whole fishery (Arias pers.comm.) and by Annibal (1983) as:

Catch y(kg) = 56937 DDFy - 164206: (r = 0.84)

for the Plagioscion fishery of the Lago do Rei of the Amazon basin, where DDF is an index of the

amount of water remaining in the system during the dry season.

The amount of water remaining in the system during the dry season may also influence the catches in

subsequent years as was shown by the University of Michigan et al. (1971) for the Kafue Flats

fishery. Here the relationship:

Cy = - 6630 + logeDDFn-1: (r = 0.77)

which was originally formulated for the years 1956–71, has continued valid through 1983 (Hayward,

1984) despite the construction of two sets of flow control dams in the intervening period.

Sufficient data are available from the Kafue and Shire floodplains and from the Central Delta of the

Niger for an assessment of the effects of the intensity of the flood on fish catch. Unfortunately none of

these sets of data include reliable estimates of changes in fishing effort, which may themselves be

responsible for some year-to-year variations in catch.

In the Kafue river records are available from 1954 until the present. However, the fishery was not

judged to have reached maximum expansion until 1958 (Muncy, 1978), and the flood regime was

changed following the closure of the Kafue Gorge dam in 1972. This was followed by the

construction of an upstream dam at Itezhitezhi and the two dams, acting together substantially altered

the duration but not the periodicity of flooding. Calculations based on the years 1958–71 gave good

correlations between catch in year y and the flood of the preceding year (y - 1) (r = 0.72) or the flood

two years previously (y - 2) (r = 0.71). Muncy (1973 and 1978) presented a complete analysis of the

various factors also finding good positive correlations between catch in year y and flood intensity in

the preceding years (y - 1 and y - 2). Although detailed analyses were not continued after 1971 the

predictions of these regressions are consistent with the greater catches that have been obtained

because of the increased flooding following the installation of the two dams.

Sets of data for water level and catch are also available for the Shire river (1969–73) and the Central

Delta of the Niger (1966–74) which were analysed by Welcomme (1975). This analysis used a simple

sum of all weekly mean water levels which exceeded the bankfull stage. The results showed a highly

significant correlation to exist between catch in year y and the flood regime in y - 1 in all three

systems. Correlations of catch with flooding in the same year were not so satisfactory. Because the

fisheries of most tropical rivers are based on fish that are one or two years old, it might be expected

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that the flood regime in the both preceding years might exert an effect on catch of any year. When the

regressions of combined and weighted hydrological indices from both receding years were tested

some improvements in correlation were noted. As a result it was finally concluded that catch in year y

is best explained by a combination of the flood histories of the two preceding years. Thus the best fit

linear regression lines which are plotted in Fig. 7.17 were as follows:

Kafue: Cy = 2962 + 70.54 (0.7 HIy-1 + 0.3 HIy-2)

Shire: Cy = 5857 + 38.11 (0.9 HIy-1 + 0.1 HIy-2)

Niger: Cy = 3239 + 32.10 (0.5 HIy-1 + 0.5 HIy-2)

Differences in Hydrological Index accounted for 57 percent of the variation in catch between years in

the Kafue, 82 percent in the Shire and 92 percent in the Niger. Subsequently the Niger fishery has

continued to show a strong correlation with flood regime and a data set for the 19 years from 1966 to

1984 yields a linear correlation:

Cy = 19.172 + 0.027 (0.7 HIy-1 + 0.3 HIy-2) (R² = 0.84)

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272

Figure 7.17 Best-fit regression lines for the relationship between catch and flood regime for: (A) the

Kafue Flats; (B) the Shire river; (C) the Central Delta of the Niger at Mopti

which HI is calculated from the mean six monthly flood discharge on the Kaulikoro gauge. A linear

relationship was not at best felt, however, and an improvement of correlation to R² = 0.87 was

obtained with the regression:

Cy = 151.73 log (0.7 HIy-1 + 0.3 HIy-2) - 428.26

The log-normal nature of this regression indicate a slight lessening of the response of fish

communities as flood densities increase over a certain level. Other rivers of the Sahel also showed

decline in catch over its period of the drought (Table 7.15)

Table 7.15

Catches of fish in the Senegal river and the Central delta of the Niger

(data from Senegal, Direction des Eaux, Forets et Chasses, 1976 and

Laboratoire d'hydrobiologie de Mopti, 1981)

Year Senegal Niger Upper Chari

1966 110

1967 30 98

1968 25 105 20–25

1969 20 107

1970 18 107

1971 18 94

1972 15 88

1973 12 73 20–25

1974 21 63

1975 25 87

1976 89

1977 87

1978 77

1979 83

1980 88

1981 75

1982 73 4.0

1983 61 1.8

1984 54 0.8

There is considerable interest in identifying the relative role of the two components of the

hydrological regime in determining productivity although it is sometimes difficult to separate them

statistically. For example, where the intensity of flooding is highly correlated with the severity of

drawdown, as in the Kafue (r = -0.78), it is difficult to determine whether the apparent dependence of

catches on the preceding drawdown is a statistical artefact. Some indication of the relative roles might

be deduced the Shire river, which shows a lesser correlation between drawdown and flood (r = -0.45)

and also shows a lower correlation between drawdown and catch. However, this should not be taken

to imply that the high water phase is the more influential of the two phases in all systems. The

difference in response could equally be due to differences in the amount of water remaining in the

system at low water relative to the amount at peak flood. In the Kafue the dry season area was about

27 percent of that during the floods, whereas in the Shire the equivalent figure was 48 percent. It may

be surmised that the more stringent the drawdown, as reflected by the lessened percentage of residual

water during the dry season, the greater the influence of the low water regime on catch in the next

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year. This is supported by the predictions of Welcomme and Hagborg's (1977) simulation of a

floodplain and its fishery which predicted that the more water remaining in a system at low water, the

more the differences induced by variations in the high water regime are transmitted to subsequent

years.

Analysis of catch relative to the flood history of a river has been carried out independently by several

workers. Catches in the Orinoco gave two relationships depending on species (Novoa, 1982). Thus the

total catch over 15 years was:

Cy = -634.8 + 0.53 (0.5 HIy + 0.5 HIy-1) : (r=0.62)

and the catch of Prochilodus mariae over 10 years was:

Cy = -389.4 + 0.19 (0.5 HIy + 0.5 HIy-1):(r=0.73)

In this river a second relationship between catch and flooding was noted for Semaprochilodus laticeps

where, because more water bodies were in contact with the river channel in higher floods, a logistic

relationship was found between an index of flood intensity (HI) and catch during the same year:

Catch was also found to be related to flood height in the Madeira river by Goulding (1981). Here

catch at low water was improved when low water flows were less than average. Heavier floods are

reported by the fishermen to have the effect of releasing fish imprisoned within floodplain water

bodies and increasing the recruitment, the fishery thus leading to better catches in the source as

subsequent years.

Flood strength has also been found to influence catches in temperate rivers. Ivanov (quoted by

Chitravadivelu, 1974) also noted similar effects in the Zofin arm of the Danube, but with a lag time of

one year.

Because of the number of factors involved, for instance changes in fishing effort or the reduction in

the age and length of first capture associated with the fishing up process, it is frequently difficult to

interpret these relationships by direct regression analysis, although visual comparison of plots of flood

regime and catch indicate some relationship to exist. Krykhtin (1975), for example, obtained excellent

relationships between flood strength and catch in the Amur River where the best correlation was

obtained with the flood regime three to four years previously. Dunn (1982) provided graphical

evidence for a correlation between an index of catch in year y and of the flood five years previously

for the Hilsa fishery of Bangladesh. Hilsa, which is a marine anadrome, is thought to enter the fishery

in its 4th or 5th years. In analysing these results fluctuations between 1966 and 1976 are best

explained by a combination of y-5 and y-6 hydrological indices (r=0.76) but between 1976 and 1981

there is little clear correlation. Similarly, graphical inspection of Rasmussen's (1979) catch data as a

function of the Upper Mississippi floods shows there to be parallel trends but this data set is less

consistent and did not yield a conclusive regression.

The lag between the year of flooding and the time when its effects are reflected in the catch are

probably dependent on the time taken for fish to grow to the size range captured by the fishery. In

tropical rivers this is very short, often less than a year, because of the small size and rapid growth of

many of the species caught, and also because of the heavy fishing for fish of the year as they leave the

floodplain. An example of this was provided by Benech and Quensiere (1984) in their studies of the

fishery of the El Beid river which drains the Yaeres of Cameroun. Here there was a strong positive

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correlation between the quantity of juvenile fish moving down river and the floods of the same year.

The same study showed the level of production to be independent of the species composition when a

regular sequence of floods exists. Thus in the El Beid the relation of production to the intensity of

floods compared well between a series of readings from 1968 and another series between 1974 and

1978, although the population structure was very different. By contrast, when the regular series of

floods is disturbed, aberrant patterns may be detected in the following years. Ivanov also remarked on

the high proportion of young year classes in the Danube fishery one year after high floods and a

corresponding drop in catch a year after particularly poor flow. In some rivers where growth is slower

and the larger species are favoured by the fishery, fish may take two or more years before they are

susceptible to the gear. In the Amur river, for instance, Krykhtin proposed that the effect on catch is

only felt after the incorporation of 20–30 percent of the new year class into the target stock.

The fisheries of some reservoirs with highly fluctuating water levels seem to behave like floodplains.

In the Parakrama Sumadra of Sri Lanka a 20 year study by Scheimer (1983) has shown the effects of

fluctuating water levels on fish catch to be described by a relationship:

Cy = 232.2 + 16.2 HIy-3

In the lake fish take from 2–3 years to enter the fishery. This data together with that of Moreau (1980

and 1982) indicate that reservoirs and shallow lakes with highly fluctuating water levels are

ecologically homologous to floodplains.

Cy = 232.2 + 16.2 HIy-3

On the basis of the obvious correlation between the intensity of flooding and catch it is tempting to try

to predict catches in future years from the flood of the year using regression formulae similar to the

ones above. To test this it may be assumed that if one can indeed predict the catch in any one year

from its flood or the flood of preceding years the accuracy of the prediction would improve as an

increasing number of years of data are added to the regression. When the data for the Niger were

treated in this way the accuracy of forecast did improve, from which it was concluded that it is

possible to validly predict catches in river systems from regression analyses of the past performance

of the fishery provided enough terms are available. In the case of the Niger, at least 14 years data were

needed to accurately predict future trends; such predictions assume that there are no major changes to

the resources base through overfishing, environmental change, etc. Likewise, correlations between

water levels in years y and y+1 and catch in the Czechoslovak reaches of the Danube, were used by

Holcik and Bastl (1977) to predict future catches from hydrological flow data. Because of the

importance of such predictions both for the regulation of the fishery and for the establishment of

criteria for mitigation of falling catches due to upstream flood control structures further work on this

topic is desirable.

CHAPTER 8

MANAGEMENT OF RIVER FISHERIES

EFFECTS OF OTHER USES OF RIVERS AND THEIR BASINS ON FISHERIES

Because the river is only one component of a larger system, the basin, fish communities in rivers are

affected not only by events occurring within the channel and its associated waters but are also subject

to a range of external influences. Precipitation falling within the basin eventually finds its way into the

275

river by surface and sub-surface flow carrying with it a variety of materials including the topsoil and

any contaminants it might contain. Changing conditions in the basin can produce differences in water

quality and quantity, as well as in loading with silt and other material which can directly affect

channel form. Many such effects are the result of natural variability, particularly climatic shifts but

more frequently such changes are the result of some human intervention. In river reaches with

floodplains the situation is more complex as the floodplain has a dry season phase during which time

terrestrial use is made of space which is covered with water during the flood. Because of the fertility

of their soils, and their proximity to water courses for irrigation and transport, floodplains are much in

demand for a wide range of activities. They are also regarded as having high development potential

and efforts to ‘reclaim’ them by flood control are widespread. This chapter examines some of these

effects and also describes the observed impacts on river fish communities of uses of the river and its

basin other than fisheries.

CHANGES IN FLOW

Rivers respond to changes in mean annual flow by a period of adjustment to the new regime after

which they stabilize in a form adapted to the altered conditions. Clearly reductions in flow will result

in the progressive restriction of the stream to a smaller bed within the original channel with a

concomitant loss of habitats for fish and other aquatic organisms. Conversely, overall increases in

flow will lead to the enlargement of the river channel by erosion of former banks and other features;

these, in time, should lead to an increase in habitat diversity through extension of the aquatic system.

Biotic sensitivity to changes in water regimes are not limited to alterations in absolute quantity but

also to the distribution of flow in time as food peaks can be moved to different times of the year or

even suppressed altogether.

The living aquatic organisms in rivers are usually adapted to the particular patterns of flow found

there; consequently changes in flow will produce changes in the biotic components of the ecosystem

quite apart from those arising from the contraction or extension of the aquatic habitat. Fish

communities tend to be either limnophilic or rheophilic, depending on the type of water regime

prevalent in the river reach in which they live. Changes in flow will, therefore, tend to favour one or

other of these communities, increasing flows leading to a larger presence of rheophilic species,

decreasing flows encouraging colonization by limnophilic species. Adequate flows are also essential

for the breeding and migration of many fishes. Physiologically fish respond to flood conditions3 by

becoming sexually ripe and by movement to breeding grounds. Conversely, should the appropriate

flow regime not occur, eggs may be resorbed, as in the case of the Rutilus rutilus, Abramis brama and

Cyprinus carpio of the Arakum lakes described by Shikhshabekov (1979). Certain critical levels of

flow are also needed to maintain certain types of breeding substrate in a suitable condition for

spawning. For this reason, there has been much concern over the determination of minimum flow

requirements for fish, particularly in rhithronic areas where high value angling resources coincide

with high demands for water. Examples of such research are discussed by Stalnaker (1980) who has

identified three main approaches to the general problem of evaluating instream flow requirements: (a)

rule of thumb and hydrographic analyses, (b) physical habitat analyses and (c) crop-flow analyses. An

example of the rule of thumb approach is the ‘Montana method’ described by Tennant (1976) (Table

8.1) whereby the effect of the various flow regimes relative to the original regime are described.

Table 8.1

Instream flow regimes for fish and related environmental resources (after Tennant, 1976)

Description Base flow regimes

(% of original average flow in periods)

October-March April-September

Flushing or maximum 200 200

Optimum range 60–100 60–100

276

Outstanding 40 60

Excellent 30 50

Good 20 40

Fair or degrading 10 30

Poor or minimum 10 10

Severe degradation 10–0 10–0

The above figures have been found to describe the effects of flow reductions on a wide range of North

American rivers. Similar series have possible applications elsewhere in the world but, unfortunately,

there is insufficient data in the tropics upon which to base such indices.

