PRIMARY RESEARCH PAPER

Assessing the effects of sewage effluents in a Mediterraneancreek: fish population features and biotic indices

Blanca Figuerola • Alberto Maceda-Veiga •

Adolfo De Sostoa

Received: 26 December 2011 / Revised: 13 April 2012 / Accepted: 24 April 2012

� Springer Science+Business Media B.V. 2012

Abstract Sewage effluents are one of the main

anthropogenic stressors in Mediterranean rivers. The

establishment of a cause–effect relationship is hin-

dered in natural systems by the existence of con-

founding factors (i.e. biotic interactions). Here we

analysed the effects that anthropogenic stressors have

on a mono-specific fish community (Iberian redfin

barbel population, Barbus haasi) inhabiting the north-

ern edge of its distribution range. In Spring 2004, a

total of 40 consecutive sampling sites were surveyed

in Vallvidrera creek, and 1,331 specimens were

measured and weighed. A principal component anal-

ysis was performed to synthesize the information

provided by 22 environmental variables. Analysis of

variance, bivariate correlation analyses and multiple

linear regressions were then used to determine the

influence of the environmental gradients built (water

quality, hydromorphology, woods and macrophytes,

and degree of silting) on fish population features (fish

size, body condition status, density and biomass).

The findings revealed that water quality was the

most significant environmental gradient for this fish

population. In particular, fish density decreases and

fish length increases in those sites exposed to sewage.

Additionally, our results showed the best body con-

dition of those specimens inhabiting fast flow reaches

which confirms the rheophilous condition of B. haasi.

However, these findings were unnoticed for the

current version of the index of biotic integrity using

fish as bioindicators in Catalonia. Resource managers

need to refine diagnostic tools in order to detect subtle

deleterious changes on fish communities before they

become evident at population scale. Conservation

measures should be focused in these small streams in

where the best preserved native fish populations

usually inhabit. This study suggests the need to change

water management policies in Mediterranean rivers to

improve the water quality of sewage effluents and

increase the dilution power of these rivers.

Keywords Mediterranean creek � Sewage effluents �Effects � Iberian redfin barbel

Introduction

Mediterranean rivers have suffered a long history of

anthropogenic impact events, which have conditioned

the conservation status of their native fish fauna. Fish

species are also shaped by the harsh climatic con-

straints of the Mediterranean area in which cycles of

droughts and floods alternate (Gasith & Resh, 1999;

Magalhaes et al., 2007; Verdiell et al., 2007; Beche

Handling editor: M. Power

B. Figuerola (&) � A. Maceda-Veiga � A. De Sostoa

Department of Animal Biology & Biodiversity Research

Institute (IRBIO), Faculty of Biology, University of

Barcelona, 08028 Barcelona, Spain

e-mail: bfiguerola@hotmail.com

123

Hydrobiologia

DOI 10.1007/s10750-012-1132-y

et al., 2009). Both anthropogenic and natural pres-

sures, which may also exacerbate the consequences of

the former, have converted Mediterranean rivers into

one of the most endangered ecosystems in the world

(Hooke, 2006; Magalhaes et al., 2008).

Native fish species from Mediterranean streams

have adapted their life-histories (e.g. fast growth, early

maturity and multiple spawning) to face the high-

hydrological variability of these systems (e.g.

Fernandez-Delgado & Herrera, 1995; Vinyoles et al.,

2010). In this regard, the most restrictive conditions

appear in small intermittent streams, because fish need

to develop the ability to resist floods and survive in

isolated pools during drought (Aparicio & de Sostoa,

1998; Pires et al., 1999; Soriguer et al., 2000;

Magalhaes et al., 2002). Pools act as a population

reservoir, allowing fish recolonization when flow is re-

established (Sedell et al., 1990; Magalhaes et al., 2007;

Pires et al., 2010). Nevertheless, flow variations are

still the main natural conditioners of the abiotic (i.e.

channel morphology) and biotic (i.e. fish assemblages)

structure of Mediterranean rivers (Magalhaes et al.,

2007; Boix et al., 2010). Natural variations in fish

assemblages and in the structure of fish populations

have been reported in these systems within seasons but

also inter-annually (Magalhaes et al., 2007; Beche

et al., 2009).

