Resource use of native and stocked brown troutSalmo trutta L., in a subarctic lake

K . K A H I L A I N E N & H . L E H T O N E N

Department of Limnology and Environmental Protection, University of Helsinki, Finland

Abstract Habitat use, food composition and growth of stocked and native brown trout,

Salmo trutta L., were studied in the subarctic Lake MuddusjaÈ rvi in northern Finland.

Stocked brown trout and native brown trout preferred littoral and pelagic areas. Trout

were stocked in October. In June stocked trout fed primarily on invertebrates while native

®sh were piscivorous. From July onwards the composition of the diet of both stocked and

native trout was similar and consisted almost entirely of small-sized white®sh. Brown trout

were already piscivorous at a length of about 20 cm. The mean length of prey consumed

was about 12 cm. Mean length-at-age was similar from the second year in the lake despite

of the larger size of stocked ®sh during the ®rst year in the lake.

KEYWORDS : brown trout, feeding, growth, habitat use, stocking.

Introduction

More than 1 million, 2±3-yr-old brown trout, Salmo trutta L., are stocked annually into

inland waters of Finland. Stocked trout are usually caught soon after release with ®ne

meshed gillnets when their length is still below the legal minimum ®shing size (40 cm)

(Niva 1999). Stocked trout are subject to inter- and intraspeci®c competion for food and

space from native trout and other ®sh species (Hegge, Dervo, Skurdal & Hessen 1989;

Hesthagen, Hegge, Skurdal & Dervo 1995). The change from hatchery to lake habitat may

make stocked ®sh susceptible to predation (L'Abe e-Lund, Langeland & Sñgrov 1992) and

they are poor users of resources in nature (Bachman 1984). Stocking of piscivorous brown

trout was most successful when suitable-sized prey ®sh were available (Vehanen 1995;

Niva & Julkunen 1998). In Finland, small-sized coregonid ®shes (white®shes, Coregonus

sp.) are the preferred prey for brown trout (Niva 1999), and it has been suggested that

brown trout stocking rates should be adjusted according to availability of suitable-sized

coregonids (Vehanen 1995; Vehanen & Aspi 1996).

Many studies have revealed that small-sized stocked brown trout can adjust to a new

environment quickly (Johnsen & Ugedal 1986, 1989, 1990; L'Abe e-Lund & Sñgrov 1991;

L'Abe e-Lund & Langeland 1995; Hesthagen et al. 1995). Among takeable-sized brown

trout, adaptation to new circumstances has been slower (O'Grady 1983) and learning

how to use natural food items has required a longer time (Hesthagen, Jonsson &

Correspondence: Hannu Lehtonen, Department of Limnology and Environmental Protection, PO Box 27, FIN-

00014, University of Helsinki, Finland (e-mail: hannu.lehtonen@helsinki.®).

Fisheries Management and Ecology, 2001, 8, 83±94

Ó 2001 Blackwell Science Ltd 83

Skurdal 1989). Growth rate of takeable-sized brown brout was variable and in many

cases there was evidence of malnutrition (Hesthagen et al. 1989; DamsgaÊ rd & Langeland

1994).

In Lake MuddusjaÈ rvi the brown trout stock consists of native and stocked ®sh.

Stocking has been criticized, because it has been suggested that stocked ®sh su�er from

malnutrition and therefore their growth rate is slow. In this study habitat use, food

composition and growth rate of stocked and native brown trout were investigated. The

basic hypothesis was that stocked ®sh di�er from native ones in habitat use, food

composition and growth rate at least during the ®rst months after stocking.

Materials and methods

Subarctic Lake MuddusjaÈ rvi is situated in northern Finland (69°N, 27°E). The surface

area of the lake is 48 km2 and the maximum depth 74 m. The study area in the northern

part of the lake is shallower, reaching a maximum depth of 30 m. The ice-free period lasts

from June to October and surface water temperatures reach 18±20 °C in July.

A thermocline is established during the summer at depths of 10±15 m. The largest

tributary, the River Kaamasjoki, drains through a swamp region and the water is therefore

humic. Secchi disc transparency is 3±5 m. Oxygen concentration is 10±14 mg L)1 and

pH 6.7±7.3. Total phosphorus concentration is 3±6 lg L)1 and total nitrogen 150±

180 lg L)1. Aquatic vegetation is sparse.