In Potamon reaches the problem of maintenance of flow is especially critical as the yield of systems

with floodplains is closely linked to the extent of flooding. Thus, if at any time the floodplain fails to

be inundated recruitment fails in that year for all those species which spawn on the plain.

Furthermore, the fertility of the plain during its dry phase is conditional on its being flooded for at

least part of the year.

Even if average flows within the system remain unaltered changes in the timing or form of the flood

may have grave consequences. In many fish species breeding success depends on a coincidence of

characters of which flow is but one. Consequently races or strains of species have adapted to a

particular timing in their breeding and displacement of the floods to a different time of year may not

permit them to reproduce. There are, however, signs that in some systems fish may adapt to the

altered flow regimes following dam construction. In the Volga River, for example, changes in the

timing of floods and in temperature of the water led to shifts in the spawning time and to an increase

in the duration of the reproductive period in many cyprinid and percid species, even to the extent that

a twin-peaked spawning period has appeared following reservoir construction (Kutznetzov and

Fedeyev, 1979). Equally, abrupt changes in flow characteristics can influence breeding success

adversely. Overly rapid rises and falls in water level can leave nests or spawning grounds dry at

critical periods or can result in eggs or fry being washed away. The precipitous decline of the flood

can result in fish being trapped in temporary water bodies for lack of time to find passage to the main

channel of the river.

CHANGES IN SILT LOAD

Increases in silt load resulting from changes in land or water use accelerate the natural evolutionary

processes of the river system, but in doing so cause a number of problems. In the rhithron deposition

of fine particles of silt on what is normally a coarse substrate suffocates the rheophilic organisms that

normally inhabit such reaches, cutting down on the availability of food. Such choking of the substrate

may also render it unsuitable for spawning by those species requiring swift, well aerated flows and

clear pebble or gravel bottoms. The silt provides an anchorage for vegetation, blocking low order

streams and even diverting them into new courses. Further downstream deposition of silt on levees

and on the river bottom may lead to progressive elevation of the whole channel until it stands above

the level of the surrounding plain. An extreme flood in a channel so encumbered may cause the river

to jump its bed changing its course by some kilometres. Excessive silting of floodplains chokes the

standing waters, which disappear faster than new ones can be generated by erosion. Similarly,

channels and dead arms are filled and new channels are cut to such an extent that the whole delta of a

river may shift along the coastline. At the same time coastal deltaic floodplains grow rapidly,

especially at their seaward end, where new land continuously appears.

Heavy silt loads also directly affect living organisms. The poor light penetration into silt laden waters

reduces the depth to which phytoplankton can develop and shade out submersed vegetation. Choking

of bottom substrates has also been implicated in the disappearance of benthic organisms in the

potamon as well as being identified as the cause of mortalities of fish scattering eggs in such areas.

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CHANGES IN WATER QUALITY

The succession of physical and chemical conditions in rivers from the headwaters to the mouth may

be regarded as a natural eutrophication process. Most downstream reaches are normally enriched, and

further loading with nitrogen, phosphorus and organic compounds from agricultural, industrial and

urban sources appears acceptable up to a point, and may even be beneficial in initially impoverished

systems. If the capacity of the ecosystem to satisfy the BOD is exceeded, however, conditions can

deteriorate rapidly. Fish communities inhabiting potamon reaches of rivers are usually adapted to

eutrophicated conditions and can support a measure of deoxygenation, although some of the more

active species can disappear, Godoy (1975) for instance, traced the disappearance of Triurobrycon

lundii from the Mogi Guassu river to deoxygenated conditions produced by such eutrophication.

General pollution with toxic substances has not so far been widely reported from tropical systems,

although the situation in the rivers of India described by Patil (1977) leaves little room for

complacency. Here several rivers are suffering from severe contamination with industrial effluents

which have adversely affected their fish stocks. Severe local pollution can occur, however, and is

commonly associated with mining where seepage and direct discharge of toxic wastes can create

fishless zones in the rivers affected.

In temperate rivers the situation has been more serious and the widespread degradation of the quality

of water in many European and North American rivers has been widely described. Even large rivers

such as the Rhine or the Vistula have deteriorated to the point where many elements of their fish

fauna have been lost. Liebman and Reichenbach Klinke (1967) also recorded bad conditions in certain

reaches of the Danube caused by domestic and industrial pollution, but the self-purifying capacity of

this large and swift flowing river has been sufficient to keep the main stream at an acceptable quality.

Studies on the floodplain, however, indicated that conditions in some standing waters can deteriorate

to a point where they damage the fish stock, although in others moderate eutrophication by sewage

can raise the productivity.

The effects of pollution on the aquatic life of the system may be summarized as:

(i) lethal toxicity which kills fish at some stage of its life history. In the case of floodplain rivers this

may be indirectly in that the reduction of dissolved oxygen in standing waters of the floodplain and

river channel may make them unsuitable for fish that normally live there;

(ii) sub-lethal effects which are usually difficult to detect or prove but which alter the fish's

behaviour in such a manner as to prevent it completing its normal life cycle, or simply to reduce its

growth or increase its susceptibility to disease;

(iii) cumulative effects which can render fish either unsafe or unpalatable for consumption. Most

pollution effects tend to be very broad affecting many different species. Whatever their immediate

effects, the response at the community level is a reduction in diversity and a shift in species

composition towards relatively smaller, shorter-lived forms. In other words they tend to mimic the

changes expected from heavy fishing and are therefore apt to reduce the amount of fish available to

the fishery. Moderate enrichment with organic substances, on the other hand, can increase the amount

of fish (ichthyomass) supported by the system.

INTERACTION WITH OTHER USES

Terrestrial Animals

Wildlife

278

Many species of game animals move on to the floodplain during the dry season in search of the rich

grazing to be found there. Certain species are more specialized to this habit than are others, and some

are virtually limited to the floodplains for their distribution. Such species as the Lechwe (Kobus leche

kafuensis) migrate from the centre of the plain toward its periphery as the flood rises, and return in the

wake of the falling water (Fig. 8.1). Their distribution is, therefore, a mirror image of that of the fish

and their dynamics are somewhat similar with a maximum in lambing as the water leaves the plain

exposing new pasture (Sayer and Van Lavieran, 1975). Before the construction of the new dams on

the Kafue river there was a stable lechwe population of about 94 000 individuals, but changes in the

regime following the closing of the Kafue Gorge dam has reduced the amount of grazing available

and most probably will cause a drop in the number of individuals supported by this plain. According

to Gonzalez-Jimenez (1977) the capybara (Hydrochoerus hyrochaeris) exists in huge numbers on the

floodplains of Latin America where it feeds on grasses, and also eats aquatic plants. Unlike the

lechwe it swims well and inhabits permanently swampy areas.

Figure 8.1 Distribution of the Kafue lechwe: (A) at maximum flood, April 1971; (B) at low water, 2–

14 September 1971. (After Sayer and Van Lavieren, 1975)

279

Floodplain wildlife is apt to be affected adversely by alterations to their environment in the same way

as the fish. As a further example of this, Attwell (1970) noted some effects of the Kariba dam. Here

the Mana floodplain is of great importance for the conservation of wildlife in the valley. Since the

closure of the dam a lesser area of the plain has been flooded and the remaining portion is under

severe pressure of utilization by the larger species of mammals which has produced changes in the

vegetation. The impact of overpopulation is reinforced by changes in the type of grasses favoured by

the new flood regime, which are tougher. Growth of plants is also less lush as the rich alluvium which

used to fertilize the plain is now removed from the Zambezi water before it reaches the plain. By

stabilizing the flow the dam has reduced the ecological dynamism of the river whose discharges are

wrongly timed, disrupting reproductive patterns of the mammals. A similar theory to this appears in

many systems where mammals and waterfowl have suffered by the drying up of the wetlands (Smart,

1976) and will also emerge when we examine the consequences of dam building to the fish.

The presence of wild ungulates and hippopotami on the floodplain has been considered by Kapetsky

(1974) to be beneficial to the fish and we have already seen the importance attached by Fittkau (1973)

to the crocodile populations of Amazonian rivers for the maintenance of balanced communities. In

fact, the amount of nutrients recycled by the terrestrial components of the system is probably very

large and must have some effect on its overall productivity. It has been suggested that the tendency

for wildlife to disappear in favour of cattle early in the development of the plains, lowers this

productivity as the cattle deposits less of their wastes directly into the aquatic system.

Cattle

Most unmodified floodplains are used as ranges for cattle during the dry season. In certain areas

seasonal migrations of the cattle herding peoples dominate the demography. Rzoska (1974 and 1976)

has described the way in which the pastoral Nuer and Dinka of the Nile Sudd migrate away from their

permanent villages on the higher ground of the swamps following the receding water. As they

progress they burn the dead and drying aquatic vegetation to obtain the fresh shoots upon which the

cattle feed. At the beginning of the flood they return to their island villages after migrating distances

of up to 80 km. Similar movements have been noted from the Central Delta of the Niger (Gallais,

1967; Fig. 8.2) and the Okavango swamps (Stannard, pers. comm.), and are a prominent feature of all

African wetlands. Patterns do differ in some plains, for instance, the Oueme, where the characteristic

“lagunair” cattle are confined to the levees which are ditched or fenced off from the back swamp

depressions. During the flood the cattle are corralled on artificial islands and fed with aquatic

vegetation cut from the floating mats on the plain. Extensive cattle ranching is also a feature of Latin

America, and at present the vast “llanos” of Venezuela and Colombia, which are drained by the

Apure, Arauca and Meta rivers, are used mainly for this purpose. Because the llanos are either

submerged by sheet flooding or extremely arid, the Venezuelan ranchers are erecting large crescent

shaped dykes called “modulos” across the plain. These trap and retain the water as it retreats towards

the main river channels and by evaporation and filtration they slowly dry out leaving a well-watered

fringe of vegetation available for the cattle. The “modulos” also tend to trap and retain fish although

their potential as fish collecting and rearing devices needs to be explored.

Large numbers of cattle are present on the plains, Rzoska (1974) recorded some 625 000 head as

being present in the Sudd. On the Barotse plain there were 310 000 head (FAO/UN, 1969), on the

Kafue 250 000 cattle grazed the plain from May to October (FAO/UN, 1968a), on the Shire

floodplains 148 000 head are present in the Elephant and Ndinde marshes, and in West Africa on the

Central Delta of the Niger, Gallais (1967) estimated some 200 000 head, whereas on the Gambia

floodplain there were 300 000 head. In general cattle and fisheries are compatible, even

complementary utilizations of the plain. The dung dropped by the cattle, estimated at about 500

kg/ha/yr (Shepherd, 1976) converts much of the dry season primary production into readily dissolved

organic and mineral nutrients which have an important impact on the chemistry of the flood waters.

Several workers have noted the enrichment of some standing waters of the floodplain by cattle that

use the lakes for drinking. The exceptional fertility of the Bangula lagoon on the Shire has been

280

attributed to this cause, and according to Gilmore (1976) lagoons of the Okavango frequented by

cattle had more than three times the standing crop of more normal pools, 700 kg/ha as against 200

kg/ha. Some adverse effects of cattle have also been noted. Extensive use of portions of the Oueme

floodplain for pasture have resulted in trampling and breaking down of the banks of the drain-in

ponds found there, with their later abandonments and filling through siltation (Hurault, 1965). On the

flooded banks of small channels and streams, clearance of vegetation for intensive grazing may result

in a lessening of cover with a consequent reduction of fish population. Gunderson (1968) cited the

case of a Montana stream where ungrazed reaches had 76 percent more cover, and the brown trout

living there were more numerous and had about 44 percent more ichthyomass than those then in the

reaches where grazing was common. The literature on the interactions between grazing in riparian

environments, particularly those associated with smaller water courses, and the quality and quantity of

water in United States streams has been surveyed by Platts (1981). It is clear that improper grazing

practices, involving high densities of animals over prolonged periods degrades bottom land and

associated water courses. Changes include loss of vegetation cover, instability and erosion of stream

banks, increase in silt load and consequently in siltation of channels and a decrease in water quantity.

Because of these changes, fish populations, in these cases consisting of salmonids, are much higher

(by 200–400%) in ungrazed than in grazed reaches of lower order streams (order 6 or less).

281

Figure 8.2 The seasonal migrations of the pastoral peoples of the Central Delta of the Niger prior to

the Sahelian drought. The numbers refer to separate ethnic groupings. (Adapted from Gallais,

1967)

Forestry

Because forests tend to conserve water, topsoil and nutrients, they exert a conservative effect on the

aquatic systems draining forested landscapes. The removal of the trees has several readily observable

effects, notably on the water yields in streams, the timing and nature of runoff and the production of

silt. Other effects are less immediately discernible but may nevertheless be important and include

increased concentrations of nutrients, changes in pH and water temperature and more recently

introduction of pollutants into the system.

Deforestation of the catchment area of rivers leads to changes in the flood characteristics whereby

flood peaks tend to become higher and shorter as run-off is decanted straight into the channels. In

forested slopes much water is retained by the vegetation and also in the top soil. As the top soil

disappears there is nothing to delay the water in its move down slope. The faster rise and fall, the

more unpredictable spiky flood regimes and the lower dry season flows are detrimental to many

species of fish which require a smoother transition from one water phase to another. The lack of top

soil and the exposure of the bedrock also tend to lower the amount of nutrients entering solution. The

conductivity of the water drops leading eventually to impoverished conditions in the river. Such

changes are summarized in Table 8.2.

Table 8.2

Changes in stream flow resulting from various land management practices

Treatment Mean

precip.

(mm)

Annual

flow (*) Annual water yield response (stream flow)

Conversion of

Forest to grassland 1 850 840 Grass cover yielded equal stream-flow as former

forest when vigorous but declining grass gave 27

mm more water than forest

Brush to grass 480 30 Increased 127 mm

Abandoned

farmland/ 1150 640 Decreased 25 mm

grassland to forest

Hardwood to conifer 1950 686 Decreased 200 mm after 15 years

Sclerophyll scrub - 490 Increased 142 mm after 8–12 years

to conifer

Natural event

Forest destroyed 580 150 Increased 89 mm

by fire

Forest management

Clearcut regrowth 1220 710 Increases of 284 mm first 3 years

prevented by

herbicides for 3

years

Clearcut 1524 534 Increased 130 mm first year

Clearcut 760 280 Increased 112 mm first 5 years

Selection cut 2030 1290 Increased 55 mm

Strip cut 1220 710 Increased 32 mm after first two- year phase and 114

mm after second two-year phase

282

Regrowth after

clearcut

1221 737 Decrease 52 mm during 7 years of regrowth

* Rise in level in mm caused by increased flow of 1 1/m²

The process of logging itself contributes to the degradation of stream quality. With bad logging

techniques excessive amounts of waste timber and soil enter the stream causing increases in stream

bed load, suspended sediments and dissolved solid concentrations (Graynoth, 1979).