With regard to anthropogenic alterations, the input

of sewage treatment plants plays a crucial role in the

water quality and flow of Mediterranean rivers, in

particular in those streams with the most restrictive

hydrological conditions during droughts (Prat &

Munne, 2000; Maceda-Veiga et al., 2009; Damasio

et al., 2011). Natural flow reduction added to water

abstractions may diminish the dilution capacity of

streams, and the effects of pollutants on fish fauna

may be exacerbated (Gasith & Resh, 1999; Prat &

Munne, 2000; Pires et al., 2010). This is particularly

important in areas with a high degree of endemicity,

such as the Iberian Peninsula, or when it affects fish

populations with great conservation value (Sostoa,

1990; Doadrio, 2001; Maceda-Veiga et al., 2010a).

NE Spain has a long history of anthropogenic

alterations, such as the modification of the morphol-

ogy of the channels, abstractions, changes in land

use and industrial and municipal sewage discharges

(Prat & Ward, 1994). In addition, its native fish

fauna have declined greatly in recent decades

(Maceda-Veiga et al., 2010a).

Barbus haasi (Mertens, 1925) is a medium-sized

cyprinid that inhabits the headwaters and middle

reaches of Mediterranean Spanish Rivers from the

Llobregat to the Turia basins. It is listed as rare (R) in

the Redbook of Spanish Vertebrates (Doadrio, 2001).

It also appears as vulnerable in international and

national IUCN criteria (Doadrio, 2001; Crivelli,

2005). Unlike main rivers, in which introduced

species tend to dominate fish assemblages, some

tributaries still preserve native fish fauna populations

in good conservation status. Previously published

studies have addressed the life-history, home-range

and habitat preferences of this population of B. haasi

(Grossman & Freeman, 1987; Grossman & de

Sostoa, 1994; Aparicio & de Sostoa, 1999). Never-

theless, the relationships between fish abundance, fish

condition and anthropogenic stressor gradients

related to habitat and water quality were not assessed.

This fish community consists solely of the Iberian

redfin barbel (B. haasi), which is an advantage for

establishing relationships between fish species and

environmental factors, since biotic interactions are

minimized (Gasith & Resh, 1999). In addition, this

population is at the edge of the northern range of its

distribution (Vallvidrera creek, Llobregat basin),

which may condition its susceptibility to anthropo-

genic impacts (Gasith & Resh, 1999; Maceda-Veiga

& de Sostoa, 2011).

Understanding the response of native fish fauna

to anthropogenic stressors is crucial to guarantee-

ing their conservation and to refining the use of

fish as bioindicators (Karr, 1991; Schiemer, 2000;

Vila-Gispert et al., 2002; Oliva-Paterna et al.,

2003; Benejam et al., 2008; Maceda-Veiga et al.,

2010b; Maceda-Veiga & de Sostoa, 2011). Fur-

thermore, environmental risk assessment studies

using fish as bioindicators have been poorly

interlinked with the results obtained by other

biotic indices (e.g. riparian coverage), which may

increase their value as diagnostic tools (Benejam

et al., 2008). Here, we studied the population of

B. haasi in a small Mediterranean creek (Vallvid-

rera), in order to determine the effects that habitat

and water quality have at the population (size–age

structure, fish density and biomass) and individual-

organism level (body condition). In addition, we

compared the suitability of two biotic indices

(IBICAT and QBR) as diagnostic tools in small

Mediterranean streams.

Hydrobiologia

123

Materials and methods

Study area

This reach surveyed is located in Vallvidrera Creek

(418260N, 2�030E) in Llobregat basin within the Col-

lserola Natural Park, close to Barcelona city in

Catalonia (northeast of Spain, Fig. 1). Vallvidrera

Creek is a first-order tributary of the River Llobregat.

