UNIVERSITETET I · som har muliggjort sammenstilling av resultater om næringsvalg ... Strandefj.11 3531 7.0 July Oct. 1977 Trout, minnow perch,whitefish 1) Filled reservoir. 2 )

Laboratorium for ferskvannsøkologiog innlandsfiske,,(LFI)

Rapport nr. 102 1988• ISSN 0333-161X

FEEDING BEHAVIOUR AND HABITAT SHIFT IN ALLOPATRIC AND

SYMPATRIC POPULATIONS OF BROWN TROUT (SALMO TRUTTA L.):

EFFECTS OF WATER LEVEL FLUCTUATIONS VERSUS INTERSPECIFIC

COMPETITION.

ÅGE BRABRAND AND SVEIN JAKOB SALTVEIT

UNIVERSITETET I 05L0ZOOLOGISK MUSEUM

FEEDING BEHAVIOUR AND HABITAT SHIFT IN ALLOPATRIC AND

SYMPATRIC POPULATIONS OF BROWN TROUT (SALMO TRUTTA L.):EFFECTS OF WATER LEVEL FLUCTUATIONS VERSUS INTERSPECIFICCOMPETITION.

ÅGE BRABRAND AND SVEIN JAKOB SALTVEIT

Laboratorium for ferskvannsøkologi og innlandsfiske (LFI),Zoologisk Museum, Universitetet i Oslo,Sarsgate 1,

0562 Oslo 5.

Forord

Laboratorium for ferskvannsøkologi og innlandsfiske ved

Universitetet i Oslo har ønsket å utnytte tidligere innsamlet:

materiale ved reguleringsundersøkelser i en regional

undersøkelse . Hensikten har vært å øke kunnskapen om virkning

av vassdragsreguleringer på fisk og næringsdyr. Det er fra

Vassdragsregulantenes Forening ( VR) bevilget forskningsmidler

som har muliggjort sammenstilling av resultater om næringsvalg

for ørret.

Den foreliggende rapport omhandler næringsvalg for ørret i

endel høyereliggende innsjøer i de sentrale østlandsområder.

Det er lagt vekt på å belyse betydningen av regulering og kon-

kurranse fra andre fiskearter , både pelagiske og littorale. For

å vurdere avkastning , utsettingspålegg og produksjonsforhold

vil det være av betydning å ha generell informasjon om til-

gjengelighet av næringsdyr i ulike innsjøkategorier.

I en redigert utgave vil den foreliggende rapport bli publisert

i et vitenskapelig tidsskrift . Redigeringen vil først og fremst

omfatte figurene 2-5, da disse er å betrakte som en present-

asjon av primærdata. Det er imidlertid i den foreliggende

rapport valgt å presentere data fra enkeltsjøer separat.

Oslo , april 1988

Åge Brabrand

Svein Jakob Saltveit

2

INTRODUCTION

Habitat utilized by brown trout (Salmo trutta L.) in lakes is

obviously influenced by the presence of potential competitors

as well as by food selection and food items available. In most

of the literature, brown trout are described as food

generalists, but mostly take their food in the littoral zone.

The availability of benthic food items is greatly influenced by

the presence of littoral fish species and the degree of water

level fluctuations. In lakes with several littoral species and

large water level fluctuations, the availability of littoral

food items may be scarce, and brown trout can behave more or

less as a planktivore. However, the presence of pelagic fish

species more specialized as planktivores may reduce the

planktivorous tendency of trout.

In a regional study we have compared gut contents in brown

trout from 15 lakes in the central part of South-Norway (Fig.

1). The lakes cover a gradient of water level fluctuations from

unregulated to 8.3 m, and support four categories of fish

communities (Table 1). In Category I, brown trout and minnow

(Phoxinus Phoxinus) are the only species present. In Category

II, brown trout, minnow and whitefish (Coregonus lavaretus) are

present, the latter as a planktivorous competitor (Sverdson

1976). In Category III, minnow and perch (Perca fluviatilis)

are present in addition to brown trout, while Category IV have,

brown trout, whitefish, minnow and perch. Perch behave both as

a littoral competitor as well as a potential predator on minnow

and juvenile stages of trout. In combination with the degree of

water level fluctuations, the influence of littoral and pelagic

food competitors on food uptake of trout is considered.