Clearing of the forests allows sunlight to fall on the aquatic system. Thus in rivers in the North

Western United States total salmonid standing crops were significantly greater (some 1.5x) in

deforested sites than in forested areas, although there were also associated charges in year class

strength and species composition of the community. Similarly the more rapid recycling of nutrients

through the repeated growth and death of floodplain grasses on floodplains free of trees would suggest

higher levels of productivity on savannah plains.

It is now apparent that many of the great open savannah floodplains of the present day were once

lined with gallery forests and the plains themselves were covered with scrub forests of the bush

savannah type. Clearance of trees for agriculture, grazing and firewood has denuded these and this

process is still continuing in some of the Latin American plains. As the process of denudation is

historically slow it is difficult to assess the impact of these changes on the fish populations although

these have undoubtedly occurred. It seems probably that the rivers have become less stable with more

frequent changes of channel. The flood regimes would also have changed as extensively forested

plains tend to retain the flood waters longer. There would be a reduction too in the amount of

allochthonous food available to the fish. Nevertheless, most studies seem to indicate that most

savannah plains are indeed more productive than those that are forested possibly because they are at a

younger stage of their succession. However, deforestation may lead to lowered productivity in some

cases such as that which took place in the Grand Lac of the Mekong. Here the surrounding forest was

cleared for agriculture and for firewood, and was accompanied by a decline of the fish catch by about

half over twenty five years. Sao Leang and Dom Saveun (1955) attributed this to erosion and siltation

in the basin and the lessened availability of allochthonous food arising from the reduced area of

forests. The silting led to increased turbidity in the lake through re-suspension of particles by wave

action in the increasingly shallow water body. This in turn led to a drop in primary productivity which

was reflected in a lowering of the fish populations. According to the information now available this

process was halted and reversed during the late 1970s when political troubles brought about the

collapse of population in the Khmer republic. The area around the Grand Lac became re-afforested

and the fish populations increased again.

Much of the increased silting noted in tropical rivers in recent years has been traced to the denudation

of land in the upper reaches of their basins. Here the steeper slope of the land encourages erosion and

the top soil is rapidly lost especially where marginal agriculture is practised (Eckholm, 1976).

Reforestation of hill slopes has been proposed for the control of soil erosion, the protection of

rhithronic streams and for the eventual diminution of siltation in the lower courses of rivers. Locally,

however, reforestation programmes may not be universally beneficial, as experience in Scotland have

shown. Here plantations of Sitka spruce (Picea sitchensis) decreased the pH of streams flowing over

quarzite, schist or slate by collection of acid contaminants from rainfall (pH c. 4.3 – 4.5). In such

streams salmon eggs die within a few weeks and trout are also absent. In adjacent unforested streams

good trout populations exist (Harriman and Morrison, 1982).

Agriculture

The new deposit of alluvial material during the floods each year, a valuable input of organic nitrogen

from blue-green algae and a mobilization of phosphorous and potash through the alternation of

283

aerobic and anaerobic conditions (Bramer, 1980) make flood-plains some of the richest of agricultural

lands. For this reason, they have attracted man's attention from early in his history as a farmer and

some of the earliest civilizations apparently arose in response to the need for communal control of the

flooding of the Nile, the Mesopotamean rivers or the rivers of the great plains of China. Where human

population density is low, simple culture is common whereby small plots are cleared of the moribund

aquatic vegetation as the floods subside. Cereal crops such as maize, sorghum or millet are usually

sown and, aided by the high water table, grow to be harvested before the next flood. At this level of

exploitation little modification of the plain is needed and the cultivated areas are dispersed among

wooded plains. As pressure on the land increases, the floodplain woodland is progressively cleared

and the environment further modified by drainage or irrigation. The needs of intensive dry season

agriculture lead to the filling in of many floodplain depressions and reclamation of permanent

swamps. To assist in this drainage canals are dug which dry out permanently wet areas, but these also

hasten the run-off from the plain so as to lengthen the growing season. Irrigation systems often have

to be installed in floodplain areas deficient in water or to compensate for water lost through improved

drainage. There is thus an increasing tendency to control the flooding of the plain either by poldering,

which keeps the water out of some areas, or artificial levee construction which keeps the river channel

within its banks and stops the annual flood. To complete the control, upstream flood control dams and

reservoirs are also built. Levels of cropping at different intensities of exploitation of bottom lands

have been summarized by Thompson (1983) as:

- Static subsistence 300–600 kg/ha/yr

- Slash and burn 1500 kg/ha/yr

- Intensive cropping 6000 kg/ha/yr

In its simple state agriculture does not conflict with fisheries and on such rivers as the Oueme in

Africa a pattern of intensive use has evolved where both activities are pursued together. On the

floodplain the maize fields are interspersed with drain-in ponds, the banks of which are used for

various types of market culture, tomatoes, peppers or green vegetables. In addition, the higher levees

which follow the river are used for grazing cattle, giving a balanced economy which supports a dense

population. Fig. 8.10 illustrates the typical arrangement of these activities according to Hurault (1965)

who described the land use patterns of this area in some detail.

Unfortunately as agriculture is intensified beyond this level and pursued as a sole objective, the

effects on the aquatic system of the flood control works that become necessary are far reaching. One

result, for instance, is that when the floods are controlled depositions of alluvial silt no longer fertilize

the soil which can quickly become exhausted. One finds a decrease in productions such as that

experienced downstream of the Kainji dam, where within ten years of its closure 50 percent of the

land was no longer suitable for agriculture (Adeniji, 1975). To compensate for this applications of

fertilizer are needed, putting up costs, and raising the risk of eutrophication and eventually pollution

of the water courses. Furthermore, in regions where there is relatively little water and where there are

high evaporation rates, salinization of the soil arises either through repeated flooding with salt water

or though the drawing to the surface of ground waters whose evaporation leaves the salts in the

surface layers of the soil. Should there be insufficient flood water to flush the accumulated salts into

the river, the ground rapidly becomes unsuitable for crops and eventually totally barren. As yet little is

known of the salinity tolerance of floodplain biota, but the salinization of lagoons definitely inhibits

most freshwater forms and, although in regions near the coast colonization with halophytic species of

plants may occur, the ecosystem is in some measure simplified.

The hydrological regime of a floodplain environment is well-suited to rice culture. This may be at the

primitive level of floating rice such as is practiced in the Central Delta of the Niger, but in Asia, the

adaptation of the lowlands of the river systems to intensive rice growing is one of the major features

of the landscape. Intensive rice culture requires a complete control over the hydrological regime.

Flood control dams, polders and networks of irrigation and drainage ditches are therefore installed for

this purpose.

284

Heckman (1979) has examined in considerable detail the ecology of rice fields in Northern Thailand

which become naturally colonized by numerous fish species which contribute to the crops of the area.

In fact the capture of fish in rice paddies in Thailand amounts to some 65-80 kg/ha/yr (figures similar

to those estimated for natural flood-plains), which means that for the 8 million ha of paddies, the total

production could be as much as 580 000 t/yr, most of which is for home consumption. His findings on

general ecology of rice fields (Fig. 8.3) may be taken as representative not only of cultivated semi-

aquatic ecosystems but also, to a certain degree, of natural floodplains as well. The nutrient flow

within the system emphasizes the enormous proliferation in the biomass of rooted aquatic plants

relative to other elements of the community. Evidently in the natural system the offtake toward man

and domestic animals would be absent and this block of nutrients would be returned to the floodplain

by the natural processes of decay.

285

Figure 8.3 General overview of ecosystem nutrient flow in rice culture: (A) early rainy season (1

May); (B) late rainy season; (C) cool season (16 October); (D) dry monsoon (5 December).

(After Heckman, 1979)

286

Because rice growing and fisheries use the same phase of the hydrological cycle there are sometimes

considerable conflicts between them. Fish are frequently accused of destroying rice and Matthes

(1977) investigated the ways in which the fishes of the Niger river interact with this type of culture.

He found that many species were present in the rice fields most of which were juveniles. Certain

species were, however, present as adults and these could be classed in three groups:

(i) fish feeding predominantly on rice, of which four emerged as the most destructive, Alestes

dentex, A. baremoze, Distichodus brevipinnis and Tilapia zillii of which the last two nibble through

the stalks at mid-height thus cutting down the whole plant;

(ii) fish feeding occasionally on rice, particularly Brycinus nurse which bites at the leaves of young

plants, and Oreochromis niloticus which nibbles epiphytic algae from the stems of the rice plants but

also tears away at the stalk;

(iii) species causing occasional damage through other activities such as Heterotis and Gymnarchus

which construct nests of the stems, or Clarias, Heterobranchus and Protopterus which uproot small

plants when probing the mud in search of their benthic food. Some species probably benefit the rice

by seeking out and eating the stem borer and other insect pests or by cleaning the stems of epiphytic

vegetation.

In the Central Delta of the Niger fishermen construct low dykes, 50 cm high, which protect the young

rice during the early part of the rising flood when it is most vulnerable to attack and these work

reasonably well, as by the time fish can penetrate the field the rice is past its most tender stage. For

complete protection, permanent dams with screens are needed to exclude fish from the field entirely,

but these all too often succeed only in sealing fish in rather than out.

Rice culture may affect fisheries adversely in two ways. First, the need to control insect pests has

encouraged the use of insecticides leading to possible pollution of the water in the rice field and

downstream of it. Second, the modifications of the environment associated with intensive rice culture

are detrimental to the fish. The fact that rice and fish are not necessarily mutually exclusive is shown

by the widespread practice of fish culture in rice fields.

Urbanization

In most parts of the world early human communities were established along the courses of rivers and

streams and about 5 000 years b.p. the systematic colonization of the flood-plains of the Nile, Indus,

Yangtse, Tigris and Euphrates rivers lead to the growth of man's earliest civilizations. With

heightened demand for land brought about by population growth, the pressures for physical

occupation of the lower part of water courses has increased despite the risks inherent in the season

flooding of such areas. This breeds the need for flood control which in turn encourages further

occupancy of the plain to the point where a considerable proportion of the population is located on the

100 year flood-plain. For example, about 16 percent of the urban areas of the United States lie within

such areas, and of this over 50 percent is developed for industry or human habitation (Sabol, 1974).

Small wonder, therefore, at a certain preoccupation with flooding in these areas as flood losses mount

annually despite the best efforts to contain them. An analysis of the world occupancy of floodplains

(UN, 1969) summed up as follows: In Western Europe new flood losses are mounting slowly as a

result of the intensified use of areas in major cities which are subject to large but infrequent

overflows. In Eastern Europe there is also an increase in flood losses though somewhat slower than in

North America. In South and Southeast Asia the intense use of long settled land and the development

of new floodplains have more than offset the effects of major river control works. In South America,

there is widespread encroachment of urban growth on floodplains. The situation in Africa is similar

for, although the older settlements were sited away from the main flood zones, new cities are

expanding into these areas. The result of this gradual invasion is a growing tendency to control floods.

287

The construction of cities tends to produce local disturbances in discharge as the existence of large

areas of impermeable surface accelerate run-off in the vicinity of the city, but such effects are still

slight in the basin as a whole. Some changes to flood-plain morphology also follow from the

communication systems which support the city and its surrounds. Navigation by commercial craft on

the river often requires some measure of regularization of flow including the installation of weirs and

locks, and the canalization of exceptionally tortuous stretches to make passage easier. Wash from

boats erodes banks and accelerates siltation and the destruction of marginal habitats. Roads and

railways cut across the floodplains, usually on raised embankments which act as polders or dams to

seal off large areas of the plain restricting movements of fish and, even more important, containing

the flood waters within a smaller area. Urbanization has two additional effects which also bear on the

aquatic environment. One of these, the growing need for power, has similar results to the flood control

measures in that it is associated with upstream dam and reservoir construction. The second, loading of

the waters with organic and inorganic substances, produces pollution and eutrophication in the waters

at the level of the city and downstream of it.

Hydraulic Engineering

Control of the flood in rivers is thought necessary for many of the foregoing uses, and is increasingly

pursued as a major objective in the development of river basins. Three main types of structure are

used either separately or together:

(i) dams and impoundments;

(ii) levees;

(iii) canals;

These all have impacts on the riverine and associated ecosystems upstream and downstream of the

structures which have caused considerable concern in a number of fora and have found expression in

such publications as Ward and Stanford (1979) or Petts (1984). The main effects on fisheries of such

structures are briefly reviewed in Table 8.3 and in the following sections.

Dams and impoundments

The storing of water behind a dam so that it may be released more slowly throughout the year is

probably the most popular of flood control devices. It has the additional advantage that the water can

do work for the generation of power and that it can be used for irrigation or cattle watering.

Furthermore, fish can be grown in the sometimes extensive water bodies that are retained. In size, the

reservoir can range from vast lakes such as that backed up behind the Akosombo dam on the Volta

river, which has a total flooded area of 8 500 km², to small dams which desiccate completely in the

dry season. Some major flood rivers such as the Volga, the Missouri and the Columbia have already

been converted into a chain of reservoirs, and others such as the Danube, the Indus or the Mekong

seem destined for that fate in the near future. The durability of such barrages is questionable as silting

is proceeding at a considerable rate in many rivers and the smaller dams have a predicted useful life of

only a few years before their ability to control floods diminishes progressively. Eventually the

reservoir will fail in its purpose, whether it be water storage or flood control. Larger reservoirs, of

course, may take much longer to silt up, and may last for over a century but the process is progressing

faster than predicted in many areas. As major sites are something of a non-renewable resource the

success of a policy of hydraulic control through the use of dams seems somewhat dubious in the long-

term. Meanwhile, dams produce their changes on the floodplain and the fish. These are brought about

by alternations in the flood regime and by changes in silt loading which in turn alter the dynamics

determining the channel shape. This in turn affects the distribution and persistence of vegetation in the

downstream stretches of river. For example, below the Volta dam the more stable hydrological

conditions and lack of scouring during the flood favoured the rapid development of extensive stands

of submersed vegetation such as Potomogeton octandrus and Vallisneria aethiopica (Hall and Pople,

1968).

288

The flood regime may be suppressed completely, or at least altered in magnitude so that it does not

flood such extensive areas. Very large amounts of floodplain have been lost in this manner as a few

documented examples will show. In the Missouri, the amount of floodable wetlands over a 145 km

sample reach of river dropped from 15 167 ha in 1879 to 7 414 ha in 1967, a loss of 67 percent of the

area (Whitley, 1974). The Illinois river, too, lost about half of its floodplain as 80 939 ha of the

original 161 874 ha were drained between 1903 and 1920. Subsequently, 3 238 ha have been restored

as lakes. Following the closure of a dam on the Peace river, the 2 560 km² of the Peace-Athabaska

delta were transformed from a thriving floodplain environment into a series of isolated mud flats

(Blench, 1972). Although drainage is not so advanced in other parts of the world the intentions are

there. Liepolt (1972) stated that all of the 26 450 km² flood-plain of the Danube will eventually be

drained for irrigated agriculture and the original floodable area of the river is already much restricted.