It is 4 km in length and has a mean stream width of

1.60 m. The highest flows usually occur in spring and

the lowest in summer, when some stretches of the

stream can lose connectivity and fish remain in isolated

pools. Other than rainfall, water flow is influenced by

the effluent from an activated sludge sewage treatment

plant located close to the source of Vallvidrera Creek

which represents over the 50–100 % of rivers’ flow

(rainy and drought conditions, respectively), high-

substrate permeability and water extractions for agri-

cultural purposes (Murria, 2003; Maceda-Veiga et al.,

2009). In addition, the stream regularly receives effluent

inputs from the sewage collectors of nearby urban

areas (physicochemical parameters are as follows:

ammonium = 21.5 mg/l, phosphates = 11.2 mg/l and

conductivity = 1,541 lS/cm, ACA 2006). The water

quality of Vallvidrera creek varies as follows:

Fig. 1 Reach surveyed

each 50 m in the Vallvidrera

creek (Llobregat basin)

(Catalonia, NE Spain)

Hydrobiologia

123

temperature ranges between 4 �C in winter and 20 �C

in summer, dissolved oxygen is always close to

saturation, with the exception of isolated pools during

the summer, when minimum values of 3–6 mg/l were

recorded and conductivity ranges between 800 and

1,100 lS/cm (Maceda-Veiga et al., 2009). The current

ichthyofauna of this stream comprises only B. haasi,

but the European eel (Anguilla anguilla) was also

found some years ago (Aparicio & de Sostoa, 1999).

Surveys were performed covering the whole 2 km

section of this creek inhabited by fish. The area

surveyed was divided in 50 m long consecutive

reaches using blocking nets.

Environmental data

Prior to fish sampling, physicochemical water quality

parameters were measured at each sampling site. A

digital multiparametric YSI� 556 MPS probe was

used for conductivity (lS/cm), dissolved oxygen (O2,

mg/l), temperature (�C) and pH. The colorimetric test

kit VISOCOLOR� was used to determine ammonia

(NH3, mg/l), nitrite (NO2, mg/l), nitrate (NO3, mg/l)

and phosphate (PO4, mg/l) levels. After sampling fish,

the hydro-morphological variables of the river (cur-

rent velocity, width and depth) were measured along

transects set perpendicular to the flow at 20 m

intervals. We then calculated the total area surveyed

and the mean depth. To assess the relative proportion

of different substrate types (%), we visually catego-

rized the substrate type in: bedrock (exposure of the

consolidated rock), rocks and pebbles (64–256 mm),

gravel and sand (0.06–64 mm), silt (0.06–0.004 mm)

and detritus (organic material including the bodies or

fragments of dead organisms). We also characterized

the potential refuges for fish and other habitat diversity

features: structural refugia (%), caves (%), aerial

coverage (%), aquatic macrophytes (%), submerged

riparian vegetation (%) and woods (%). To further

characterize habitat, we also applied the riparian

vegetation quality index (QBR) (Munne et al., 1998).

The QBR is a summation index of four parts: total

riparian cover, cover structure, cover quality and

naturalness of river channel. Each part is calculated

independently, and the individual score of each part

cannot be either negative or higher than 25. Then, the

final score is the sum of these four parts and ranked

between 0 and 100, in where 100 indicate the highest

degree of conservation.

Fish sampling

Fish were captured with a portable electro-fishing unit,

which generates a voltage up to 400 V, 2 A, and uses

the single-pass electric fishing method (CEN, 2003).

Block nets were used to avoid fish movements

between reaches. All fish were identified at species

level and counted, and a subsample of 50 randomly

selected fish (when it was possible) was measured. To

avoid management stress, fish were anaesthetized with

MS-222 (3-aminobenzoic acid ethyl ester; Sigma-

Aldrich�) and were returned alive to the river after

examination. We determined the fork length (FL, mm)

and total wet weight (mg). All specimens were

examined in situ by the naked eye for external

macroscopic parasites or anomalies. Relative fish

density was calculated, using the total number of fish

captured, the area surveyed and the fishing time

(CPUE, Capture per Unit of Effort). On the basis of the

fish captured at each sampling site, its ecological

status was determined, following the index of biotic

integrity developed for Catalan Rivers (IBICAT)

(Sostoa et al., 2003). The IBICAT’s methodology, as

other IBIs, consists of comparing the ecological status

of two body masses (the reference and the tested one)

based on selected features named metrics (e.g. fish

density, % introduced species, % of insectivorous

species, % of tolerant species, etc.) (Karr, 1991;

Sostoa et al., 2003; Pont et al., 2007).