3

STUDY AREA

All lakes studie.d are situated in the alpine/subalpine areas in

the central part of South-Norway (Fig. 1). Some hydrological

and biological data concerning the lakes are given in Table 1.

The outlet river of all the regulated lakes is unavailable for

spawning, although recruitment is ensured through spawning in

the inlet river or by stocking. Most of the lakes are

classified as oligotrophic to ultra-oligotrophic, with total

phosphorus concentrations below 30 Ng/L. Temperature

stratification occurs in the period June-September, with

maximum epilimnetic summer temperatures in July.

MATERIAL AND METHODS

Trout for stomach analysis were collected overnight by sets of

gill nets (1.5 x 25 m) in the littoral zone during summer

(June, July) and autumn (August, September) from each lake. The

following mesh sizes were used: 19.5, 22.5, 26, 29, 35, 39, 45,

and 52 mm. When whitefish was present, pelagic gill nets (6 x

25 m) with mesh size 19.5, 22.5, 26, 29, 39, and 45 mm, and

covering the depth interval 1-7 m below the water surface, were

always used. However, in Rossjoen (r_.at.I) pelagic gil]. nets

were also used.

Captured fish were measured, weighed, sexed, and their st.omachs

removed. Gut contents were analysed according to the volumetric

method described by Hynes (1950). Fish from the littoral or

pelagical zone were kept separate, and gut contents of up to 20

individuals from each 5 or 10 cm length groups were analysed.

Benthic communities were sampled from the stony littoral in all

of the lakes during the period of fishing using the kick-method

(Hynes 1961). Survey zooplankton sampling was carried out in

all lakes with whitefish. Samples from the soft profundal and

sublittoral bottom were taken using core samplers or Ekman

grab.

4

Table 1. Investigated lakes, their altitudes (m a.s.l.), water

level fluctuations (in metres), time of investigation and fish

communities.

Lake m a.s.l Fluct. Investigated Fish community

Dokkvatn 1 776 N.R.2 July Sept.1978 Trout, minnow

Øyangen 2 676 8.3 July Oct. 1977 Trout, minnow

VåsjØen 3 870 3.0 June Sept.1986 Trout, minnow

Djupen 4 914 3.0 Sept.1982 Trout, minnow

Sept.1983

Mjogsjoen 5 887 N.R. July Sept.1979 Trout, minnow

whitefish

Synnfjord 6 796 N.R. Sept.1979 Trout, minnow

whitefish

Goppollen 7 977^ 2.2 Sept.1982 Trout, minnow

Sept.1983 whitefish

Rossjcien 8 895 N.R. July Sept.1978 Trout, minnow

1979Jul erchy p .

Volbufjord 9 429^ 3.0 July Oct. 1977 Trout, minnow

perch

Dokkfloyv.10 696 N.R. July Sept.1978 Trout, minnow

perch

Strandefj.11 3531 7.0 July Oct. 1977 Trout, minnow

perch,whitefish

1) Filled reservoir.

2 ) Not regulated lakes.

5

RESULTS

The main results clearly demonstrates the variability of food

items observed in trout gut contents. This was highly related

to food availability and inter- and intraspecific conditions

both within each lake and between the investigated lakes.

Brown trout and minnow lakes (Category I).

Gut contents of trout in unregulated and regulated lakes where

trout and minnows are living sympatrically is given in Fig. 2.

In the only unregulated lake in this c.ategory, Dokkvatn, uptake

of zooplankton by trout was insignificant, and was in summer

completely dominated by nymphs of Ephemeroptera ("group other"

in Fig. 2). In autumn, trout showed a piscivorous tendency,

feeding to a large extend on minnows, which made up to 100 0 of

total gut volume in some length groups (Fig. 4). All other

studied lakes in this category were regul.ated, and zooplankton

was obviously an important food component at specific times of

the productive period. However, during early summer, a diverse

benthic animal community is also available in these lakes,

while the zooplankton peak is somewhat later. Only in Lake

Øyangen, brown trout fed on zooplankton as early as Jul.y,

probably caused by the relatively higher water level

fluctuation in this lake, giving low benthic animal densities,

forcing trout to feed on zooplankton even in periods of low

zooplankton abundance. In Lake Djupen, Daphnia longispina was

of considerable importance in small trout, while the proportion

of Bythotrephes longimanus increased in larger fish. Also in

the regulated VåsjØen, the zooplankton component was dominated

by D. longispina and B. longimanus. However, predation on the

two species was more similar.