The native ®sh fauna comprises 10 species: brown trout, Arctic charr Salvelinus alpinus

(L.), white®sh Coregonus lavaretus (L.), grayling Thymallus thymallus (L.), perch Perca

¯uviatilis L., pike Esox lucius L., burbot Lota lota (L.), minnow Phoxinus phonixus (L.),

three-spined stickleback Gasterosteus aculeatus L. and nine-spined sticleback Pungitius

pungitius (L.). Based on gillraker frequencies, the white®sh stocks consist of three di�erent

morphs. These are small sparsely rakered, large sparsely rakered and densely rakered

white®sh (Kahilainen & Lehtonen 2001).

In Lake MuddusjaÈ rvi, brown trout (o�spring from the original stock) were released at

age 3+ in October. To improve adaptation to lake conditions, trout were kept in net pens

from June to October and fed with a commercial dry diet. During this period, the average

length increased from about 20±30 cm. Weight at release in 1995±1998 was 292±344 g. The

number of ®sh stocked in 1995±1998 was 2700±9000 brown trout per year. Native brown

trout reproduce in the River Kaamasjoki and smolts descend to the lake between

June±August at lengths of 20±25 cm.

Two gillnet series were used to sample brown trout. One series consisted of ®ve, 60-m

long ´ 5-m high nets. Mesh sizes were 35, 40, 45, 50 and 55 mm (bar). Fishes were

removed from the nets both in the morning and evening. Catch-per-unit-e�ort (CPUE)

was expressed in grams of brown trout 100 m2 net in a 12-h period. Day and night catches

were assumed to be similar. Fish were immediately cooled in ice. Three habitats were

distinguished according to ®shing depth zones (Table 1). The pelagic zone included

midwater and surface nettings at depths of 0±5 and 5±10 m in the open water area. The

littoral zone included bottom nettings at depths of 0±5 and 5±10 m in near shore areas.

The profundal zone included bottom nettings at depths of 10±15 and 15±20 m. Di�erences

84 K. KAHILAINEN & H. LEHTONEN

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

in CPUE were studied by ANOVA. The CPUE data were log transformed because

assumptions of homogeneity of variance (Bartlett's test) did not prevail. Pairwise

comparison between depth zone, month, origin of trout (native or stocked) were

performedwith Tukey's test. All statistical analyses were considered signi®cant atP < 0.05.

Length and weight were measured to the nearest 1 mm and 1 g. Scale samples were

taken from the area between the adipose ®n and the lateral line. Otoliths were also taken.

Both scales and otoliths were used for age determination. In further data analyses, only the

lake age of stocked and native trout were considered. Native trout were separated from

stocked ones according to growth rate during the ®rst year of life. Native trout spend an

average of 5 yrs in the river and the growth rate is slower than that of cultured trout.

Di�erences between mean lengths were tested with Student's t-test. In growth studies, the

®rst growing period for stocked ®sh was the summer following release and for native ®sh it

was the same summer they descended to Lake MuddusjaÈ rvi.

After basic data were obtained (length, weight, sex, maturity), stomachs were removed

and frozen at )20 °C. The number of brown trout sampled was 604, of which 177 were

native and 427 were stocked. Stomach contents were examined at 8´ magni®cation. Prey

items were identi®ed to species and white®sh to morph by gillraker counts if possible.

Stomach contents were analysed using the points method (Hynes 1950). Stomach fullness

was visually estimated using a scale 0±10, where 0 means empty stomach and 10 a

distended full stomach. Food categories were identi®ed and their relative contribution to

stomach fullness was estimated.

The lengths of prey ®sh were estimated from total otolith lengths when possible. The

relation between white®sh length and otolith length was calculated using the linear

equation (K.1 Kahilainen & H. Lehtonen, unpublished observations):

y � 3:36x� 0:52; r2�0:97; n�198

y is white®sh length and x is otolith length in mm.

Prey ®sh lengths were compared with predator lengths using linear regression. If the

predator had many ®sh in its stomach each prey ®sh was considered to be one observation.