Data analyses

Normality and homoscedasticity were tested by the

Shapiro–Wilk and Levene tests, respectively. Data

transformation did not allow normality and homosce-

dasticity to be attained. To avoid redundancy in further

analyses, the bivariate relationship between environ-

mental variables was checked by Spearman’s corre-

lation coefficient (one variable for each pair-wise with

r [ 0.7 was removed). Principal component analysis

(PCA) was then applied as an indirect ordination

technique to describe the main sources of variation and

relationship among the selected environmental vari-

ables. PCA reduces the dimensionality of the envi-

ronmental variables to a few principal synthetic

gradients. The ‘‘varimax’’ rotation method was used

to increase the interpretation of axes; and the number

of PCA axes examined was determined by Kaiser’s

Hydrobiologia

123

rule, which states that the minimum eigenvalue should

be 1 when correlation matrices are used (Legendre &

Legendre, 1998). The PCA scores for each gradient

were used for regression analysis and for grouping

sampling sites based on their factor loadings. The

Kruskal–Wallis test was used to compare between fish

features (FL, weight, condition factor and density) and

groups. The somatic condition of B. haasi was

calculated with Fulton’s condition factor (e.g. Clavero

et al., 2009): K = W � 105 � FL3, where W is the wet

weight (in g) and FL is the FL (in mm). The U Mann–

Whitney test was used for paired comparisons. The

bivariate relationship between environmental gradi-

ents (i.e. PCA scores) and fish features was checked by

Spearman’s correlation coefficient. To further

describe the relationship between biological and

environmental data, multiple linear regression models

were performed to test the relationship between fish

density, biomass and the average length, weight

(W) and condition factor (CF) of fish and the

synthetic PCA gradients. We followed a manually

conducted backward stepwise procedure to construct

minimum adequate models (MAM). Retention of

independent variables in the models relied on

statistical significance (P \ 0.05), a more restrictive

criterion than information criteria (e.g. Akaike

information criterion), which tend to leave more

independent variables in final models (Crawley,

2002; Clavero et al., 2009).

All analyses were performed using the R package

(R Development Core Team, 2010) and the library

statistics, lawstat (Nogouchi et al., 2009) and psych

(Revelle, 2010), assuming the alternative hypothesis

at P \ 0.05.

Results

A total of 40 sampling sites were surveyed, and

fish were captured at all of them. 1,331 specimens

were measured and weighed, and 22 environmental

variables were determined (Table 1). The strongest

correlations were detected between the following envi-

ronmental variables: nitrates with nitrites (r = 0.712,

P \ 0.001), pH with alkalinity (r = 0.705, P \0.001),

gravel and sand with rocks and pebbles (r = 0.734,

P \ 0.001), structural refuge with coves (r = 0.707,

P \ 0.001) and submerged riparian coverage with

natural woods (r = 0.701, P = 0.01).

Environmental gradients

The PCA produced four significant axes, which

explained 70.13 % of variation in our environmental

data set. The suitability of this analysis was deter-

mined by the Kaiser–Meyer–Olkin test (KMO =

0.703) (Table 2). PC1 accounted for 25.52 % of

Table 1 Mean, standard deviation and minimum maximum

values of the environmental and biological variables recorded

at the sampling sites surveyed, without removal of redundant

environmental variables

Mean ± SD Range

(min–max)

Biological data

Density (ind/ha) 6781.9 ± 4133 178.2–25163.4

Biomass (kg/ha) 202308 ± 167401 5542–698440

FL (mm) 108.30 ± 29.01 23–263

W (g) 32.49 ± 24.13 0.2–314

CF 0.138 ± 0.02 0.12–0.13

Biotic indices

QBR 33.62 ± 11.20 10–55

IBICAT 5 ± 0 5–5

Environmental data

Ammonia (mg/l) 0.355 ± 0.255 0–1.50

Nitrite (mg/l) 0.162 ± 0.167 0–0.50

Nitrate (mg/l) 7.79 ± 5.49 5–18

Phosphate (mg/l) 0.62 ± 0.23 0.20–1.20

Alkalinity (� KH) 12.90 ± 0.42 10–15

pH 7.55 ± 0.23 7.20–8.00

Depth (m) 0.941 ± 0.16 0.55–1.35

Current velocity (m/s) 0.272 ± 0.18 0.07–1.00

Temperature (�C) 12.04 ± 2.86 8.20–17.00

Conductivity (lS/cm) 936.9 ± 229.99 534–1148

Bedrock (%) 1.55 ± 2.86 0–10

Rocks and pebbles (%) 14.5 ± 8.21 0–25

Gravel and sand (%) 73.30 ± 8.50 61–93

Silt (%) 2.85 ± 1.25 1–6

Detritus (%) 1.00 ± 0.32 0–2

Structural refugia (%) 7.28 ± 3.84 2–20

Caves (%) 6.55 ± 5.54 0–15

Aerial coverage (%) 51 ± 42 10–90

Aquatic macrophytes

(%)