Brown trout, whitefish and minnow lakes (Category II).

Gut contents of trout coexisting with whitefish and minnow are

given in Fig. 3. Three lakes are included in this group, two

6

of which are unregulated. Zooplankton were not observed in the

gut r_.ontents of trout from any of the lakes, either in July or

in September/October. Trichopteran larvae were the dominating

benthic group, together with a relatively high proportion of

minnows, even in trout of size 15 - 20 cm. Fishing with pelagic

gill nets showed a strong horizontal habitat segregation

between pelagic areas completely dominated by whitefish, and

the more littoral areas dominated by trout. This confirms the

presence of a Strong feeding segregation between trout and

whitefish when c.oex.isting.

Brown trout, minnow and perch lakes (Cateaory III).

Gut contents of trout living together with minnows and perch

are given in Fig. 4. In all the investigated lakes, the

zooplankton species, Bythotrephes longimanus, was an important

Component of total gut volume, even in larger fish in some of

the lakes. Compar_ed to lakes with minnow, but without perch,

trout predated on minnows to a much lesser extent, and when

doing this, trout seems to be of larger size. In the

unregulated lakes RossjØen and Dokkfloyvatn, high availability

of large benthic animals, such as Gammarus lac.ustris and

trichopteran larvae is reflected in the high consumption of

these food items.

Brown trout, perch, minnow and whitefish lakes (Category IV).

Gut contents of brown trout in the regulated lake, Strande-

fjorden, are shown in Fig. 5. Here brown trout coexist with the,

littoral living perch and minnow and the planktivorous

whitefish. Regulation obviously forces trout to utilize both

littoral and pelagic areas. However, water level fluctuations

of 7 m will also have a very strong impact on the availability

of benthic prey items. High densities of minnows and perch

force trout to feed planktivorously in this lake, in spite of

strong competitive pressure from a truely planktivorous

7

population of whitefish. Both species fed to a large degree on

Daphnia and Bosmina, but trout also fed on benthic organisms

such as Trichoptera and Chironomidae, as well as fish (largely

minnows). Indeed, many lakes that posess perch, minnow and

whitefish, have extremely low densities of trout, indicating a

strong interspecific interaction.

DISCUSSION.

The availability of food items for fish is influenced by a

number of factors, both abiotic (Aass 1973) and biotic

(Klemetsen 1967, Langeland 1982, Nilsson 1960). General lake

productivity, which influences status and species composit.ion

of both zooplankton, benthos and prey fish is of basic

importance.

The most important feeding areas for brown trout in oligo-

trophic lakes are in the littoral zone. The high zoobenthic

production here is due to vegetation, allochtonous inputs and

temperature conditions. This zone is, however, vulnerable to

drawdown of the water level, and important fish food organisms

typical of this zone, such as Gammarus, snail.s and l.a.rger

insects are affected (GrimAs 1962). In general, impoundment can

be divided into short term and lonet term effects. By increasing

the water level, new areas rich in nutrients become available

for production (impoundment effect). Few animals are however

adapted to large seasonal fluctuations in water level. In the

long term ice and wave e.rosion aided by freezing will lead to

an inpoverished littoral zone, lacking vegetation and dead

organic material for food and cover. There is therefore both a

quantitative and a qualitative decrease in benthic animals in

littoral areas (Grimås 1961, 1962). However, animals living in

the profundal zone below the drawdown limit are not affected

and may indeed profit by the organic material sedimented from

the littoral zone. These benthic animals (chironomids, oligo-

chaetes, mussels) are, however, less available to fish preda-

tion due to their habit of living down within the sediments.