In the Mekong the intention is to eliminate flooding from the 49 560 km² delta area and 1 480 km²

have already been lost below the Pa Mong dam site. Similarly, in the Senegal river most of the 5

000km² valley floodplain will become dry after the dams which are at present being planned in the

head-waters are built. This presumably is going to be the fate of most of the world's great rivers, at

least temporarily. Many smaller water courses are being modified in a similar manner, and although

this is happening with less attention being drawn to individual cases, the effects of the accumulation

of environmental modifications brought about by many small dams may well be impressive. A typical

example of such an intervention is the Strydom dam on the Pongolo river, South Africa, which has

resulted in the disappearance of 100 km² floodplain and its associated lakes, although in this case

there has been an attempt to keep the wetlands intact through controlled discharges (Coke, 1970). In

the case of the Mogi Guassu-Rio Grande system in Brazil described by Godoy (1975) there has been a

progressive loss of feeding areas on the floodplains of the Rio Grande, by the construction of a series

of dams for industrial use.

Table 8.3

Summary of the effects of hydraulic works on river fish communities

Changes in flow

Temporal changes

Disruption of spawning patterns through

inappropriate stimuli or unnatural short-term

flows

Changes in community structure away from seasonal spawners to

species with more flexible spawning

Shift from pulse regulated to stable system

dynamics

Diminished productivity at community level

Changes in velocity

Increases in flow rate (usually due to

channelization)

Young fish in drift swept past appropriate sites for colonization

Local shifts in species composition in tail race with accumulation

of rheophilic predators

Decreased flow rate Shifts from rheophilic to lentic communities in reservoir

upstream and in controlled reaches downstream Changes in

flushing rate resulting in accumulation or low dilution of toxic

wastes or anoxic conditions leading to fish mortalities

Loss of habitat

Prevention of flooding by dams and levees Loss of floodplain area available for spawning growth; loss of

habitat diversity; change in species composition with loss of

obligate floodplain spawners

General diminution in productivity of whole system

Drowning of spawning substrates upstream of

dams or in channelized reaches

Variable effects usually involving decline of lithophils or

psammophils although new wave washed shore or rock rip-rap

may simulate rhithronic habitats

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Blocking of channel

Interruption of migratory pathways by dam

walls or by the creation of unsuitable

conditions for passage

Elimination of diadromous or obligate migrants by preventing

movement to upstream breeding sites by adults and slowing

down- stream movements of juveniles

Changes in silt loading

Changes in channel form (due to

channelization or to changes in

deposition/erosion process)

Reduction of habitat and community diversity: loss of species

Increased rate of silt deposition (usually

upstream of dams but also in newly cut off

portions of channel or channelized reaches

downstream)

Choking of substrates for reproduction leading to failure to

reproduce in lithophils/ psammophils

Changes in density of vegetation usually in favour of phytophilis

Changes in quantity and type of food available and in the benthos

leading to restructuring of the fish community toward

iliophagous

Decrease in suspended silt load Changes in fish community reduction in number of non-visual

predators and omnivores

Lack of sediment (downstream of dams) Changes in nutrient cycle and in the nature of the benthos leading

to loss of iliophagous and increase in benthic limnivores

Changes in plankton abundance

Increases in phytoplankton in reservoir or

downstream due to slower flow and higher

water transparency

Increase in abundance of planktonivorous fish

Changes in temperature

Changes in mean temperature caused by low

flow regimes

Increasing temperature variation can cause shifts in success of

spawning due to adverse temperatures either for cold or warm

water spawners

Stratification in reservoirs Difficulties of passage for migrant species

Elimination of fish in deoxygenated hypolimnion

Mortalities downstream of dams due to emission of anoxic

waters and H2S

Uptake of water

Induction of water into power stations or

through pumps or irrigation canals

Entrainment of fish into currents diverting them; impingement of

fish on turbines and pumps resulting in loss of fish particularly

juveniles

Water transfers between river systems Transfer of species and disease organisms from one system to

another

The loss of floodplain area for feeding and breeding has serious effects on the fish populations, and

the reaction of the fish communities of the Chari, Niger and Senegal rivers to flood failures provoked

by natural climatic variations such as has occurred in the Sahel also confirm the highly detrimental

effects of suppressing the flood. The significance of floodable plains and backwaters as breeding and

nursery areas in North American rivers is documented by Guillory (1979) and was early appreciated

by Richardson (1921) who found a mean fish yield of 199 kg/ha where 90 percent of the water areas

of the Illinois river was in backwaters, 146 kg/ha where 83 percent was backwater and 78 kg/ha where

63 percent was backwater.

In the Missouri river, Whitley (1974) traced the steady decline of catch from 680 t in 1894 to 122 t in

1963 mainly to the loss of fish habitats following the construction of the reservoirs. On this river, too,

there have been modifications of the main channel by dykes on side channels and backwaters have

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been blocked. The river has been trained by reveted banks into a series of bends. Prior to 1900

Ictalurus punctatus and Ictiobus spp. made up the major part of the catch, but since then the

introduced common carp Cyprinus carpio has become dominant.

Widespread changes occurred in the Colorado River following its conversion into a cascade system

(Holden and Stalnaker, 1975; Holden, 1979). A combination of hindrance to migration, reduction in

river flow and lowered mean water temperatures seriously reduced populations of endemic cyprinid

species such as the Colorado squawfish (Ptychocheilus lucius) hump back chub (Gila cypha), bonytail

chub (Gila elegans) and the razorback sucker (Xyrauchen tecarus) all of which are obligate riverine

species. These have to a large measure been replaced by species exotic to the Colorado basin such as

Micropterus salmoides, Lepomis cyanellus and Notropsis lutrensis which are more tolerant of lentic

conditions. A similar replacement of endemic fish species by introduced species which are better

adapted to the changed hydrological conditions has also been described from Soviet rivers. In the

Volga Chikova (1974) has noted changes in species composition below the V.I Lenin Volga

Hydroelectric station (Kuibyshev reservoir) where the phytophilous fishes, Abramis brama, A.

ballerus and Rutilus rutilus decreased in number, whereas Stizostedion (=Lucioperca) lucioperca and

“Sichel” (Pelecus cultratus ?) increased. Eliseev and Chikova (1968) concluded that the decline of the

phytophilous species is due to the failure and unpredictability of the flood, which when it does

inundate the plain, does so in a sporadic manner which arbitrarily strands young fish and spawn in

isolated pools. Main stream spawners such as the various sturgeons, Stizostedion and “Sichel” are not

so affected. Furthermore the flood peak of the new regime is now somewhat retarded favouring the

later spawning species which generally belong to the second group. Long distance migrant species

also declined in the Volga river following its conversion into a cascade reservoir system. Such species

as Huso huso, Caspialosa kessleri or Stenodus leucicithys disappeared from all but the lowest reaches

of the system whereas the regulated flows and reservoirs favoured such lacustrine species as

Clupeonella delicatula which populated large areas (Poddubnyi, 1979). In the Volga delta the

reduction of duration of flooding of the floodplain water bodies reduced the time available for their

occupation by young fish from 50–70 days before the closure of the dams (1950s) to only 10–15 days

afterwards (1960s–70s). This meant that the downstream migration of the young has been advanced

with less time for growth and survival on the plain. As a result there has been a great reduction in year

class strength (Koblitsbeya, 1985). At least 40 days of regular annual flood are judged necessary for

the maintenance of an adequate fish community in the lower reaches of this river. According to Lelek

and El Zarka (1973) and Adeniji (1975) the changes in the fish fauna of the Niger river below the

Kainji dam are also traceable to the unpredictable nature of the flood. Catch in the reach between

Jebba and Lokoja fell by about 50 percent in three years from about 4 400 t/yr (1967–69) (Otobo,

1978). This was accompanied by changes in species composition whereby the Characidae,

Mormyridae and Clariidae declined but predatory species such as Lates and Bagridae increased in

abundance (Sagua, 1978). Changes were also felt further downstream where the Lower Anambra

basin fisheries declined by about 60 percent (ie. 4 000 t) because of desiccation of much of the

floodplain following the closure of the dam (Awachie, 1978; 1979).

A localized increase in the abundance of fish, particularly of predatory species, immediately below

dams has been noted from many rivers. In the Nile below the Owen falls dam populations of Barbus

altianalis and the recently introduced Lates niloticus were particularly abundant. In the Niger, as we

have seen, Lates also appeared in quantity especially in the reach immediately below the Kainji dam

(Kainji to Jebba); although more recently it declined in abundance (Sagua, 1978) in favour of various

species of Mochokidae, Characidae and Cyprinidae. Chikova also commented on the maximum

accumulation of fish in the Volga as occurring in the same regions downstream of the dams. Whitley

(1974) explained this locally increased production in terms of the enriched water from the reservoir

which, in passing through the sluices, carries with it zooplankton, insects and fish. This enrichment

does not persist for any great distance, however, and in the Missouri was only detectable for about 2

km downstream. Such concentrations are probably associated primarily with feeding, but fish also

accumulate below dams where they are interrupted in their migrations and form the basis of rich

tailrace fisheries such as that described by Otobo (1978) for the Kainji dam.

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Fish may be entrained by the flow out of dams either through the sluices or through the associated

power stations. This usually results in their death by gas disease caused by the explosive

decompression from high pressure upstream to low pressure downstream of the dam.

A history of diminishing catches in the Columbia river from a peak production of 22 440 t in 1911 to

only 6 800 t at present, is mainly due to the blocking of passage to migrating fish and changes in the

flow characteristics of the river, rather than to the loss of floodplain area (Trefethen, 1972). In the

Murray river, Australia, the 800 km migrations of the golden perch (Plectroplites ambiguus) have also

been stopped by a combination of water management practices, including flood control weirs, which

have left few of the original characteristics of the uncontrolled river system unchanged (Butcher,

1967). The Sakkur dam of the Indus did not affect populations of the migratory Hilsa ilisha, but the

construction of the Gulam Mahommed dam further downstream of it deprived the fish of 60 percent

of their previous spawning areas. Further upstream in the same river the Tarbela dam poses a threat to

Tor putitora, a large cyprinid which migrates from the floodplain to the foothills of the Himalayas to

breed. Hilsa stocks have become depleted in several other rivers of the Indian sub-continent. In the

Godavari river a combination of upstream dams and silting through erosion are restricting the species

increasingly to the estuarine delta (Nagaraja Rao and Rajalakshmi, 1976); in the Cauvery H. ilisha has

ceased to ascend the river following the regulation of flow by the Stanley reservoir (Sreenivasan,

1977). Following the construction of the reservoir, Puntius dubius has disappeared from the Cauvery

and Tor khudree is now much more restricted in its distribution. Species have also declined in

abundance in Brazilian rivers where dam construction has been intensive in recent years. Many of the

“Piracema” species have largely disappeared from the Rio Grande and its affluents in the Sao Paulo

state, and Pseudoplaytstoma coruscans was eliminated from the Tiete river soon after the closure of a

series of barrages there. In Venezuela the construction of the Guri dam on the Caroni River impeded

the upstream migration of many species of Pimelodid catfishes as well as the prochilodontid,

Prochilodus mariae and Semaprochilodus spp.

The importance of changes in silt and nutrient discharge from rivers on larger aquatic systems

following damming has been noted from several areas. Ryder (1978) traces the decline of the East

Mediterranean pelagic fishery to the withholding of nutrients in the Nile within Lake Nasser which in

turn has led to some increase in the yield of the lake (Fig. 8.4). Increases also occurred in Lake

Manyala in the Nile Delta where the reduction in flow caused by the Aswan High Dam led to the

silting of the channels connecting the lake to the sea. The freshwater environment then created with

the eutrophicating conditions originating from organic pollution has in recent years (1962–1973)

supported a greater fish population than previously (1908–1935) (Shaheen and Youssef, 1979). A

disastrous decline has been noted in fish production in the Black Sea and Sea of Azov since the main

inflowing rivers, the Danube, Dnieper and Dniester have been subject to increasing control in the

form of cascade systems (Tolmazin, 1979). This has led to a reduction in the inflow of nutrients which

used to support large stocks of fish and shellfish in the estuarine seas. It has also prevented the

discharge of freshwater which formed a well oxygenated layer over the deeper, saline and largely

anoxic waters. A similar decline in water quality and fisheries of the Caspian followed the

construction of the dams of the Volga River. Berdicevsky (1975) (in Carre, 1978) showed that while

the overall catch remained the same a change occurred in community structure away from higher

valued species to small pelagics of less commercial interest.

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Figure 8.4 Regression of the annual fish catch from Lake Nasser on the annual yield of the Eastern

Mediterranean fishery four years earlier. (After Ryder and Henderson, 1975)

The importance of river inputs into larger aquatic systems is also demonstrated by the influence of

river flow on pelagic fish catches in Lake Kariba. Here highly significant relationships between the

wet season flow of the nutrient rich tributaries (Gwaai and Sanyati rivers) and the catch in the

subsequent year were established by Marshall (1982). Variations in the flow of the larger but nutrient

poor Zambezi did not affect catches to the same extent. Although these relationships originated in a

reservoir, the great flow to volume ratio makes nutrient conditions more closely resemble a river.

Thus by extension fluctuations in discharge of nutrient rich tributaries are also liable to influence the

productivity of larger but nutrient poor major rivers. This indicates the possible dangers of even

comparatively small tributary dams for lowering the overall productivity of river, lake or even marine

ecosystems.

Levees

The man-made levee is a type of linear dam which heightens the natural levee upwards to prevent

water spreading laterally on to the plain. The main impact of this is of course to deny fish access to

the feeding and breeding grounds that are necessary for their survival. Catch and specific diversity

drop in much the same way as they do for dams. As the water can no longer spread over such large

areas, sediment and pollution are concentrated into the river and such standing waters as remain,

much as described for the Illinois river by Starret (1972) (Fig. 8.5). Here the lack of spawning

grounds caused species such as pike (Esox lucius), large-mouth bass (Micropterus salmoides) and

yellow perch (Perca flavescens) to diminish in abundance compared to new species such as Cyprinus

carpio which were introduced into the system. The commercial catch declined from 2 613 t in 1950 to

182 in 1973, and angling success showed a similar reduction; effects which were attributed directly to

loss of habitat and siltation following the leveeing of the river and the draining of the bottom lands

(Sparks and Starrett, 1975).