3.60 ± 2.34 1–6

Submerged riparian

vegetation (%)

15.88 ± 12.32 10–35

Woods (%) 18.00 ± 15.54 10–30

Hydrobiologia

123

variation and loaded mainly on physico-chemical

water parameters and the quality of riparian coverage.

PC2 explained 20.71 % of variation and included

hydromorphological variables (current velocity, struc-

tural refugia, % bedrock). PC3 accounted for 13.51 %

of variation and loaded on the percentage of woods

and macrophytes; whereas PC4, accounting for

10.39 % of variation, included the degree of silting.

Sampling sites were grouped, based on their scores

in the two main environmental stressor gradients

(Fig. 2). The effects of the effluents from the sewage

treatment plant and nearby urban collectors condi-

tioned the location of Groups I and III in the PC1

gradient. Group II mainly reflected reaches from

headwaters with high-current velocity and low-detri-

tus deposition.

Fish population features and biotic indices

The characteristics of specimens sampled at each

group (I, II, III) were provided in Table 3. All

specimens differed in the average FL, W) and body

CF between groups. In particular, individuals from

Group III had, significantly, the lowest values in FL

and W (P \ 0.001). There were significant differences

between CF and all groups (P \ 0.001). Fish density

decreased significantly in locations from Group III

(P \ 0.001), but no significant differences were

detected in biomass (P = 0.448).

Groups also differed in the quality of the riparian

coverage (QBR). Locations in Group II have signif-

icantly the highest values in this index (P \ 0.001). In

contrast, as all sampling sites received the same score,

based on IBICAT, there were no significant differ-

ences between groups or a positive correlation

(P [ 0.05).

Environmental gradients and fish population

features

The multiple linear regression analyses explained 62.9

and 38.9 % of the variation in Iberian redfin barbel

density and the specimen’s length. Both models are

selected, as a significant independent variable, PC1

(b = -0.782; P \ 0.001; b = 0.618; P \ 0.001,

respectively). Bivariate correlation analyses also

revealed the high-significant relationship between

PC1 and fish density, and fish size compared to the

others (Table 4). After removal of the non-significant

factors (MAM), the relationship between these bio-

logical data and PC1 revealed that Iberian redfin

barbels tended to be scarce and specimens big in

Table 2 Main environmental gradients (PC1, PC2, PC3 and

PC4) defined in the study area by means of PCA

Environmental variable PC1 PC2 PC3 PC4

Ammonia 0.63 0.09 0.23 -0.22

Nitrite 0.51 0.28 0.54 0.14

pH 0.82 -0.05 -0.18 0.33

Average depth 0.64 0.14 0.33 0.09

Average water current -0.01 0.74 -0.07 0.49

Water temperature -0.82 -0.18 0.18 -0.42

Conductivity 0.90 0.04 -0.05 0.12

Bedrock 0.12 0.82 0.09 -0.11

Gravel and sand -0.46 -0.51 -0.36 -0.13

Silt 0.24 0.15 0.13 0.77

Detritus -0.02 -0.82 0.28 -0.07

Structural refuge 0.03 0.72 0.40 0.24

Macrophytes -0.19 0.15 0.72 0.06

Submerged riparian

coverage

-0.08 0.61 0.46 0.45

Natural Woods 0.25 -0.24 0.72 -0.05

QBR 0.68 -0.24 0.05 -0.40

The greatest factor loadings in absolute value are marked in

bold

Fig. 2 Sampling scores on the first two component axes: PC1

mainly loaded on water quality and riparian coverage (25.52 %)

and PC2 mainly loaded on hydromorphology (20.71 %).