8

More arctic condi_tions in reservoi_rs due to freezing in the

littoral zone and deeper intrution of cold epilimnetic water

during drawdown in winter, may lead to an increase in the

importance of the crustacean Lepidurus arcticus in some lakes

(Aass 1969, BorgstrØm 1973, Brabrand and Saltveit 1980).

Zooplankton product.i_on will increase due to increase in

nutrients from the impounded areas and reduction in loss by

closing the outlet. In the long term, this positive effect will

disappear through grazing and sedimentation, but production of

zooplankton will probably not be negatively affected (Elgmark

1970).

Despite the limits placed by the general productivity of lake

ecosystems in determining the avai_lability of food items, a

number of authors have document.ed the influence of the fish

populations themsel.ves on prey abundance (Shapiro et al. 1975).

Fish species have differing ability to suppress planktonic

crustaceans and benthi.c organisms by their selectivity and

changing predator pressure (Garnås et al. 1983).

In the subalpine lake, Øvre Heimdalsvatn, where trout were the

only fish species until. 1969, they feed on Bvthotrephes

longimanus and DaOinia longispina, and obviously selected food

items from the zoopllankton community at cer_tain times of the

year (Lien 1980). In the littoral zone, trout fed on Lepidurus

arcticus and Gammarus lacustris, which are both sensitive to

predator pressure (Brabrand et al.in prep.). The same

planktivorous tendency is also shown by Klemetsen (1967) in

Lake Jolstervatn. However, when whitefish are present, a switch

in feeding habit in brown trout from partly planktivorous to a,

true benthic habit occurs, which only can be explained by

interspecific segregation. This shift occurs both in regulated

and unregulated lakes, because whitefish are more truely

planktivorous than trout. Interesting, in most of the regulated

lakes inhabited by whitefish, the traut population is small or

even absent. An extreme environment is lake Trevatn, which is

regulated approx. 7 m (Helluer & Saltveit 1981). In this lake,

9

smelt are present in addition to whitefish, the former being

even more planktivorous than whitefish, and probably forcing

whitefish to feed more on benthic animals compared to the

situation where whitefish are the only planktivorous species.

The presence of smelt will therefore hamper further

inteaspecific segregation between trout and whitefish, which in

turn can leed to virtual absenc_e of trout in this type of lake

(Sverdson 1976). The only way for trout to survive in

whitefish-smelt lakes is to switch to a higher trophic level,

and changing to fish diet as often can be observed in a number

of large lowland lakes in Scandinavia (flass 1973).

However, as brown trout have the ability to utilize both

pelagic and littoral habitats, it is the total environmental

conditions which are of decisive importance for brown trout

populations. In the present study, it is shown that trout can

feed to a greater or lesser extent on planktivorous food,

which is probably of special importance for immature trout.

This will increase the intraspecific trout niche separation

between immature and matu.r_e individuals. Also the effect of

perch, a littoral predator, will be reduced, as they are

primarily littoral predators (Popovn & Sytina 1977). When

regulated, the trout population of such lakes will adapt to an

even more planktivorous behaviour, still maintaining high

densities or even increasing, if spawning areas are not

destroyed. This seem to occur independently of the presence of

perch or minnow.

However, when perch and minnows are present, the potential of

minnow as prey for larger brown trout is greatly reduced,

compared to the situation when brown trout and minnow live

alone. This aspect is probably of great importance, since

minnow can obviously be an important food item for trout larger

than approx. 15 cm, producing inc_reased growth rates and

intraspecific segregation for the trout population.

Introduction of perch as a more effec_tive minnow predator than

trout, therefore seems to reduce the prey availability for

trout themselves.

10

In conclusion, regulation has a serious negative impact on the

brown trout populations when planktivorous fish species are

present, notably regarding competition for food. The key

elements here are a reduced benthic animal production and

increased competition for pelagic food. Increased pelagic food

competition together with wide water level fluctuations or

presence of littoral fish species will reduce or even inhibit

the trout population. However, trout may occupy a true pelagic

predatory food niche, and can despite low abunda-nce, reach

large individual size.