Food overlap between wild and stocked trout was calculated with Schoener's (1970)

diet overlap index:

Table 1. Number of experimental gillnet ®shings in di�erent habitats during June±September 1999

Depth zone June July August September

Pelagic (m)

0±5 10 8 7 11

5±10 15 3 6 6

Epibenthic (m)

0±5 4 8 0 0

5±10 11 14 6 19

10±15 16 10 13 7

15±20 5 7 6 6

Total 61 50 38 49

RESOURCE USE OF BROWN TROUT 85

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

a � 1ÿ 0:5Xn

i�1jPxi ÿ Pyij

!where Pxi is the proportion of food item i used by native brown trout x and Pyi is the

proportion of food item i used by stocked brown trout y and n is the number of prey

categories. A value of zero indicates no overlap, and a value of 1.0 suggests complete

overlap. Empty stomachs were excluded from the analyses.

Results

About 70% of all trout caught originated from stocking. The CPUEs of stocked and

native trout di�ered signi®cantly in all months (ANOVA, P < 0.05). The highest CPUE for

stocked trout was 160 g 100 m)2 and that for native trout was 65 g 100 m)2 (Fig. 1).

Mean weights of stocked and native ®sh were 535 and 552 g, respectively. Di�erences in

CPUE between June and September were statistically signi®cant (Tukey, P < 0.05).

Vertical distribution of native and stocked trout did not di�er in June±September (ANOVA,

P > 0.05). Both native and stocked ®sh were caught mainly in the pelagic (0±10 m) and

littoral (0±10 m) zones where catches were signi®cantly higher than in the profundal

(10±20 m) zone (Tukey, P < 0.05).

Mean lengths of 0-group native and stocked trout were 24.1 and 31.8 cm, respectively

(Fig. 2). In June, the mean length-at-age of native and stocked trout di�ered signi®cantly

in lake age groups 0 and 1 (t-test, P < 0.001) and were equal in lake age groups 2±4. In

September, mean lengths of 0-lake age native and stocked trout were 28.8 and 34.9 cm,

respectively (P < 0.001). Mean lengths of other lake age groups did not di�er signi®cantly

in September.

Brown trout diet comprised mainly white®sh, surface insects and trichopterans

(Fig. 3). Other food items were chironomids, ephemeropterans and dipterans. Stocked

trout stomachs also contained a few stones and twigs, and perhaps remains of trichopteran

cases. Food composition of native and stocked trout was di�erent in June especially in

lake age groups 0 and 1 (Fig. 3). In June, small-sized native trout ate more white®sh than

stocked trout. Older ®sh of both groups had similar food composition in June to

September. The food composition in lake age groups 0 and 1 became more similar from

June onwards and in July all age groups ate mainly white®sh. The proportion of densely

rakered white®sh in the brown trout diet was about 90% and that of the large sparsely

rakered white®sh formed only 10%. No specimens of the small sparsely rakered white®sh

were found in brown trout stomachs.

The degree of stomach fullness of native and stocked trout was similar in all months

(Table 2). Any small di�erences were connected with small sample sizes. Stomach fullness

in all lake age groups increased toward autumn and this was associated with the time when

all age groups had shifted to feeding on white®sh.

Altogether 459 white®sh were found in brown trout stomachs. Mean prey lengths for

stocked and native brown trout were 12.7 and 11.8 cm, respectively. Prey length varied

between 2±18 and 3±16 cm in stocked and native trout, respectively, and a weak positive

correlation (P < 0.001) between predator and prey length were found for both groups

86 K. KAHILAINEN & H. LEHTONEN

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

Figure 1. CPUE of native (black column) and stocked (white column) brown trout from di�erent habitat

(P � pelagic, E � epibenthic) and depth zones in June±September 1999. Note that data are missing from the

epibenthic zone (0±5 m) in August and September.

RESOURCE USE OF BROWN TROUT 87

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

(Fig. 4). The linear regression equation for native trout was y � 0.20x + 4.39, r2 � 0.28

and for stocked trout it was y � 0.20x + 4.76, r2 � 0.17. The relative minimum and

maximum prey length vs. predator length varied between 12 and 51% in native and 6±51%

in stocked trout. Mean relative prey length of native and stocked trout were 32.5 and

31.9%, respectively.