Groups (I, II and III) were based on the proximity of sampling

sites in the PCA

Hydrobiologia

123

locations with high conductivity, ammonia and ripar-

ian quality index (QBR) values, and low-water

temperature, depth and pH (R2 = 0.60; b = -0.782;

P \ 0.001; R2 = 0.36; b = 0.618; P \ 0.001; respec-

tively) (Fig. 3). A more detailed analysis of fish

lengths showed that the minimum FL of specimens

increased along PC1 (R2 = 0.37; b = 0.622;

P \ 0.001) (Fig. 3). In contrast, no significant rela-

tionship was found between the maximum FL and

habitat gradients (R2 = 0.10; P = 0.100). The same

habitat gradient (PC1) was selected as significant in

the case of fish weight (R2 = 0.32; b = 0.511;

P \ 0.001). In addition, the model explained 40.9 %

of the variation in the body CF, and the significant

variable selected was PC2 (b = 0.598; P \ 0.001).

The regression between the body CF and PC2

excluding the rest showed that fish had a good

condition in locations with high-water current, struc-

tural refuges, submerged riparian coverage and low

detritus (R2 = 0.34; b = 0.598; P \ 0.001).

Discussion

Environmental parameters

Physico-chemical water parameters indicated a

decrease in water quality downstream. The poor water

quality was mainly due to the increase in ionic levels

and organic pollution, denoted by an increase in

conductivity, ammonia, nitrite, nitrate and phosphate

values downstream of the sewage inputs. The presence

of reduced forms of nitrogenous compounds (ammo-

nia, nitrites) shows the input of organic sewage

effluents without treatment or from an inefficient

sewage treatment plant (e.g. Welch & Lindell, 1992;

Mariz, 2011). The eutrophication detected may also be

responsible for the proliferation of algae (i.e.Ta

ble

3M

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and

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on

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axim

um

and

min

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mv

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db

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cin

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esp

rov

ided

by

gro

up

sb

ased

on

PC

Asc

ore

s

Gro

up

IG

rou

pII

Gro

up

III

Sta

tist

ics

Pv

alu

e

Mea

n±

SD

(min

–m

ax)

Mea

n±

SD

(min

–m

ax)

Mea

n±

SD

(min

–m

ax)

KW

Den

sity

(in

d/h

a)4

32

0±

13

56

(33

61

–5

27

9)a

41

89

±3

84

6(1

78

–1

56

86

)a1

45

34

±5

41

9(6

28

6–

25

16

3)b

18

.17

\0

.01

Bio

mas

s(k

g/h

a)1

88

98

3±

54

47

6(1

50

46

2–

22

75

03

)a1

79

37

7±

15

10

90

(55

42

–5

03

97

2)a

26

91

78

±2

14

41

9(3

07

32

–6

98

44

0)a

1.6

10

.45

FL

(mm

)1

20

.51

±4

.32

(36

–2

21

)a1

25

.4±

22

.72

(23

–2

62

)a8

4.7

6±

23

.72

(23

–2

63

)b1

94

.19

\0

.01

W(g

)4

3.7

8±

1.1

7(0

.7–

19

6.5

)a4

2.6

6±

24

.37

(0.2

–3

14

)a1

8.2

4±

10

.04

(0.2

–2

42

)b2

17

.47

\0

.01

CF

0.2

0±

0.0

8(0

.11

–1

.95

)a0

.15

±0

.01

(0.0

1–

1.9

3)b

0.1

3±

0.0

1(0

.05

–0

.6)c

31

0.9

9\

0.0

1

QB

R1

0±

0(1

0–

10

)a3

8.3

9±

9(1

5–

55

)b2

5±

0(2

5–

25

)a1

8.4

0\

0.0

1

IBIC

AT

5±

0(5

–5

)a5

±0

(5–

5)a

5±

0(5

–5

)a0

.00

1.0

0

Th

ele

tter

s(a

,b

,c)

con

sid

erth

atg

rou

ps

wer

eh

om

og

eneo

us

atP

\0

.05

Table 4 Correlation matrix (Spearman’s coefficient) of the

fish data and the environmental gradients built in PCA

PC1 PC2 PC3 PC4

Fish density -0.793** -0.227 -0.003 -0.016

Fish biomass -0.399* -0.207 0.130 0.068

FL 0.555* 0.112 0.141 0.016

W 0.464* 0.152 0.223 0.074

CF 0.461* 0.198 0.051 0.097

* P \ 0.05, ** P \ 0.001

Hydrobiologia

123

Cladophora). This fact may also explain the increase

in pH in those reaches due to the photosynthetic

activity of these algae mass.