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Figure 8.5 Schematic drawing demonstrating the impact man has had during the past century on the

ecology of the Illinois river and two of its adjoining bottom-land lakes near Havana. (After

Starret, 1972)

The flow during the floods is also increased and as a result fish can be swept out of protected

positions into unfavourable environments. The construction of levees in low lying areas of the Danube

delta has increased the rate of descent of the larvae of many of the migratory fishes. Instead of

spreading out over the floodplain, to grow in the standing waters, these are ejected into the sea. As a

result catches of such species as Alosa pontica, Aspius aspius, Tinca tinca, Blicca bjoerkna and

Abramis brama, are much reduced relative to other arms of the river where no levees have yet been

constructed (Zambriborsch and Nguen Tan Chin, 1973). In the Middle Danube the Yugoslavian

fishery yields have declined from 20 000 t/yr to 1 200 t/yr following the enclosure of the flood-plain

zones by levees. A similar loss in fish production from Rumanian tributaries of the Danube has also

been noted by Bacalbasa following the reclamation of floodable bottom land for agriculture. Here not

only has the catch declined but the remaining population has come to be dominated by Carassius

auratus gibelio which is useless to the fishery (Bacalbasa and Popa, 1978). Similar losses are

described by Butcher (1967) from the Murray river, and pose a potential source of concern when

levees are to be constructed on floodplains supporting “Subienda” or “Piracema” types of migrants.

Effects comparable to those produced by levees can be anticipated from polders which essentially

enclose areas of floodplain within dykes to prevent them from uncontrolled flooding. In this way

many potential breeding areas may be removed from the ecosystem especially when the poldered area

is of any great extent.

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Standing stocks of fish in the poldered Northern portion of the Atchafalaya basin, a distributory of the

Mississippi, has a lower standing stock at 555 kg/hr than the less controlled southern basin with 860

kg/hr. Land clearance and agricultural runoff leading to eutrophication are also implicated in the

lowered yield from the North whereas, in the South, the original allochthonous detrital based food

chains in the annually inundated habitats have been maintained (Bryan and Sabins, 1979).

Gaygalas and Blatneve (1971), traced the poor recruitment of bream, Abramis brama to the poor

spawning conditions in the 23 000 ha² of summer polders, which were constructed to reclaim the

floodplain depressions of the Nyamunas river delta. Summer polders are normally inundated during

the winter floods but are emptied during the spring. Overly rapid drainage does not allow the fish to

breed successfully, whereas with correct management, a slight prolongation of the flooded phase

improved recruitment in other years, and large fish stocks survived in the canals within the enclosed

area. Polders can also be flooded in a controlled manner for rice culture, forms of irrigated agriculture

or, as in Venezuela, for grazing reserves for cattle. Such practices can be combined with extensive

aquaculture. Where polders occupy a large proportion of the plain, the reduction in area available for

the expansion of the floodwaters can result in the acceleration of flow which can alter conditions on

the remaining flooded areas in a way unfavourable to fish.

Channelization

The channel is frequently used as a system for simplifying the natural complexity of a river. The

banks are smoothed and straightened in the interest of the more rapid and contracted evacuation of

water. Whilst channelized reaches of river may protect a particular area their net effect is to increase

the energy content of the water making it potentially more troublesome downstream. Flow patterns

are concentrated in time, producing shorter, higher spates. The rivers themselves may be shortened to

some degree by the evening out of meander bends; about 120 km have been subtracted from the

Missouri river between 1890 and 1947 in this way and the surface area of water has been reduced

from 49 250 ha in 1879 to 24 645 ha in 1972. Trees have largely been eliminated and the flood-plain

forest coverage has declined from 76% (1826) to 13% (1972). By contrast the percentage of land

under cultivation has increased from 18 to 83 over the same period (Burke and Robinson, 1979).

Channelization has much the same effects on species composition and abundance of fish and other

organisms as other flood control measures. In the United States workers such as Congdon (1973) and

Adkins and Bowman (1976) have found that the channelization of streams considerably reduces both

species diversity and biomass when compared with unchannelized stretches of the same river, and

Gorman and Karr (1978) also found that the general reduction in habitat diversity following

disturbances to the ecosystem such as channelization produced a corresponding reduction in both

diversity and stability of the fish community. As examples of this effect Groen and Schmulbach

(1978) reported that fish catch per mile was 2 to 2.5 times greater in the unchannelized reaches than in

channelized reaches of the Missouri. In channelized reaches many of the larger species such as lake

sturgeon (Acipenser fulvescens) paddlefish, (Polyodon spathula) and blue catfish (Ictalurus furcatus)

have almost been eliminated (Burke and Robinson, 1979) and catch has fallen by 80% between 1897

and 1963. Tarplee et al. (1971) found an average ichthyomass of 174.0 kg/ha in unchannelized and

55.3 kg/ha in channelized streams in N. Carolina. Jahn and Trefethen (1973) found 632.8 kg/ha of fish

in unchannelized, 502.9 kg/ha in lightly channelized and 146.7 kg/ha in heavily channelized reaches

of the Blackwater river, Missouri where the mean size of the fish was also much reduced in

channelized reaches. Many organisms other than fish are affected by channelization and Bulkley et al.

(1976) and Griswold et al. (1978) among others, have shown the drift organisms, which constitute the

bulk of fish food, are also much reduced by the siltation of the bottom in channelized reaches. Further

references describing the reduction of aquatic fauna in U.S. rivers due to this type of management

were listed by Schneberger and Funk (1971) and Schoof (1980). Schoof found that in addition to the

effects on the biotic elements of the system channelization can affect the physical environment by

draining wetlands, cutting off oxbow lakes and meanders, reducing ground water levels and the

recharge of ground waters from stream flow and by increasing sedimentation rates. Channels can also

contribute to the diminution of flooded area by deflecting the flow from one river to another, as in the

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case of water transfers, or by bypassing major floodplain areas. The greatest projected work of this

nature is probably the Jonglei Canal, which if completed will lead much of the Nile water past the

Sudd to decant it back into the main river downstream of the swamp at Malakal. This will make much

more water available for irrigation in the arid Nile basin, but the estimated effects of the altered

regime within the Sudd will result in a reduction of maximum flooded area of between 26 and 36%

depending on the water regime adopted (Mefit-Babtie, 1984).

An early phase in the channelization process is the clearance of streams of snags, logs, boulders and

other large debris. This effectively reduces habitat diversity and has been responsible for a great loss

of fish. Even in torrential streams from the North-West United States floodplain habitats resulting

from such debris represented 6 – 29% of the summer rearing habitat and 55 – 70% of the standing

crop of coho salmon juveniles. As a result large log-jammed side channels had eight times the density

of coho than side channels lacking such features (Sedell, Yuska and Speaker, 1983).

Not all changes associated with channelization are detrimental to fish stocks. For example, Hesse et

al. (1982) noted that the rock rip-rap, wing dykes and stone revetments used to contain the flow

enormously increase the area available for periphyton. This offsets the lack of benthic food organisms

present in the accelerated flow of the river.

DEVELOPMENT AND MANAGEMENT OF RIVER SYSTEMS FOR FISHERIES

OBJECTIVES AND STRATEGY

Development and management are here considered as those intentional activities which are directed at

the manipulation of living aquatic resources, the environment in which they live and the human

communities associated with them. The nature of these activities depends on a variety of factors

among which are the purpose for which the resource is to be utilised, the objectives of management,

the type of fish community and the developmental context within which management is pursued.

Strategies for effective implementation of management plans also have to be conceived at different

levels. At the level of the basin, priorities must be set among a number of competing uses of which

fisheries is but one. At the level of the fishery the resource may have to be allocated among several

interested groups and direct management techniques applied to control the type and amount of fish

being caught. At the level of the fish community certain management measures can be employed to

improve the productivity of the fishery. Less directly fisheries may be managed by inducing social

changes or by the development of new types of fishery.

Objectives

Objectives, as opposed to uses, are here considered as those socio-economic criteria which underlie

the planning process. The first level of decision is the assignment of the resource among several

possible alternative uses. Such decisions usually arise from social or economic pressures. For

example, whereas production of fish for food may have high priority throughout the developing

world, the earning of foreign currency through export of luxury items may be judged more important.

Also in industrialised countries the commercial fisheries of the past have largely been supplanted by

sporting interests. Questions of access dominate recreational fisheries, and whether the resource is

open to all-comers or restricted to a certain group is a matter of deep concern to the recreational

fishery manager. Several main objectives consistently recur in development and management plans

for food fisheries; first the production of the maximum quantity of food at an appropriate price,

second the maintenance of the quality of the fish caught, third the provision of employment and fourth

the improvement of the standard of living of the fishing community. Such objectives are frequently in

conflict and the attainment of one in its entirety will impede the realization of the others. For example,

production of the maximum amount of cheap fish protein can only be at the cost of the quality of the

fish caught and usually of the well being of the fishing community. Alternatively, regulating access to

the fishery reduces employment opportunities but increases economic returns from the fishery.

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Decision makers should, therefore, give same weighting to the various objectives to allow for a more

realistic orientation of management plans.

One of the commonest concepts for the regulation of fisheries is that of maximum sustainable yield

(MSY), which predicts that any fish stock has a constant surplus production which may be removed

by a fishery each year. The concept is applicable mainly to single stocks whose abundance is

relatively unaffected by changes in the environment. Few river fisheries are typical of this situation,

and because the concept introduces severe economic weaknesses in that to reach MSY costs incurred

exceed the revenues added by increasing effort. MSY has been also somewhat discredited in recent

years from the population dynamics standpoint. The hunt for a successor concept has proved

unrewarding and MSY is still cited in a loose way as representing the general productive capacity of a

fishery. The concept is particularly inappropriate to those river fisheries which are based on multi-

species communities and here the type of model described is more applicable. As an illustration of the

interaction between the objectives selected and planning, three points may be considered as critical on

the yield curve in this figure.

A. Represents a strategy based upon the management of the fishery for a few high valued species

only. It implies some loss in total potential catch and a great limitation in employment opportunities

but is often coincident with the maximum value to individual fishermen who maintain an elevated

catch per unit effort of a highly priced product. The fishery must be highly regulated to remain in this

state for an extended period.

B. Represents a compromise at which catch is maximal while conserving acceptable quality.

Individual welfare is high but access to the fishery must be restrained. In practise a fishery generally

fails to remain long at this point as social pressures tend to encourage drift towards the right hand end

of the curve.

C. Represents the other extreme. It provides maximum employment and a near maximum yield of

protein but a poor quality product and low level of individual welfare. Given the nature of the catch

per unit effort curve, however, where decreases in catch per unit effort decline and effort intensifies,

the welfare of the individual fisherman may not fall excessively when compared with point D for

instance. This strategy accepts the risks inherent in pushing the fish community near to its limits of

tolerance whereby any further pressure, environmental disturbance or climatic change can readily

destroy the fragile equilibrium and bring about the collapse of the community.

Because it is apparently more difficult for the fishery to move from the right-hand side of the model to

the left for socio-economic and biological reasons, the early setting of objectives is highly desirable to

avoid arriving inadvertently at a position where options are limited. Furthermore, because impacts of

human induced changes in the ecosystem tend to accelerate the fishing-up process, anticipated

changes in water quality or quantity should also be taken into account when setting objectives.

Uses of Fish Resources

The various species of fish that are found in rivers can be used for a variety of purposes.

(a) Food: This is by far the most widespread and important use of fish resources, especially in

tropical and subtropical areas far removed from the coast where freshwater fish often contribute a

significant proportion of the total animal protein in the diet. Fish for food may be either captured as

adults for direct consumption, or may be caught while still in the juvenile or fry stages for stocking

into other bodies of water or for rearing in ponds or cages through a variety of aquaculture techniques.

Most of the world's rivers contain stocks of fish that are used to a greater or lesser extent for food but

such fisheries are usually diffuse with relatively low yields at any one point. However, most of the

great lateral expansion areas, where extensive floodplains exist, support intensive fisheries which are

among the most productive in the world. The location of most of these are indicated in Figs. 1.13–

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1.19. It is difficult to separate the production obtained from rivers from that of lakes and intensive

aquaculture in ponds in any given geographical region. It seems fair, however, to assume that about

half of the world's inland fisheries production of 8.6 million tons (1983 estimate) comes from running

waters, their floodplains and from aquaculture associated with lands inundated for rice culture or other

purposes. Certainly in Africa just under half of the 1.4 million tons is attributed to these sources

(Welcomme, 1979) and in S. America most of the 323 000 t comes from rivers as there are very few

lakes there. By far the greatest proportion of inland fish is derived from Asia, and in particular China,

where various types of extensive aquaculture are common, but river fisheries are also pursued. Fish

rearing in floodplain rice paddies is widespread throughout Southeast Asia and the production from

major rivers such as the Ganges, Brahmaputra, Mekong and Indus alone combine to give some 1.25

million tons.

(b) Sport: Recreational fishing is particularly common in the industrialised countries of the temperate

zones although this type of utilization is now spreading to other areas of the world where there is

heavy industrialization. It is often difficult to define the limits between recreational and food fisheries

as most fish caught for sport is also eaten, thus providing a valuable addition to diet. Furthermore

there is sometimes a recreational element in subsistence fisheries even in non-industrial economies.

Sport and commercial fisheries often compete for the same resource and, because the value of the

same fish is considerably greater to the sport fishery than to the commercial fishery, there is a trend to

favour the recreational interest with subsequent declines in the commercial catch.

(c) Ornament: The beauty and the interesting behaviour of many of the species of riverine fishes,

particularly those of the tropics, together with their small size have favoured their use for ornament or

hobby. Several hundred species are regularly captured or cultured for export providing a small but

significant income to some river communities. The magnitude of the world trade in aquarium fish has

been described by Conroy (1975) and I.T.C. (1979). From these it emerges that large numbers are

caught to compensate for wastage through disease and transport mortalities. Colombia, for instance,

exported some 10 million fish in 1974, Peru 15 million, Brazil 3.5 million, Venezuela 10 million - all

from the Amazon and Orinoco basins. As mortalities between capture and export range between 50

and 70 percent, this represents an offtake of at least 70 million fish, most of which are withdrawn

from such restricted environments as floodplain pools or small feeder streams. Local disappearances

of species due to overexploitation have already been reported from some countries. To compensate for

this, and to reduce the costs of capture and collection from the wild, many of these species are now

cultured in suitable warm waters. The transfer and spread of species by escape from such farms has

resulted in several of the commoner forms becoming pan-tropical in distribution.

(d) Other uses: There are a number of minor uses for riverine fish species including disease vector

control, where fish such as Gambusia or Lebistes have been introduced to control mosquito larvae.

Other species notably Ctenopharyngodon idella have been transferred widely for the control of

undesirable aquatic vegetation.