Regulation of trout lakes lacking planktivorous fish species,

give trout a pelagic feeding refuge, almost independent of

water level fluctuations. Presence of littoral fish species or

higher water level fluctuations increase the pelagic tendency

of trout. In extreme cases, trout can turn into a completely

planktivorous behaviour, also encompassing changes in

morphology (Solheim 1987). A large population of trout in such

lakes can still be ma.intained, or even increased after

regulation. However, individual fish size are usually small,

due to deficit of larger food items.

11

LITTERATURE

Aass, P. 1969 . Crustacea, especially Lepidurus arcticus Pallas,

as brown trout food in Norwegian mountain reservoirs. Rep.

Inst. Freshwat. Res. Drottningholm 49: 183-201.

Aass, P. 1973. Some effects of lake impoundments on Salmonids

in Norwegian Hydroelectric reservoirs. Acta Univ.

Upsalensis Abstr. Uppsala Diss. Sci. 324: 1-14.

Borgstrom, R. 1973. The effect of inc_reased water level.

fluctuation upon the brown trout population of Mårvann, a

Norwegian reservoir. Norw. J. Zool. 21: 101-112.

BorgstrØm, R., Brabrand, A. and Solheim, J.T. 1986. Tilslamming

og redusert siktedyp i Ringedalsmagasinet: Virkninger på

habitatbruk, næringsopptak og kondisjon hos pelagisk aure.

Rapp. Lab. Ferskv. Økol. Innlandsfiske. Oslo. 90: 36 pp.

Brabrand, A. og Saltveit, S.J. 1980. Skjoldkreps, Lepidurus

aret.ir_.us (Pallas), i Volbufjorden 434 m o.li. i Øystre

Slidre, Oppland. Fauna 33: 105-108.

Brabrand , A., Bremnes ,J., Brittain, J.E. and Saltveit, S.J. The

biology of Gammarus lacustris in relation to the

introduction of minnows , Phoxinus phoxinus, in Øvre

Heimdalsvatn , a Norwegian subalpine lake. In prep.

Elgmork, K. 1970. Plankton og_Planktonproduksjon i regulerte

innsjØer. Kraft og MiljØ, Norges vassdrags og elektrisi-

tetsvesen, Nr. 1: 11-15.

Grimås, U. 1961. The bottom fauna of natural and impounded

lakes in northern Sweden. (Ankarvattnet and Blåsj6n). Rep.

Inst. Freshw. Res. Drottningliom 42: 183-237.

Grimås, U. 1962. The effect of increased water level

12

fluctuations upon the bottom fauna in Lake BlAsj6n,

Nor.thern Sweden. Rep. Inst. Freshw. Res. Drottningholm 44.

14-41.

Hellner, D. og Saltveit, S.J. 1981. Fiskeribiologiske

undersØkelser i forbindelse med endret regulering av

Trevatn, Oppland. Rapp. Lab. FerskvØkol. Innlandsfiske

Oslo 50: 60 pp.

Hynes, H.B.N. 1950. The food of freshwater sticklebacks

(Gasterosteus aculeatus) and Pygosteus pungitius), with a

review of methods used in studies of the food in fishes.

J. Anim. Ecol. 19: 36-58.

Hynes, H.B.N. 1961. The invertebrate fauna of a Welsh mountain

stream. Arch. Hydrobiol. 57: 344-388.

Klemetsen, A. 1967. On the feeding habitat of the population of

brown trout (Salmo trutta L.) in JØlstervatn, West Norway,

with special.. reference to planktonic crustaceans. Nvt.t.

Marx. Zool . 15: 50-57.

Langeland, A. 1982. Interactions between zooplankton and fish

in a fertilized lake. Holarct. Eco].. 5: 273-310.

Lien, L. 1980. The energy budget of the brown trout population

of Øvre Heimdalsvatn. Holarct. Ecol. 1: 279-300.

Nilsson, N.A. 1960. Seasonal fluctuations in the food

segregation of trout, char and whitefish in 14 North-

Swedish lakes. Rep. Inst. Freshw. Reg Drottningholm 41:

185-205.

Nilsson, N.A. and Pejler, B. 1973. On the relation between fish

fauna and zooplankton composition in North Swedish lakes.

Rep. Inst. Freshw. Res. Drottningholm 53: 51-77.