Habitats and diet of native and stocked brown trout were similar. Schoener's indices of

dietary overlap between the same lake age groups in all months were high (Fig. 5). The

lowest values (0.2±0.5) were observed in June when small-sized stocked trout (lake age 0)

fed mainly on invertebrates. Older age groups (ages 1±3) of both native and stocked trout

were piscivorous in June and dietary overlap was high (>0.7). In July±September all age

groups fed mainly on white®sh, and the indices were correspondingly high (>0.7).

Discussion

CPUE of stocked brown trout was higher than for native ®sh which indicated that

stocking density (0.5±2 brown trout ha)1) in 1995±1998 was higher than natural

recruitment into the lake. Another explanation is that stocked trout exhibited shoaling

behaviour and their catchability was, therefore, higher than that of native trout. Both

stocked and native trout used the same habitats. CPUE was greatest near the surface in

littoral and pelagic waters. The littoral zone provides refuge areas and better benthic food

Figure 2. Mean empirical length of native and stocked brown trout in June and September 1999.

88 K. KAHILAINEN & H. LEHTONEN

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

resources (Hiisivuori & Honkasalo 1977) for small-sized brown trout compared with the

profundal zone. Larger trout shift to use the pelagic zone (Jonsson & Gravem 1985;

Hegge, Hesthagen & Skurdal 1993). The brown trout is a visual predator which is

probably one reason it dwells near the surface (Hegge et al. 1989). Water temperature was

highest at the surface, reaching a maximum of 15.5 °C in July and for most of the summer

Figure 3. Food composition of stocked and native brown trout age groups in June±September 1999. Figures

above bars indicate the number of trout.

RESOURCE USE OF BROWN TROUT 89

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

it was between 11 and 14 °C. This is close to 13±15 °C which is the optimum growth

temperature of brown trout (Elliott 1975; Jensen 1990).

Stocked trout were larger than native trout during their ®rst year in the lake. However,

mean lengths were the same during the second growing season. Thereafter length-at-age

was equal for both stocked and native trout. Because the onset of piscivory takes place

early in Lake MuddusjaÈ rvi, the growth rate is high when suitable ®sh prey are available

(Garman & Nielsen 1982). Similar observations from Norway con®rmed these results

Table 2. Mean stomach fullness (scale 0±10) of stocked and native brown trout in June±September 1999

Age group June July August September

Stocked brown trout

0 2.3 3.2 3.6 5.2

1 3.9 3.7 4.0 5.5

2 4.6 3.0 4.6 5.1

3 2.6 0.5 4.0 4.4

Native brown trout

0 1.5 3.2 2.3 5.2

1 3.5 3.8 3.8 5.6

2 2.8 4.0 ± 5.3

3 4.8 0.0 2.0 1.6

Figure 4. Predator length and prey length for stocked (white circles) and native (black circles) brown trout.

90 K. KAHILAINEN & H. LEHTONEN

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

showing that the growth rate of stocked and native trout is similar after a given time

(L'Abe e-Lund & Sñgrov 1991).

Food composition of small-sized stocked and native trout was di�erent in June, but

toward autumn both fed on small-sized white®sh. Learning how to feed on natural food

items occurs during the ®rst lake summer, after which the diet of native and stocked brown

trout tends to be similar (O'Grady 1983; Johnsen & Ugedal 1986). White®sh was the main

food item used by both stocked and native trout. According to stomach fullness data the

feeding activity became more intensive in August±September when water temperature

started to decrease or the consumption time was reduced as water temperature decreased.

The growing season of brown trout continues in northern Finland to late November,

especially in years when ice cover is late forming (Niva 1999).

Figure 5. Diet overlap of native and stocked brown trout in June±September 1999 (age in lake years).

RESOURCE USE OF BROWN TROUT 91

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

According to experimental gillnet catches in 1998±1999, the abundance of small-sized

densely rakered white®sh was high (Kahilainen & Lehtonen 2001). This was probably why

brown trout over 20 cm started to feed almost exclusively on white®sh. The mean size of

white®sh prey was about 12 cm which is similar to the prey size as observed in Norway

(L'Abe e-Lund et al. 1992; Nñsje, Sandlund & SaksgaÊ rd 1998). In Finnish lakes, vendace

Coregonus albula (L.), <10 cm are usually the preferred prey item for brown trout

(Vehanen, HyvaÈ rinen & Huusko 1998; Niva & Julkunen 1998). Vendace can grow beyond

the size at predation during their ®rst growing season (Vehanen et al. 1998), but this is not

the case for the slow growing white®sh stocks of Lake MuddusjaÈ rvi.