Developmental Stage

Four main stages can be identified in the modification of river systems and their fisheries which call

for different approaches to research and management (Table 8.4) and three cutting points seem

significant between one developmental stage and the next. The first of these occurs with the

introduction of systematic agriculture to the riparian area which marks the end of the unmodified

stage. Agriculture demands at least some attempt at drainage and regulation of floodplains and also

may involve some abstraction or diversion of water courses. As the support capacity of the basin rises

population increases and fishing pressure rises to the point where it produces a noticeable effect on

the fish stock. The second cutting point seems to arrive with the installation of flood control structures

which begin to modify the flood, impound water in lower order streams and restrict the flood area

available to the fishery. At this stage exploitation of the wild stock is very intensive and its effects,

added to the stresses induced by environmental modifications, can bring about rapid changes in

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species composition and abundance. The last cutting point is reached with the installation of one or

more barrages along the course of the river. These change the nature of the downstream reaches from

flood rivers to reservoir rivers with the consequent loss of much of the previously flooded area. The

four stages listed do not necessarily represent an evolutionary sequence although most rivers in

Europe and North America have passed successively through these steps to reach their presently

highly controlled state. Similar fates are projected for many tropical rivers, but other rivers will

inevitably stay at a slightly or extensively modified stage without ever becoming completely

controlled. Nor does the developmental process necessarily proceed at an even pace along the whole

length of the river. Indeed in many large river systems, such as the Danube or the Mekong, different

reaches and tributaries may be at different developmental stages.

Table 8.4

Stages in the modification of rivers

Stage Basin use Fisheries Research and

management problems Unmodified

Channel and floodplain

show most

characteristic features,

flood regime

unhindered by direct

human interventions,

but indirect effects of

activities elsewhere in

the river basin may be

apparent, e.g., Sepik,

Niger, Sudd

In wild state often forested,

supports game and later used

for grazing cattle. Vegetation

modified by burning. Seasonal

occupation by nomadic

fishermen, hunters and

pastoralists. Slash and burn

agriculture practised in basin.

Fish stocks largely in

original condition of

diversity but size structure

may be modified by fishing

in both river channels and

standing waters. Whole

channel and plain available

for fisheries. Accidental

introductions could result

in the presence of several

exotic fish species.

Exploratory fishing for

description of composition

of fish stock. Identification

of major resources. Studies

of biology of individual

species and their

geographical and seasonal

distribution. Studies on local

fishing methods and

introduction of appropriate

additional techniques.

Establishment of simple

regulatory measures for

protection of major stocks.

Improve access and

marketing network.

Slightly modified

Some drainage

channels for more

rapid and efficient

removal of flood

waters from floodplain.

Smaller depressions

filled or regularized.

Flood still largely

unaltered in timing and

duration. Some small

dams on lower order

streams, e.g., Senegal,

Oueme.

Floodplain largely cleared of

forest, extensive drawdown

agriculture some floating rice in

suitable depressions. Some

areas reserved for grazing and

zonation of flood-plain for

different uses often highly

developed. Settlement on levées

and higher ground, or on

artificial islands and stilt

villages.

Fish stock largely unaltered

although larger species may

be becoming rarer and size

structure heavily biased

toward smaller individuals.

Some depressions may be

dammed as holding ponds,

or for extensive

aquaculture, or fish holes

may be excavated. Whole

floodplain available for

fisheries.

Population dynamics of

major elements of

community to give refined

estimates of potential yields.

Continue studies on biology

to identify possible sub-

populations and to describe

ecological interactions

between species.

Monitoring of fishery to

detect potential over-fishing

of major stocks coupled

with intensification of

regulatory measures to

protect fish stock.

Investigation of simple

forms of extensive

aquaculture. Improvement

and concentration of fish

landings and preservation

techniques.

Extensively modified

Lower order streams Flood agriculture(usually rice) Some modification to fish Examination of general

299

largely dammed for

flood control or

irrigation. Drainage

and irrigation common,

some flood control

through dams and

levées which contain

main channel.

Depressions usually

filled or regularized.

Flood often modified

in timing and duration,

e.g., Chao Phrya,

Mekong.

and intensive dry season

agriculture. Moderately

extensive occupation of the

dryer areas of the plain for

habitation - beginnings of

urbanization. Much of plain still

subject to flooding. Degradation

of lower order streams through

degradation of environment

produced by deforestation and

intensive agriculture, mining,

industrial pollutants and urban

sewage entering river

frequently without treatment.

Pesticides and herbicides from

large-scale monoculture

treatment also enter the river.

stock with disappearance of

larger species. Wild

fisheries often very intense

in main river channels, with

some new fisheries in

reservoirs. Disappearance

of most long-distance

migrant species. Rice fish

culture in suitable areas.

Drain-in ponds and some

intensive fish culture in

regularized depressions.

River area available for

fisheries restricted.

dynamics of fish community

to judge reaction to various

sources of loading.

Intensification of

monitoring of fisheries with

increased control of

catching methods by

licensing and legislation.

Examine impacts of other

activities in the river basin

on the fishery and

endeavour to ensure that

suitable conditions are

maintained. Investigation of

intensive aquaculture

methods. Consider

development of reservoir

fisheries and seek

alternative employment to

reduce fishing pressure on

main river.

Completely modified

Flood control by large

upstream dams and by

levées. Main channel

sometimes

channelized. Flood-

plain largely dry

although still subject to

occasional catastrophic

floods. River often

reduced to a chain of

reservoirs, e.g.,

Mississippi.

Urbanization, intensive use of

river basin for agriculture,

industry, habitation. Mining,

industrial and urban pollution to

some degree controlled,

eutrophication usual. Pesticides

and herbicides input.

Fish stock changed by loss

of some species through

pollution and

channelization and

sometimes by introduction

of exotic species. Some

sport fisheries in main

channels or in few lakes

that have been retained on

flood-plain. Some intensive

aquaculture in specially

constructed ponds. River

area available for fisheries

very small, but intensive

fisheries may be developed

in the reservoirs.

Investigation of

eutrophication and pollution

and other management

impacts to establish criteria

for maintenance of fish

stock. Regulation of

discharge and effluent

according to these criteria.

Contemplate introduction of

new elements to fish

community or stocking to

support threatened species.

Study access problems to

fishery to resolve

conflicting demands of sport

and commercial fishermen.

Intensify development of

aquaculture and reservoir

fisheries.

The four stages proposed correspond approximately to those presented by Yon and Tendron (1981)

(Figure 3.7) for European rivers. A similar series of categories has been proposed by Scudder and

Conelly (1984) on the basis of socio-economic criteria, which to a certain extent parallel the above

classification. Their four categories are:

(a) Primarily subsistence; in which fishing methods are limited to traditional, locally manufactured

gears and both men and women participate in the fishery. The catch is consumed entirely within the

boundaries of the fishing community.

(b) Incipient commercialisation; where fishing methods are still diversified but more modern twines

and gear are used. Division of labour is more apparent with the men fishing and the women carrying

out support activities. The fishery usually produces sufficient surplus for part of the catch to be sold

outside the fishing community.

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(c) Primarily commercial; gill nets and seines are now the most common gear and the traditional

methods have largely disappeared. The fishery is pursued almost entirely for markets external to the

fishing region.

(d) Increasing marginalization of traditional communities; a period marked by a decrease in total

catch and decline in the fishery corresponding to a depletion in the resource.

MANAGEMENT OF THE RIVERINE ENVIRONMENT

There is an increasing trend at the present time to attempt to master rivers and to control their floods

so as to make their basins more productive for agriculture and safer for human occupation. Whatever

the long-term benefit and success of these attempts they do modify the aquatic environment in such a

manner that the fish stocks are adversely affected. If rivers are to continue to produce fish, it must be

within an entirely different framework from the capture fishery as practised in unmodified rivers.

PRESERVATION OF THE NATURAL SYSTEM

The biology and ecology of the majority of fishes inhabiting rivers show them to be extremely

sensitive to modifications of the flood cycle and to the environmental changes that occur in modified

rivers. In order to retain something of the natural diversity of the stocks of fish certain areas have to

be maintained in their wild state and it is hoped that the setting aside of land or some river systems for

suitable reserves be considered in the planning of land use within a river basin. As has been discussed

various types of agricultural engineering and construction works also interfere with the natural pattern

of flooding. Thus management policies involve planning of land use within the river basin as a whole.

Levees, polders and embankments for transport systems prevent the water reaching floodplain

depressions, and the remaining water is restricted within main channels in such a way as to modify its

flow. An essential part of planning for fisheries is, therefore, to make sure that any lateral expansion

zones which are designated as reserve areas should be kept free of such structures. A second

important feature is the provision of sufficient water to produce a flood which will have the

characteristics needed for the reproduction of fish species. This is both a question of quantity and

timing which have to be based on an adequate knowledge of the biology of the fish species concerned.

Controlled releases of water have been tried in some systems, and on the Pongola river, a series of

experimental floods have successfully filled the lagoons and induced breeding in the species

inhabiting them (Coke and Pott, 1970; Phelines, Coke and Nicol, 1973). Similarly, sufficient water

has been released from the Shire river dam to fill the lagoons of the Elephant and Ndinde marshes,

with satisfactory results for the fisheries. Kenmuir (1976) showed that the juveniles of several species

of fish, including Clarias gariepinus, Brycinus imberi, B. macrolepidotus, Labeo altivelis, L.

cylindricus, Eutropius depressirostris, Synodontis zambezensis, Oreochromis mortimeri and Tilapia

rendalli appeared in lagoons of the Mana floodplain of the Zambezi river following flood releases

from the Kariba reservoir. Furthermore sexually active adults of these species were observed moving

on to the plain during flood releases that corresponded to the time when the Zambezi would normally

flood. He therefore concluded that artificial releases of water (simulated floods) can stimulate

breeding in riverine fishes provided that they occur during the previously normal flood period.

The correlations between drawdown factors and catch detected in some floodplain rivers, as well as

the theoretical predictions of Welcomme and Hagborg's (1977) simulation indicate that standing stock

and yield do not depend solely on the flooded area, but also vary according to the amount of water

retained in the system during the dry season. Because of this, the shortfall in catch following a

reduction in flooded area can to a certain extent be compensated for by increasing the area of residual

water, provided the reduced floods still occur at the appropriate time. Where water management

actions are being planned which will result in the loss of areas of floodplain the provision of large

permanent bodies of water may be considered at an early stage. In all probability fish community of

systems managed in this way will undergo changes in species composition probably in favour of the

blackfishes much as in ordinary reservoirs.

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The management of the natural system should not be restricted to the floodplain, but should be

extended upstream and downstream as part of the manipulation of the whole basin. As we have seen,

activities in the headwaters of the basin can alter the siltation, runoff and flow patterns of the potamon

reach, often with far reaching consequences to the fishery. Equally, pollution or damming

downstream can prevent species from moving up to the floodplains to feed or breed.

INSTREAM IMPROVEMENT STRUCTURES

A variety of structures can be used to restore more natural patterns where they have been destroyed or

to stabilize river channels whilst maintaining the diversity of habitats therein. Although such

structures tend to diminish the effectiveness of channels modified for drainage, irrigation, flood

control, etc. they can be used in a controlled fashion to improve the biological productivity of the

systems concerned.

According to Swales and O'Hara (1980) three main categories exist:

(a) structures which impound or modify flow;

(b) structures which provide direct cover;

(c) improvement of spawning areas.

Structures which impound or modify flow

Traditionally groynes or wing deflectors have been used for many years to improve salmonid streams

and have been proved effective in introducing artificial sinuosity into a straightened channel. They

also create slacks in the flow shadow immediately downstream and by scouring and deposition help

re-establish pool riffle sequences along the channel. As a pool riffle sequence characteristically recurs

at intervals of five to seven channel widths such devices should be installed with this spacing between

them. Low dams and nets are even more effective in the restructuring of the bed of the river to create

pool-riffle sequences. Both these types of device should be accompanied by bank stabilization to

prevent selective erosion by the deflected current. The use of structure of this type is not limited to the

rhithronic reaches of rivers. Burke and Robinson (1979) and Schick et al. (1982) describe a number of

structures which have been used to restore habitat diversity to channelized potamonic sections of the

Missouri and Mississippi rivers. These include (i) notched dykes, where portions of transverse barriers

are lower than the rest of the structure. These allow water to flow through the barrier so as to develop

or preserve side channels or to prevent further land accretion; (ii) Low elevation structures, which are

below the water surface for much of the time and which prevent island formation that can support

vegetation. They also lead to diversified depths particularly in that a deep pool develops downstream

of the structure; (iii) rootless structures are transverse dykes which do not abut on to the back, a

variant, vane dykes, are angled relative to the current. Such structures encourage multiple sub-channel

formation and create more bank area; (iv) chute closure structures are placed in such a way that side

arms, fringing lakes etc are maintained in contact with the main river channel. They also combat

permanent land accretion and the loss of spawning areas. On the whole such structures appear

successful in increasing habitat diversity and although there is no immediate evidence for increases in

biomass which would in any eventuality be difficult to detect in such a large river, various species do

appear to be showing distinct preferences within the new richer ecosystem.

Structures which provide direct cover

At the most primitive, such structures consist of fixed or floating objects which provide shade in

simulation of missing vegetation cover in larger river channels. Submerged objects may also serve a

similar purpose. Thus logs, boulders, branches etc, may be placed adjacent to the bank, a principle

elaborated by many tropical fisheries into brush parks or artificially created or anchored masses of

vegetation.

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Because snags and other large organic debris are demonstrably important in maintaining habitat

diversity in small to moderate size streams (order 1 to 4) Sedell et al. (1982) proposed that, where

channels had been cleared, such structures be either artificially reintroduced or be allowed to

accumulate in order to increase the productivity of such waters.

Spawning area improvements

Several systems have been attempted for the improvement of bottom texture for fish which spawn

over gravel, particularly where siltation has resulted in the loss of the normal substrate. These

methods often consist in mechanical or hydraulic disturbance of the gravel causing deposited

sediments to be washed out, although the more costly and arduous physical replacement of the

substrate has also been tried. Letichevsky (1981) for instance, quotes the success of the recreation of

sandy spawning substrate of Stenodus leucichithys in the lower course of the Volga, which has now

supplanted a more expensive intensive fry rearing and stocking programme.

Fish passes

One of the remedies commonly proposed for blockages to migrations caused by dams is the

construction of fish ladders or passes similar to those which work so satisfactorily for the salmons in

North Temperate rivers. In some cases fish passes have been successfully installed in the tropics. One

such, at Cachoeira de Emas on the Mogi Guassu river, has been functioning since 1936 with little

apparent detrimental effect on the stock. Here many characin species leap up a series of shallow steps.