Previous biomonitoring studies of macroinverte-

brates and algae also revealed a medium level of

organic pollution in this creek (Cambra et al., 2000;

Murria, 2003). In addition, these studies did not show

significant variations in water chemical parameters

between years, which indicate that aquatic fauna are

chronically exposed to this pollution gradient. In this

regard, chronic pollution events have been reported to

enhance the susceptibility of fish to disease (Maceda-

Veiga et al., 2009).

Fish population and environmental parameters

The effects of chronic pollution on native fish

populations have been poorly studied in small Med-

iterranean rivers (Vila-Gispert et al., 2002; Oliva-

Paterna et al., 2003; Damasio et al., 2007; Maceda-

Veiga et al., 2010b). Our findings revealed changes in

population structure, fish density and body condition

along the pollution gradient analysed. A significant

decrease in fish density was found in sites with high-

anthropogenic pressures (e.g. pollution). Neverthe-

less, fish density fluctuations can be common in

Mediterranean fish populations due to natural climatic

Fig. 3 Relationship between redtail barbel features (density, size and body condition) and the habitat gradient selected in the minimum

adequate model analyses (MAM)

Hydrobiologia

123

constraints (i.e. floods–drought) and it is often difficult

to discriminate between the factors affecting fish

populations (Magalhaes et al., 2007; Beche et al.,

2009).

Biases in population structure have been reported in

those fish populations subjected to strong environ-

mental stress (Adams et al., 1992; Gafny et al., 2000).

The life-history traits of native fish have evolved to

cope with natural climatic constraints, and these traits

may also be an advantage for surviving a pollution

event. Fish population is skewed to small sizes in

polluted sites, but native fish species from such

streams tend to have high fertility and multiple-

spawning events and reach sexual maturity at a

younger age (Herrera, 1991; Fernandez-Delgado &

Herrera, 1995; Vinyoles et al., 2010). Sexual maturity

is reached at 45 and 98 mm FL in males and females,

respectively, in this stream and fish lengths recorded

were always over these values in all sampling sites

(Aparicio & de Sostoa, 1998). Nevertheless, it is worth

nothing that small females have a less fecundity than

the larger ones (Aparicio & de Sostoa, 1998).

Sewage effluents usually contain a mixture of

xenobiotics (e.g. drugs, heavy metals, polychlorinated

biphenyls, etc.) that are responsible for a wide range of

physiological dysfunctions on fish fauna (e.g. geno-

toxic effects, tissue damage, metabolic anomalies,

endocrine disruption) (Di Giulio & Hinton, 2008).

Indeed, endocrine disruptors may shift the population

sex-ratio towards the females which are larger than

males, and this fact could be an alternative explanation

to this tendency detect in our study (Devlin &

Nagahama, 2002). However, the sexual dimorphism

of this species is not evident out of breeding season

(Aparicio & de Sostoa, 1998). Above all, the persis-

tence of this population may be guaranteed whether

pollutant levels are maintained within the tolerance

range of this fish species. B. haasi is considered

intolerant to poor water quality, and these conditions

may increase its susceptibility to disease, as has already

been reported (Maceda-Veiga & de Sostoa, 2011). In

any case, it should be pointed out that hydromorpho-

logical parameters (i.e. water level) seem to be a

greater fish size conditioner than chemical parameters

alone, particularly in small streams (Aparicio & de

Sostoa, 1998; Gasith & Resh, 1999; Pires et al., 2010).

In this regard, large fish tend to be more susceptible to

diseases in these streams in low-water flow conditions

(i.e. drought) (Maceda-Veiga et al., 2009).

General condition indices provide a first approach to

determining the health status of a fish population (Van

der Oost et al., 2003). Nevertheless, as reported in

similar studies (Van der Oost et al., 1998), the CF values

of fish exposed to sewage effluents in our study were not

significantly different from those registered in the

reference sites. In contrast, a significant positive

relationship was found in those specimens from loca-

tions with high-water current, structural refugee,

submerged riparian coverage and low percentage of

detritus. These findings corroborate previous knowl-

edge of the habitat preferences of B. haasi (i.e.

rheophilous) and confirm that hydromorphology

strongly conditions the distribution of fish in a Medi-

terranean river (Sostoa, 1990; Doadrio, 2001). Though

the CF may serve as an initial screening biomarker in

fish health assessment, it mainly indicates the nutritional

status of fish and prey availability (Van der Oost et al.,

2003). In this regard, macroinvertebrate assemblages

change along a pollution gradient, but potential prey for

B. haasi are expected to be available in all conditions

due to its generalist feeding habits (e.g. chironomidae,

ephemeroptera, etc.) (Sostoa, 1990; Murria, 2003).