Popova, O.A. and Sytina, L.A. 1977. Food and feeding relations

13

of Eurasian perch (Persa fluviatilis) and pike-perch

(Stizostedion lucioperca) in various waters of the USSR.

J.Fish. Res. Board. Can. 34:1559-1570.

Ricker, V.E. 1975. Computation and interpretation of biological

statistics of fish populations. Bull. Fish. Res. Board

Can. 191: 323 pp.

Shapiro, J., Lamarra, V. & Lynch,M. 1970. Biomanipulation: An

ecosystem approach to lake restoration. In: Water Quality

Management Through Biological Control (P.L. Brezonik and

J.L. Fox, eds). ENV-07-75-1 Gainesv.ille, Florida:

University Florida.

Solheim, J.T. 1937. Auren i Ringedalsmagasinet. Thesis Inst.

for naturforvaltning, Norges LandbrukshØskole. 46 pp.

Sverdson, G. 1976. Interspec.ific population dominance in fish

communities of Scandinavian lakes. Rep. Inst. Freshwat.

Res. Drottningholm 55: 144-171

Fig. 1. Location of the investigated lakes , all situated in

alpine / subalpine areas in South-Norway. For

identification of lake number , see Table 1.

FISH COMMUNITY

Bo^

TUPE-B INNSJO-DOKKVATH AAR-78 MND-4 INNSJO-V1SJOEN AAR-84 MND-7 INNSJO-ØANOEN AAR-77 MND-7100 ^ 100^ ^ .,...

I.

U-20 20-20 20-70

LENGTH GROUP

D0-2e

TYPE-9 INN3J0-MJ003J0E AAR-79 MNO- }

100

II.

10-20 20-20 2e-20

BENTHIC F00D COMPONENTS

^

16-20 20-20 20-00 30-oe ^ 10-20 00- .Jo 2e-4eLFNGTH GROUP

LENGTH GROUP

TYPE-11 INNSJO-DOKKfLØ MR-78 LIND-1 INNSJO-RIEISSJ6ENinn

LENGTH GROUP

TYPE-8 INNSJO-STRANDEF AAR-77 UNO-7

Too ^

IV.

t0-20

GRUPPE

20-00 70-40

LENGTH GROUP

roo

eo

m

40

O

10-te 10-2e

LENGTH GROUP

AAR-79 MND-7 INNSJO-RrSSJOEN AAR-70 MND- } F

100,

904

604

40

20

20-23 20-30 00-20

LENGTH GROUP

INNSJO-STRANDEF AAR-77 UND-+ f

010-20 20-20 2e-00 30-20

LENGTH GROUP

INNSJO-VOLBUFJO AAR-77 MND

100.

m

V So0L

s 40

20

10-20 JO-40

LENGTH GRGUP

FISH COMMUNITY.,

1. TROUT, MINNOW

II. TROUT, MINNOW ,WHITEFISH

III. TROUT, MINNOW,PERCH

IV. TROUT, MINNOW ,PERCH,WHITEFISI

Fig. 2. Percentage of benthos of total gut contents of brown

trout in early summer in lakes with different fish

communities. Regulated lakes are indicated with an

asterix.

FISH COMMUNITY

TYPE-9 INN9JO-VjSJfEN AAR-66

100 1 .L

so 4

•

I.

II.

LC::Ie

BENTHIC FOOD COMPONENTS

I2NO-e iNNSJO-DOKKVATN AAR-7e

100.

e04

so 4

16-20 20-28 28-30 30-38

LENGTH GROUP

TYPE-9 INN3J0-MJOGSJmE AAR-70

100,

604

C so^

L

2 407

204

0

toa

- M .18-20 20-23 28-30 30-38

LENGTH GROUP

MND-9 INN3JO-00PPOLLL . AAR-82

100 1T'

604

F

M

m

40

20

I PC>L1

" 18-20 20-28 23^0 20-23 23-30

LENGTH GROUP LENGTH GROUP

MND-9 INNSJO-Rø9SJ4EN AAR-76 IJND-9 INNSJO-VOLBUFJO AAR-77 MND-(GTYPE-9 INN3JO-O0KKFLpY AAR-79

100 100, 100,

804

III.