In this study, prey length increased slightly with increasing length of predator. In that

respect, the results are similar to Lake Femund where the majority of white®sh consumed

by trout were between 8 and 17.9 cm (Nñsje et al. 1998). In Lake MuddusjaÈ rvi predation

focused on 8±17 cm white®sh. Most preferred was the densely rakered white®sh (90% of

white®sh consumed) which is the only pelagic white®sh form in the lake (Kahilainen &

Lehtonen 2001). CPUE of densely rakered white®sh was about 16-times higher than that

of large sparsely rakered white®sh. According to optimal foraging theory, predators

choose the prey which is energetically pro®table to chase (Townsend & Win®eld 19852 ),

which in this case seems to be the densely rakered white®sh.

The high indices (>0.7) of diet overlap showed similarity in feeding behaviour of

stocked and native trout. It might be concluded from this that stocked trout compete for

food and space with native trout and this suppresses native trout. However, because

length-at-age has not changed after stocking (Mutenia 1984) suitable amounts of prey ®sh

appear to have been available for both native and stocked trout.

This study demonstrated that resource utilization (habitat and food) of stocked and

native trout becomes similar during the ®rst summer after stocking. Similar growth pattern

of stocked and native brown trout indicate that stocked trout are able to become a

substantial part of the brown trout population in Lake MuddusjaÈ rvi.

Acknowledgments

We would like to thank Mr Jukka Pohtila, Mr KyoÈ sti MaÈ enpaÈ aÈ and Mr Oskari Aikio for

help in the ®eld work. MuddusjaÈ rvi Research Station provided excellent facilities for the

study. The ®nancial support of the Municipality of Inari and Ministry of Agriculture and

Forestry are gratefully acknowledged.

References

Bachman R.A. (1984) Foraging behavior of free-ranging wild and hatchery brown trout in a stream.

Transactions of American Fisheries Society 113, 1±32.

DamsgaÊ rd B. & Langeland A. (1994) E�ects of stocking of piscivorous brown trout, Salmo trutta L.,

on stunted Arctic charr, Salvelinus alpinus (L.). Ecology of Freshwater Fish 3, 59±66.

Elliott J. (1975) The growth rate of brown brout, Salmo trutta L., fed on maximum rations. Journal

of Animal Ecology 44, 805±821.

92 K. KAHILAINEN & H. LEHTONEN

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

Garman G.C. & Nielsen L.A. (1982) Piscivority by brown trout (Salmo trutta) and its impact on the

nongame ®sh community of Bottom Creek, Virginia. Canadian Journal of Fisheries and Aquatic

Sciences 39, 862±869.

Hegge O., Dervo B., Skurdal J. & Hessen D. (1989) Habitat utilization by sympatric arctic charr

(Salvelinus alpinus (L.) and brown trout (Salmo trutta L.) in Lake Atnsjù, south-east Norway.

Freshwater Biology 22, 143±152.

Hegge O., Hesthagen T. & Skurdal J. (1993) Vertical distribution and substrate preference of brown

trout in a littoral zone. Environmental Biology of Fishes 36, 17±24.

Hesthagen T., Jonsson B. & Skurdal J. (1989) Survival, exploitation and movement of takeable size

brown trout, Salmo trutta L., in a Norwegian river. Aquaculture and Fisheries Management 20,

475±484.

Hesthagen T., Hegge O., Skurdal J., & Dervo B.K. (1995) Di�erences in habitat utilization among

native, native stocked, and non native-stocked brown trout (Salmo trutta) in a hydroelectric

reservoir. Canadian Journal of Fisheries and Aquatic Sciences 52, 2159±2167.

Hiisivuori C. & Honkasalo L. (1977) InarinjaÈrven pohjaelaÈintutkimus 1976. Helsinki: Finnish Game

and Fisheries Research Institute (In Finnish, mimeographed)3 .

Hynes H. (1950) The food of freshwater sticklebacks (Gasterosteus aculeatus and Pungitius

pungitius), with a review of methods used in studied of the food of ®shes. Journal of Animal

Ecology 19, 36±58.