The siluroids, which do not jump, migrate past the barrage through special tunnels. The several

fishways at dams on the Tigris and Euphrates rivers have also been used successfully by many of the

migratory cyprinid species in these rivers (FAO/UN, 1956). Furthermore fish passes in dams blocking

the Himalaya tributaries of the Ganges also appear to function adequately for Tor spp. and Indian

major carps to the extent that the upper chambers of the ladders are frequently used as fish traps.

Most attempts at using fish ladders with tropical species have been less successful. Bonetto et al.

(1971) claimed that the efficiency of the fish ladders which bypass a series of dams on the Caracana

tributary of the Parana was very low and that Salminus maxillosus, one of the major migratory species

of the basin, was unable to negotiate them. Bonetto (1980) repeated the pessimistic view in describing

the anticipated performance of fish ladders to be installed in the Salto Grande and Yacyreta dams,

although subsequent experience with the Borland lifts installed in the Salto Grande dam showed that

certain species including: Salminus maxillosus, Pseudoplatystoma coruscans, Colossoma mitrei,

Doras spp., Oxydoras spp. and Prochilodus platensis did in fact use them to pass upstream. The fish

ladders installed at the Markala barrage on the Niger also did not fulfil their intended function

satisfactorily. Here enormous quantities of fish were blocked during their upstream migration at low

water although some fish did get through. The fisheries above the dam declined considerably after it

was closed as these reaches depended on a replenishment of their stocks from fish moving out of the

Central Delta. The failure of the ladder was attributed by Daget (1960) to insufficient capacity in view

of the very large numbers of fish wanting to pass over it. Furthermore, the migration of the species

concerned was a simple dispersal movement rather than a breeding migration and the stimulus to

surmount obstacles was possibly relatively low as a consequence. The failure of conventional

salmonid ladders in many tropical rivers is hardly surprising in view of the complex behavioural

factors which contribute to the functioning of such structures. For example, it has been shown that the

form of the standing wave downstream of weirs is essential for salmon to be able to generate enough

speed to clear the crest of the weir. Presumably other species have very different requirements which

need to be studied before the installation of effective fish passes can be contemplated. In any case the

provision of a fish pass or ladder for upstream movement of fish would only be justified where the

migration is absolutely essential for the maintenance of the fish stocks.

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MANAGEMENT OF THE FISH STOCK

In principle a fishery exploits a community which in its unexploited state exists in some kind of

equilibrium with itself and with the environment. Under normal circumstances such a community may

be assumed to tend to maximize its biological productivity and to continue to do so when subjected to

a reasonable level of exploitation. In such a situation manipulation of the community in all probability

adds little or nothing to the yield of the fishery. There are two circumstances, however, under which

such interventions may be desirable. First, where the fish community is lacking some element to

exploit a food resource or habitat (i.e. a ‘vacant niche’) it may be considered advisable to introduce

one or more new species into the waterway. Second, where the fish community as a whole, or some

preferred element of it, is overfished a policy of stocking may be adopted.

INTRODUCTION OF NEW SPECIES

In their unmodified condition most rivers support fish communities that are sufficiently diverse to fill

most of the available trophic and spatial niches. Nevertheless some systems do exist which, for

zoogeographic reasons, have poor or incomplete faunas. Furthermore as systems are modified there is

a tendency for certain indigenous species to disappear and for others to take their place. This is most

noticeable in impoundments where many of the migratory riverine species are lost soon after the

stabilization of flow. In many rivers species which are adequately adapted to the new conditions are

absent from the original fauna and to overcome this additional species may have to be introduced

from other river basins. In some cases introductions have been performed uncritically, and such

species as the carp (Cyprinus carpio) or Oreochromis mossambicus have themselves been blamed for

degradation of the environment or fish community structure. These and others, for instance Salmo

gairdneri, have been implicated in the elimination of sometimes delicate native resources, such as the

unique faunal associations of Northeast California (Cooper, 1983) or the endemic galaxiids and

Protroctes oxyrhynchus from New Zealand. Such cases often arise when there has been a

contemporary disturbance of the environment which may have led to the elimination of the species in

any case. The presence of introduced species may thus be considered an advantage as they are able to

colonise the disturbed habitats and thereby maintain or increase species diversity (Leidy, 1983).

Fishes such as these, which are among the main elements for intensive aquaculture, find their way

into natural waters by accidental release from farm ponds. Any introduction of a new species into the

fish fauna of a river should, therefore, be preceded by a very careful analysis of its anticipated

impacts. The procedures for such an analysis have been defined by EIFAC/CECPI (1984).

While widespread or impetuous transfer of new species is to be decried, the great success of these

same species under other circumstances points to the role of properly considered introductions. Thus

most of modern aquaculture is founded on a comparatively small number of introduced species, the

tilapiine cichlids have been invaluable in increasing the yield from Asian reservoirs (Fernando, 1976)

and the common carp supports fisheries in many highly eutrophicated streams. Typical of such

successful transfers is that of the chinese carps into the Amu Darya river. A combination of the

macrophyte eating Ctenopharyngodon idella with the planktonophage Mylopharyngodon piceus,

Hypophthalmichthys molitrix, Aristichthys nobilis and Parabramis pekingensis colonised the system

including the adjoining Kara Kum canal thereby laying the basis for a rich fishery.

STOCKING

Three main motives for stocking exist:

(a) Stocking to maintain production in the face of intensive exploitation

(b) Stocking to mitigate or compensate for adverse effects of some activity within the river basin

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(c) Stocking to increase production from individual components of the aquatic system, usually

through extensive aquaculture or ranching;

With the exception of trout streams and some sport fisheries in the temperate zones systematic

stocking of rivers to maintain or improve their fish stocks is not yet a widespread practice. In fact the

tendency has been more for rivers to act as a source of supply of juvenile fish seed for stocking into

reservoirs and fish culture installations. The requirements for successful stocking of salmonids have

been extensively studied and rules of thumb for numbers of fish and age of fish to be stocked have

been derived for various waters (EIFAC, 1982). The requirements and indeed the rationale for

stocking with other species is less well understood and in many cases current practice appears

arbitrary and counterproductive. There is considerable room for the expansion of stocking as a tool for

the management of river systems.

MANAGEMENT OF THE FISHERY

Traditionally a number of practices exist for the control of the fishery. These usually involve direct or

indirect control on the fishermen by limiting access to the fishery, or on the practise of fishing through

control of season, location or type of gear (Fig. 8.6). Usually attempts at direct control through

legislation result in costly and sometimes oppressive enforcement programmes, whereas less direct

manipulation of the economic forces regulating the fishery may be more effective. Whichever the

approach adopted, if controls do not correspond to biological realities regulating the fish community

attempts at regulating the fishery will be counterproductive.

REGULATION OF ACCESS

As shown in Fig. 7.13 the total catch is related to the number of fishermen operating on the river.

Furthermore the individual artisanal fisherman seems to have a limited fishing power in that he is

physically capable of removing only a certain quantity of fish from the system in any one year.

Because of this the solution that seems most appealing in this type of fishery is a simple restriction on

the number of fishermen operating in a certain region. Such control of access through licensing

remains one of the most important managerial tools, particularly in commercial or recreational

fisheries. An allied technique, the fixing of catch quotas for individual fishermen poses enforcement

problems in artisanal fisheries but may be practical where more intensive industrialized practices are

adopted. Many traditional systems also practice forms of access control by allowing only certain

groups within the community to fish the waters. In many rivers, especially in Asia, such control is

achieved by dividing the river and the plain into lots, for which individuals or groups of fishermen

compete at auction each year. The job of policing the lot rented then devolves upon the fishermen

groups. The renting of stretches of river to fishermen for restricted periods has been criticized on the

grounds that any group with only temporary tenure of a fishery will attempt to gain the maximum

profit within the time available to it. This leads to ruinous fishing practices where the stock is

exploited to the point of exhaustion. On the other hand, where areas are held by tradition or contract

for a number of years as in many parts of Africa, the fishermen are free to husband the resources and

develop the fishery. A final form of access control is usually less direct by manipulating the economic

conditions of the fishery so as to dissuade fishermen from entering the fishery. This may take the form

of taxation, price control on fish landed or regulating the supply of fishing gear all of which are aimed

at lowering net revenues to the fishermen.

Unfortunately the regulation of access to the fishery by traditional or by planned management tends to

break down under the pressure of unstable socio-economic conditions. This occurs particularly where

there is a high rate of rural unemployment and landlessness and fishing is viewed as a relief

occupation producing an influx of numerous new and uneducated persons into the fishery.

305

Figure 8.6 Diagram of the relationships between the main techniques for fishery management

INCREASING THE CATCH CAPACITY OF FISHERMEN

Improvements in the catch capacity of the individual fisherman can be brought about in several ways.

Introduction of new types of gear or the amelioration of existing ones enables the individual to

capture larger numbers of fish of a greater range of species. Better materials mean that the gear is less

liable to damage; consequently less time has to be spent in its maintenance and more on the fishing

grounds. Proper boat design and motorization of craft can get the fishermen to and from the fishing

grounds faster. Historically these processes have already occurred in many rivers, first with the

introduction of perfected gill nets, seines and cast nets, second by the introduction of nylon twine and

netting, and third by the adoption of the outboard motor. In some fisheries too, traditional methods,

especially those centred on cross river barrier traps which capture migrating fish, are as efficient as

the more modern techniques that seek to replace them. The effectiveness of the fishery may also be

conditioned by geographical accessibility or by the lack of adequate facilities for treatment which

limit the amount of fish that can be handled. Because of this some fisheries do operate below their full

capacity. However, the efficiency of much of the traditional and improved modern gear is such that

individual stocks have collapsed in some river systems and other whole communities are near

maximum exploitation, if not actually overfished. Improving the efficiency of the individual

fisherman does not, therefore, appear to be appropriate in many cases and should be regarded more as

a means of fine tuning the fishery as development of other sectors occurs. In other words, in areas

where there is a great demand for rural employment it is probably advisable to restrain the

introduction of more efficient methods and to concentrate on better marketing and other support

facilities. In such circumstances, where better gear is introduced, the improved performance is either

dissipated in a diminished catch per unit effort as the stock declines, or the less prosperous and

efficient fishermen, who cannot use or adapt to the new gear, are forced out of the fishery. As the

general economy of a country increases on the other hand, there is a tendency for labour to be

withdrawn from the rural sector including fishing. In such cases, fisheries have declined due to

306

fishermen leaving their trade, which in any case becomes less and less profitable. Here a rapid

increase in the individual efficiency of a few fishermen can to a certain extent halt the decline and

preserve a reasonable way of life for those who choose to remain in their previous profession.

Closed Seasons

By the nature of their biology, most river fisheries have built-in closed seasons. In the floodplain this

lasts from just after bankfull on the rising flood to peak floods when the fish population is too

dispersed and individual fish are too small in size for them to be readily available to the majority of

methods of capture. In the torrential upper reaches flows are frequently too high for effective fishing

during some of the year. This generally discourages fishing throughout the period as yields are low,

effort needed to capture fish high, and there are additional dangers of loss of gear in the currents and

floating vegetation masses. This effective closed season makes biological sense in that it allows the

fish to reproduce relatively undisturbed and for the young to grow to a reasonable size before they are

exposed to the fishery. Closed seasons outside this time are of limited value, although restrictions on

the overly heavy fishing of migrating adult fish to the spawning grounds may be necessary in some

places, as experience with Labeo in Africa have shown.

Reserved Waters

As fishing of the floodplain and river channel in the dry season becomes more intensive there is a risk

of local over-exploitation of the stock. For this reason, traditional fisheries have long been based on

the designation of certain floodplain depression lakes and reaches of the river as reserves which

remained unfished. In larger systems, there are usually inaccessible areas which form reserves as they

are infrequently exploited and it is probably from such areas that the Niger and Senegal rivers were

re-colonized during temporary reversals of the arid conditions of the Sahelian drought. As a

management measure the conservation of certain areas is probably a wise move and gains force when

other pressures are being applied to the system.

Mesh Regulations

The use of mesh selective gear almost always entails a consideration of the mesh sizes to be adopted

which can only be viewed relative to the characteristics of the stock to be exploited. As has been

described, the fishing-up process almost always involves a drift downwards in mesh size which needs

considerable enforcement of legislation to stop. Any lower limit on mesh size, therefore, has to be

imposed in the face of a natural trend to disregard it. It also poses the classic dilemma of how to

manage multi-species stocks consisting of species with a range of sizes. If the objective of the fishery

is to exploit only the large species of the community, the imposition of mesh limits which protect the

immature fish is probably the only way to do it effectively. However, one almost certainly neglects a

considerable proportion of the potential ichthyomass in this type of fishery as is shown by the number

of species caught by progressively lower mesh sizes (see Fig. 8.7). If the mesh size is lowered to take

advantage of the smaller species, then almost automatically the larger ones will disappear. One

possible solution to the dilemma is the limitation of mesh size in major gears such as seine nets, gill

nets, etc., coupled with a use of a variety of minor gears which are aimed at particular smaller

elements of the stock. In other words, in a fish community which is highly diverse it is as well to

maintain an equally diversified fishery.

Banning of Certain Gears

The restriction or complete outlawing of more destructive fishing practices is most important.

However, even such methods may be appropriate in some circumstances. Poisoning of water courses

is of course liable to damage the stocks of fish when carried out in the main channel of the river,

whereas its use for removing fish from temporary floodplain pools or for eradicating undesirable

307

species may be quite permissible. Unfortunately were the use of poisons allowed in one habitat it

would rapidly extend to others. Gear is often prohibited for reasons other than those bearing directly

on the fish stock. Long lines, for instance, are regarded with disfavour by users of cast nets which may

become entangled in the hooks. Barriers which completely block the river channel, thus stopping fish

migrations, are as likely to be removed for reasons of navigation as for fisheries.

Figure 8.7 The number of species captured by gill nets of different mesh sizes (after Reizer, 1974)

Accessibility

One of the main factors determining the degree to which the fisheries of the world's rivers are

exploited is their accessibility and proximity to major centres of population. Many of the tributaries of

major rivers such as the Ganges or the Amazon are still relatively unexploited due to the absence of

access roads, and some of the greatest floodplains are still relatively little utilized by reason of their

distance from suitable markets and lack of living space for the fishermen. The Okavango delta, the

Gran Pantanal of the Paraguay river or the Central flood regions of the Congo river are typical

examples of areas where development is conditional on the provision of suitable access routes and

centres for the marketing of catch, as well as artificial mounds or other appropriate locations for the

fishermen to settle near the fishing grounds during the flood.

DEVELOPMENT OF NEW OR ALTERNATIVE FISHERIES

Aquaculture

308

Natural production of rivers can be exceeded by various forms of husbandry, intensive or extensive

aquaculture. Such methods are widespread particularly on floodplains, and various techniques are

available for the different developmental stages of rivers.