0 so so

LR 40- M 40

Fk^li

20

- n p

20

0 0

204

18-20 20-23

LENGTH GROUP

0

76-20 20-28 30-33 1Y20 20-23 23 -30 30-38

LENGTH GROUP LENGTH GROUP

TYPE-9 INNSJO-STRANDEF AAR-77 MND-j()

1001 .^

LENGTH CROUP

018-20 20-23

LENGTH GROUP

FISII COMMUNITY:

IV.

MND-9 INNSJO-0JUPEN AAR-e3 UNO-9 iNN3J0-0YANOEN

100 1 y

so4

eo4

100.

so4

404

0

T

100

204

16-20 2020 20-30 30-06

LENGTH CROLIP

T

20-30 3e-40

AAR-77 MND-10

.. I MPM torn ID

MND- 9 INNSJO-OOPPOLLL AAR-63 HIND-q INNSJO-SYNNFJOR AAR-76 MND- 9

1 100,

804

604

40 4

204

404

20

T

boa

10-20 20-30 30-40

LENGTH GROUP

I. TROUT , MINNOW

II. TROUT , MINNOW,WHITEFISH

III. TROUT , MINNOW,PERCH

IV. TROUT, MINNOW,PERCH,WHITEFISH

Fig. 2 . Percentage of benthos of total gut contents of brown

trout during autumn in lakes with different fish

communities . Regulated lakes are indicated with an

asterix.

FISH COMMUNITY ZOOPLANKTON FOOD COMPONENTS

TYPE-2 INNSJO-DOKKVATN AAR-7e 11ND-^7. INNSJO-VASJ1fEN AAR-88 MND-7 1NNSJ0- YANOEN AAR-77 4ND-7100^ 1oo 100

004

1.0

V so0L

S 40

TYPE-2 INNSJO-144003.I;E AAR-73 UND- ;

100 ,

eo a

II.

0

0 :01

Lo

m

ED

m

40^

204

0 ,--,+Fas !e-xs a3-33 fw3

CROUP LEHCTH GROUP

13-20 20-23 39-30 30-33

LENGTH

m

10-20 20-23 23-30

LENGTH GROUP

TYPE-2 INNSJO-DOKKFL9Y AAR-7e MND-7INN9J0-Rf99J0EN AAR-7e

1o0 i to0,

m

0 eo^

LR 10

20

0

IV.

I=b7---i_YL^f.XI

so

eo

40

20

^

LENGTH GROUP

MND-; INNSJO-11038JCSEN AAR-73MND- } F INNSJO-100 VOLRUFJO AAR-77 MND-7

I 100 ,

'- -- ----- .°-u° 10-13 13-20 20-x3 aw0 3w3

LENGTH GROUP LENGTH CROUP

TYPE-2 INNSJO-STRANDEF AAR-77 1aND-:h -STRANDEF AAR-77 MND-

1001 1001 .^

eoJ

0 eo1

LS 40

0

so

IN

401

204

10-20 20-30 30-40

LENGTH GROUP

oRUPPE

0j

u*1o•^g R.'

toa

0

eo.

$04

40

m20

+0-20 30-10 --

IENGTH GROUP

FISH COMMUNITY:

I. TROUT, MINNOW

II. TROUT , MINNOW,WHITEFISH

III. TROUT, MINNOW,PERCH

IV• TROUT, MINNOW ,PERCH,WHITEFISH

Fig. 3 . Percentageof zooplankton of total gut contents of

brown trout in early summer in lakes with differentfish communities . Regulated lakes are indicated withan asterix.

^

WW

eo:

m

40

W

+e-.23 20-:3 25_33 30-33

LENGTH GROUP

FISH COMMUNITY

i

•

ZOOPLANKTON FOOD COMPONENTS

INNlJO-DOKKVATN AAR-7e MND-Q INNRJo-VxlJ6EN AAR-04 Ø-0 INF

100 l / 100 ^ ^". 100

IR

0 [Z_10-20 20-25 25-30 30-39

LENGTH GROUP

M

m

40

m

040-20 20-29 25-30 30-38

LENGTH CROUP

00 a

C :01

0

20 J

0.18-20 20-25 29-30 30-38

LENGTH GROUP

INN3Jo,fYANo[N AAR-77 MND-10100 ,

IF

0 oe

LR 40

m

0

INNSJO-NJOOlJpf AAR-70 YN0-9 INNSJO-OOPPOLLL AAR-02 MND-9 INNSJO-OOPPOLLL AAR-es MND- 9 INNSJO-SYNNFJOR AAR-79 NND- Q100 1 /00 ^ _ .^.- 100 1001

110a

o so^II. L

R 40

m

rrr.