Jensen A. (1990) Growth of young migratory brown trout Salmo trutta correlated with water

temperature in Norwegian rivers. Journal of Animal Ecology 59, 603±614.

Johnsen B. & Ugedal O. (1986) Feeding by hatchery-reared and wild brown trout, Salmo trutta L., in

a Norwegian stream. Aquaculture and Fisheries Management 17, 281±287.

Johnsen B. & Ugedal O. (1989) Feeding by hatchery-reared brown trout, Salmo trutta L. released in

lakes. Aquaculture and Fisheries Management 20, 97±104.

Johnsen B. & Ugedal O. (1990) Feeding by hatchery- and pond-reared brown trout, Salmo trutta L.,

®ngerlings released in a lake and in a small stream. Aquaculture and Fisheries Management 21,

253±258.

Jonsson B. & Gravem F. (1985) Use of space and food by resident and migrant brown trout, Salmo

trutta. Environmental Biology of Fishes 14, 281±293.

Kahilainen K. & Lehtonen H. (2001) Habitat use and growth of three sympatric white®sh forms,

Coregonus lavaretus (L.), in the subarctic Lake MuddusjaÈ rvi. Archiv fuÈr Hydrobiologie Special

Issues (in press)4 .

L'Abe e-Lund J. & Sñgrov H. (1991) Resource use, growth and e�ects of stocking in alpine brown

trout, Salmo trutta L. Aquaculture and Fisheries Management 22, 519±526.

L'Abe e-Lund J., Langeland A. & Sñgrov H. (1992) Piscivory by brown trout Salmo trutta L. &

Arctic charr Salvelinus alpinus (L.) in Norwegian lakes. Journal of Fish Biology 41, 91±101.

L'Abe e-Lund J. & Langeland A. (1995) Recaptures and resource use of native and non-native brown

trout, Salmo trutta L., released in a Norwegian lake. Fisheries Management and Ecology 2,

135±145.

O'Grady M. (1983) Observations on dietary habits of wild and stocked brown trout, Salmo trutta L.,

in Irish lakes. Journal of Fish Biology 22, 593±601.

Mutenia A. (1984) Kaamasjoen kalatalousselvitys kalastuksen ja kalakantojen hoidon suunnittelua

varten. Finnish Game and Fisheries Research Institute, Monistettuja Julkaisuja 24, 1±55.

(In Finnish).

Nñsje T., Sandlund O. & SaksgaÊ rd R. (1998) Selective predation of piscivorous brown trout (Salmo

trutta L.) on polymorphic white®sh (Coregonus lavaretus L.). Archiv fuÈr Hydrobiologie Special

Issue 50, 283±294.

RESOURCE USE OF BROWN TROUT 93

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94

Niva T. (1999) Ecology of stocked brown trout in boreal lakes. (Doctoral Dissertation Thesis)

Biological Research Reports of University of JyvaÈskylaÈ 75, 1±26.

Niva T. & Julkunen M. (1998) E�ect of population ¯uctuation of vendace (Coregonus albula) on the

diet and growth of stocked brown trout (Salmo trutta). Archiv fuÈr Hydrobiologie Special Issue 50,

295±303.

Schoener T.W. (1970) Non-synchronous spacial overlap of lizards in patchy habitats. Ecology 51,

408±418.

Townsend C. & Win®eld I. (1985) The application of optimal foraging theory to feeding behavior in

®sh. In: P. Tytler & P. Calow, (eds). Fish Energetics, New Perspectives. Sydney: Croom-Helm,

pp. 67±98.5

Vehanen T. (1995) Factors in¯uencing the yield of brown trout, Salmo trutta m. lacustris L., in

northern Finnish lakes. Fisheries Management and Ecology 2, 121±134.

Vehanen T. & Aspi J. (1996) Classi®cation of northern Finnish lakes and the suitability for the

stocking for brown trout (Salmo trutta (L.) m. lacustris). Fisheries Research 27, 37±49.

Vehanen T., HyvaÈ rinen P. & Huusko A. (1998) Food consumption and prey orientation of

piscivorous brown trout (Salmo trutta) and pikeperch (Stizostedion lucioperca) in a large

regulated lake. Journal of Applied Ichthyology 14, 15±21.

94 K. KAHILAINEN & H. LEHTONEN

Ó 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 83±94