Fish population characteristics and biotic indices

Diagnostic discordances were detected between the

three approaches (i.e. water physicochemical param-

eters, general biotic indices and fish features) used to

determine the ecological status of Vallvidrera creek

and also the health status of this fish population.

Furthermore, the diagnoses of the two biotic indices

IBICAT and QBR were not consistent with each other.

This may be explained because QBR is an index of

riparian coverage (QBR in Munne et al., 1998) and

IBICAT a fish index, and fish do not directly depend

on the quality of riparian coverage, as long as they

have somewhere that acts as a refuge.

QBR scores drop at the most polluted sites.

Locations near built-up areas have the most altered

riparian coverage, but in addition the stream received

the inputs of urban collectors at these sampling sites.

The low score was due to the replacement of riparian

coverage by cultivated lands or the presence of

opportunists and allochthonous species that down-

weighted the index (i.e. the giant cane, Arundo donax)

(Munne et al., 1998; Murria, 2003).

IBICAT scoring did not vary between sampling

sites despite the impairments detected in this fish

Hydrobiologia

123

population. The selection of metrics in IBIs is

generally conditioned in Mediterranean rivers by the

low-species richness and the covariation of natural and

anthropogenic perturbations (Oberdorff et al., 2002;

EU FAME, 2004; Noble et al., 2007; Pont et al., 2007;

Benejam et al., 2008). In this case, the poor sensitivity

could be due to the selection of the reference water

body for this small river and the monospecific

condition of this fish community (Benejam et al.,

2008). This fact may explain that IBIs only reflect

strong changes at the population or community scale

(i.e. native species extirpation, big fluctuations in fish

density), without taking into account the subtle

changes that appeared in the population before (i.e.

biases in population structure). In addition, the metric

of fish density and the thresholds established may be

quite controversial since the capacity of these ecosys-

tems is not calculated, and natural big fluctuations are

reported in fish populations from Mediterranean rivers

(Magalhaes et al., 2007). Our findings argue for the

need to incorporate new metrics (e.g. fish population

structure) in the present version of IBICAT, in order to

detect the impact on fish populations that are over-

looked by the traditional biomonitoring protocols in

Mediterranean streams (Benejam et al., 2008; Mace-

da-Veiga et al., 2010b). The use of biomarkers using

non-invasive sample techniques (i.e. blood parame-

ters) may be a complementary tool in order to detect

sub-lethal effects on these fish populations, in which

low-fish richness difficult the development of power-

ful sensitive biotic indices.

Conclusions

This study revealed that the chronic exposure to the

effluents from a sewage treatment is responsible for

the deleterious effects detected on a population of

B. haasi in Vallvidrera creek. Skewed fish population

structure and low-fish density were the main changes

reported on this fish population. This study also

confirms the preference of B. haasi for fast-flowing

reaches. However, these effects were unnoticed when

IBICAT was applied as diagnostic tool. The selection

of the reference community for this small creek

together with its monospecific condition may explain

this finding. However, we suggest the incorporation of

new metrics (i.e. fish age or size structure) in the

present version of IBICAT to increase its diagnostic

power. Additionally, we recommend the use of

complementary non-lethal diagnostic tools (i.e. bio-

markers). Resource managers should be able to detect

subtle changes in fish communities before the effects

of anthropogenic impacts become evident at popula-

tion scale. Our findings also argue the need to change

water management policies in order to improve the

water quality of the effluents from sewage treatment

plants and to guarantee the dilution capacity of

Mediterranean rivers.

Acknowledgments We thank the team that performed fish

surveys and Robin Rycroft for revising the English. We also

thank the Collserola Natural Park and the Catalan Water Agency

for financial support and Dr Alex Richter for his ideas in the

previous draft of this study.

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