Blocking of natural floodplain depressions

Complete blocking of small channels draining water from depressions is common in many systems.

Fish are conserved in the pools thus created and are fished later in the seasonal cycle at times when

fish from other sources are becoming scarce. This method is often the first systematic attempt at

husbandry or extensive aquaculture. Experiments with more permanent dams have been carried out by

Reed (FAO/UN, 1969a) in the Niger and by Reizer (1974) in the Senegal. Reed's work showed that

the area of standing water on the plain was increased by such installations (Fig. 8.8), and Reizer

showed how the level and area of such pools differ before and after damming (Fig. 8.9). Harvest from

such pools may be quite respectable - about 185 kg/ha/yr in the case of otherwise unmanaged pools in

the Niger and up to 500 kg/ha/yr in depressions which are stocked with fry and fed.

Figure 8.8 Tracing from aerial photograph of the Niger floodwater retention dams, showing the

original water area and the area flooded after the placing of the dams. (After FAO/UN, 1969a)

Drain-in ponds

In more advanced culture systems on floodplains depressions are regularized and depended and

eventually their number may be increased by digging new ponds into the surface of the plain. Three

river systems particularly have benefitted from this form of management. In the Oueme about 3

percent of the 1000 km2 surface area is occupied by drain-in ponds some of which are several

kilometres long (Fig. 8.10). Similar constructions on the Mekong floodplain ranged from 20 to 100 m

in length and 2–3 m in depth. In Bangladesh some 30 000 ponds have been formed from the borrow-

pits which were excavated during the construction of the artificial islands upon which much of the

rural population lives. Drain-in ponds are usually fished by blocking a portion with a bamboo barrier,

309

successively removing the accumulated vegetation and advancing the barrier until the fish are

confined in a small space from which they are easily removed. Drain-in ponds are also used in

conjunction with rice fields where, as the water is drained prior to harvesting, the fish may retreat into

ponds or ditches constructed for this purpose. Yields from drain-in ponds may be high. In 1955–58,

34 ponds from the Oueme produced a mean of 2.1 t/ha and even though the catch dropped the same

installations were still producing over 1.5 t/ha ten years later. In the Mekong this type of pond was

suppressed following the recommendations of Chevey and Le Poulain (1940) who felt that the

stagnant waters retained at the end of the dry season damaged the crops as they spread out over the

plain at the beginning of the next flood. A similar system is used in Equator where the “Chameras”

fisheries for Dormitator latifrons of the Chone river involve the construction of ponds on the

floodplain, either by dyking or by excavation. The young of D. latifrons stocked into these are grown-

on through the dry season.

Husbandry and fish culture in backwaters

Backwaters and other residual waterbodies separated from the main channel as a result of such river

control works as reverting and channelization may be used for rearing fish. In Hungary, for example,

such waterbodies are being developed for aquaculture (Pinter, 1983) and compare very favourably

with the more traditional rearing of fish in ponds. Yearling fish are stocked into such waterbodies

which are particularly suited to the raising of herbivorous species. Little additional feed is required

over and above the natural production, although this is encouraged with a limited application of

fertilizers. The success of this method is indicated by comparison with pond farming (Table 8.5).

310

Figure 8.9 Difference in changes in (A) depth and (B) area of a floodplain pool before and after

damming of the main access channel. Encircled letters refer to individual illustrations in Figure

1.8

Refuge traps

In the Oueme river vegetation masses may be planted deliberately at the end of the flood, either

attached to the bank (Fig. 8.10) or recessed into it at the mouth of the channels which drain the plain.

They are left to collect fish for about two months after which they are fished and replaced to be

emptied again towards the end of the dry season. Harvest from such “refuge traps” or “fish parks” can

be quite high and in the Oueme river 15 installations of this type of mean area 440 m² gave a mean

harvest equivalent to 1.88 t/ha (of park) per fishing or 3.88 t/ha/yr between 1958 and 1968.

The practice of deliberately planting vegetation or branches on the water to attract fish is in fact very

widespread in rivers as well as in coastal lagoons having been recorded by Chevey and Le Poulain

(1940) from the Mekong, Stauch (1966) and Reed et al. (1967) from the Niger and Benue systems and

Meschkat (1972) for Equador. Welcomme, (1972a) carried out an evaluation of this method of fishing

as it is practised in the lagoons associated with the delta of the Oueme river. The refuge traps in this

river and its associated lakes and lagoons are of two types in addition to the masses of floating

311

vegetation described above. There are small circular types about 22 m² in area which give up to 2.8

t/ha for each harvest. As they are harvested up to ten times during the 7 month dry season an annual

yield as high as 28 t/ha can be obtained without apparently affecting the catch by other fishing

methods in the area. Larger rectangular parks or “acadjas” are harvested less frequently, but also

achieve very high yields. The yield of brush parks is directly related to the frequency of fishing and to

the density of planting with greater yields of fish coming from parks with the greater density of

branches. In the freshwater zone, a variety of species are attracted to the Oueme fish parks from which

up to 32 species were recorded. By contrast, in the brackish water zone, only two species,

Sarotherodon melanotheron and Chrysichthys nigrodigitatus made up to 95 percent of the individuals

present. These breed actively in the park throughout the year and as a consequence the population

builds up rapidly (Fig. 8.11) thus accounting for the very high yields of up to 8 t/ha/yr in 1957–59.

312

313

Figure 8.10 Portions of the Oueme floodplain showing the distribution of different activities: (A)

river channel with brush parks and drain-in ponds; (B) a dense aggregation of drain-in ponds.

In both illustrations the plain is divided into fields. (By courtesy of IGN, Paris)

Table 8.5

Comparison between production parameters in Hungarian fish ponds

and in backwaters used for aquaculture (From Pinter, 1983)

Fish ponds Backwaters

Stocking rates Kg/ha 422 488

Cyprinus carpio 314 223

Herbivorous fish 98 257

Others 10 8

Yield Kg/ha 1305 1301

Cyprinus carpio 946 476

Herbivorous fish 334 749

Others 25 76

Total yield 883 813

Of which: fed

ponds 588 (66.6%) 316 (38.9%)

fertilised

ponds 295 (33.4%) 497 (61.1%)

Amount of feed in

starch value

(Kg)/1 kg

total yield) 2.3 1.3

More recent investigations have shown that the fish stock increased in a similar manner in 1969–70

although yields were somewhat lower as a result of changes in the lagoon environment brought about

by the construction of a port. The only other such installations that have been systematically

investigated are the ‘Samra’ parks of the Grand Lac which according to Chevey and Le Poulain

(1940) would appear to have given comparable yields to the Benin type of installation. Brush park

fisheries are regarded by many as a somewhat mixed blessing. There is little doubt that they put up the

overall productivity of the body of water in which they are used, but they also may shorten its life

through accelerated silting.

Rice-fish culture

The widespread practice of fish culture in rice fields may be considered an almost ideal method of

land use which produces both a carbohydrate and protein crops from the same piece of land. It has

reached very high standards in Asia and Madagascar but is still in the experimental stage in Africa

and Latin America. Coche (1967) has dealt comprehensively with the situation up to 15 years ago.

The combined culture of fish and rice is widespread throughout Southeast Asia and represents a

logical, integrated approach to the management of these types of wetlands. Ruddle (1980) classifies

the various ways in which rice and fish can be managed as crops from the same piece of land.

314

Figure 8.11 Relation between yield and length of time of installation of fish parks in the Oueme

Delta: (•) means for 1957–59; (x) means for 1968–70

Ricefield fisheries capture systems

culture systems concurrent cultivation

of fish and rice

rotational cultivation

of fish and rice

alternate intermediate

cropping cropping

He estimates that capture yields are about 135 kg/ha in the Malaysian Peninsula.

Two main types of exploitation can be distinguished:

(i) capture systems where the fish that gain access through natural channels may be trapped.

(ii) culture systems where the field is deliberately stocked with fry.

The fish may be taken as a single annual crop with a single rice crop, an intermediate crop between

one rice harvest and the next planting or during the growing season of the rice. Frequently fish are

trapped casually during the growing season, and are then drained into specially constructed sump

ponds or canals as the fields are dried (see Fig. 8.12). Yields from trapping during the growing period

have been measured at about 132 km/ha by Tang Cheng Eng et al. (1973). If the rice is cropped only

once per year, the sump ponds yield about 162.8 to 262.9 kg/ha (mean 200 kg/ha) but if the field is

double cropped fish yields drop to 68.2–143.0 (mean 95.7) kg/ha. At the same time between 2 277

and 2 975 kg/ha of rice were produced. Fish represents between 12–50 percent of the total income in

single cropping and 6–27 percent in double cropping systems. Similar yields have been recorded in

315

Thailand in the Mekong basin where 433 hm² of paddy fields were sampled giving fish harvests

ranging between 50 and 1 710 kg/ha (mean 433 kg/ha) (Thailand, Department of Fisheries, 1974). In

Sri Lanka rice-fish culture has not been practised because of the short (3–4 weeks) duration of the

flood phase.

There has been a long term decline in yield from ricefield fish production. The main reasons for this

are: First the increasing use of pesticides, weedicides and fungicides associated with the modern

advanced agrarian practices needed to grow the high yielding varieties of paddy. The effects of

pesticide treatment of rice for the control of stem borer insects, has caused concern and such workers

as Kok (1972) or Saanin (1969) have attempted to define the toxic and sub-lethal effects of the

common pesticides used (which include Endrin, Methyl Parathion, Dieldrin, Diazinon and -BHC).

While no definite conclusions can yet be drawn as to the precise effects of such environmental

contaminants, it is evident that the use of less toxic products can result in improved yields of rice

whilst safeguarding the supply of fish.

Figure 8.12 Adaptations of the paddy field for concurrent rice-fish cultivation (after Ruddle, 1980)

Second the concept of multiple cropping in rice cultivation leaves little scope for fish culture because

of the short duration of the flood over the rice field (Kassim, et al. 1979). The production of a plant

and a protein crop from the same area of floodable land is undoubtedly of great value in maintaining a

nutritionally balanced yield pattern, although one that is not necessarily maximal in terms of absolute

tonnage consequently projects have been suggested for the extension of this type of culture. For

instance, the management of the 32 000 ha Candaba swamp. This plain is flooded by the Pampanga

and Angat rivers and is one of a similar series of floodplains in the Philippines. The plain has been

enclosed in a series of dykes which include a gate for the harvesting of fish. The fields are at present

used for water melon or rice growing during the dry season when the fish remain in the canals. Fish

316

yields based solely on natural productivity range from 300–500 kg/ha in 7 months. The melon crop

produces 5 000 kg/hm. Delmendo suggested ways in which this area could be improved with the

inclusion of fish canals at the foot of the main dykes and an intensification of fish culture by stocking

and manuring the ponds.

Intensive aquaculture in ponds

The intensive culture of salmonids in ponds associated with the valleys of rivers is very common in

temperate countries, but the use of the potamon in the tropics for the purpose is more recent.

Once the flood is completely controlled the former floodplain is converted to irrigated agriculture.

Aquaculture farms, with ponds or pools specially adapted to the rearing of fish, can be associated with

the irrigation net-works. Such farms can either rear fish to a size where they can be sold directly for

consumption, or can raise fry for stocking into such permanent lakes as remain in the plain or into

fields where fish are reared as a secondary or alternative crop. Developments of this type are perhaps

best seen in Thailand, although they are also common in many other parts of Southeast Asia. In the

Chao Phrya river basin for instance, the control of the water regime over much of the floodplain has

decreased natural fish production. To compensate for this loss many state and private fish farmers rear

various species of cyprinid and siluroids as well as freshwater prawns (Macrobrachium rosenbergii).

The impervious nature of the alluvial soils, the flat terrain and the ready access to water supplies

through irrigation canals makes this type of modified floodplain particularly suitable for aquaculture

in ponds.

Cage culture in rivers

Once flow is stabilized within the main river channels, or even before such stabilization occurs, fish

may be cultured in floating cages. This type of culture usually applies to species of high commercial

value, such as Pangasius sutchii which is cultured in this manner in Thailand. The fish are usually

caught as fry and reared to marketable size in such structures.

Reservoir Fisheries

Reservoirs support fish populations and fisheries where yields are comparable to or higher than the

rivers they submerge. The net gain is usually far higher in rapids reaches where the relatively low

productivity of the main channel is easily exceeded by the new lake, but in any system where a

reservoir replaced a floodplain either by submerging it or by stopping its flooding the benefit or loss

to the fishery as a whole has to be carefully evaluated. In the case of the Upper Mekong, many of the

impounded tributary rivers have little floodplain either in the submerged reaches or downstream of the

dams (Sidthimunka, 1972). However, according to the Mekong studies the mainstream Pa Mong

reservoir will eliminate flooding for some 700 km downstream of the dam, resulting in a loss of catch

of about 2 150 t from the river, whereas the reservoir is expected to produce only about the same

amount of fish. Similarly, at least 6 000 tons of fish have been lost below the Kainji dam in Africa

which must be deducted from the 5 400 t produced by the reservoir behind the dam.

In many other reservoirs the amount of fish lost downstream of the dam surpasses the potential of the

dam itself, although such considerations are usually secondary where power generation or irrigated

agriculture are also taken into account.

The fish community that establishes itself in the reservoir is based entirely on that pre-existing in the

river basin, unless exotic species are introduced. There are usually considerable modifications in

community structure as many species are unable to adapt to the new environment. Earliest to

disappear from the body of the reservoir are the migratory whitefishes although they might survive in

the upper portion of the lake from which they can readily move upstream to breed. Blackfishes tend to

adapt better, and the main body of the lake is often colonized by species that were relatively

317

insignificant elements of the pre-impoundment fauna. In some rivers, for example, those of the Indian

subcontinent, Fernando (1976) suggested that few suitable blackfish occur. Consequently, lacustrine

fish communities are not readily established with the species native to the system, and, failing the

introduction of exotics, populations have to be maintained by continual stocking.

ACKNOWLEDGEMENTS

Although published under one name this book is the result of the work of many people. The

knowledge has been acquired not only by scientists working on the topic as an academic problem but

also by the fishermen who depend on their wisdom for the food they eat. During the course of travel

to and on many of the worlds rivers I have benefitted from the friendship and cooperation of many

who work directly with the fisheries described here and to them I owe an enormous debt. The

scientists and fishery administrators of many countries have contributed considerably to this work

through technical meetings of various types including the working parties of the FAO Regional

Fishery Bodies dealing with inland waters. Much of the stimulus to collect and interpret this

information comes from my colleagues in the FAO Department of Fisheries without whose help and

encouragement it would be difficult to proceed. In this respect the many hours spent in the physical

preparation of the paper by a number of secretaries should be particularly acknowledged.

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