019-se so-:9

LENGTH GROUP

m

m

40

20

019-20 20-25 2"0

INN3J0-00KKFIJ6Y AAR-711 AMD- q INN3J0-RØSJOCN AAR-7e MND-100 .1 1 oo ,

m

V eo0t.

R 40

®'

IV.

16-20 20-29 30-09

LENGTH CROUP

0

INNlJO-STRANDEF AAR-77 MND-'of

100 l *

604

C so^

LR 40

toa

0 N ii20-30 30-40

LENGTH CROUP

.ORUPPE ixt

el

^^^i•r

80 4

004

404

20 a

019-20 20-28 23-30 30-39

LENGTH GROUP

m

IR

40

20 a

0

T

20-25 25-30

T

^

^.\^20-30 30-10

LENGTH CROUP

15-20 20-29

LENGTH GROUP LENGTH CROUP

INN3J0-V0L19UFJ0 AAR-77 MND-10

loe.

Be a

eea

40

m'

010-20 20-30 30-40

LENGTH CROUP

^

FISH COMMUNITY:

1. TROUT, MINNOW

II. TROUT , MINNOW , WHITEFISH

III. TROUT , MINNOW,PFRCH

IV. TROUT, MINNOW,PERCH , WHITEFISH

Fig. 3. Percentageof zooplankton of total gut contents of

brown trout during autumn in lakes with different

fish communities. Regulated lakes are indicated withan asterix.

LE

p s0

LR 40

20

FISH COMMUNITY

I.

INN1J0-v^aJdaN AAR-11

1001 y

V e0

it 40I

e04

m

0

II.

IV.

e

m

te-ae 2e-ae ae-4e-u

Ø GROUP

m

e1e-te te-9e ae=u

LEHGTH GROUP

m

0

Ie1D- }F

IENGTH GROUP

40

AAR-71 YND-

1o-2e t4-a u-oe a.=a1

IENGTH GROUP

100

F 111N3J0-VOL0UFJ0 AAR-77 YND-7

m

m

40

20

0

F I S H COMMUNITY:

o-ae ae-4e -

IlNGill GROUP

I. TROUT, MINNOW

II. TROIJT, MINNOW,WHITEFISH

III. TROUT, MINNOW ,PERCH I

IV. TROUT, MINNOW,PERCH,WHITEFISf1

Fig. 4 .Percentage of forage fish of total gut contents of

brown trout in early summer in lakes with different

fish communities .Regulated lakes are indicated with

an asterix.

n

Ø-7

FORAGE FISH COMPONENTS

INNaJO-QOKKVATN AAR-71 IØ-2^ INNTJ"TANO[N AAR-77 IMD-7

100

m

100

m

e0 1

404

!OI

11-24 :0-22 2a-a0 a.-a. e/e-20 ae-ae 3 0-10

U-.0 20-25 1sa0 30-35 -

LPNOTH CROUP

TYPL-P INN1Jo-YJODIJR[ AAR-7e 114D-iIoo 1

004

V .D

L

s 40

s0

1Fao 1.-t. 2f-1e

IENOTH GROUP

TYPt-► INNaJo-DORxf{0Y AAR-70

100 ,

V •o^

i. 40

TYPE-f IN.

1o0

m

m

e1.-ae ae-:i a.-ae

LENGIH GROUP

V so0L

n 40

IEIIGTH GROUP

/0a

tMD-^ INNaJe-ReaaJQCN AM-7s YND-* IHNaJOMjSaJf[N

/00 ^ f 00 .

s0^^

m

40

i 20

ei10-19

nto-t. a.-ae ae-a.

1E71G111 GROUP

mm-; INN>IJMeiRANDIP AAR-77

100 T ^.

so a

804

m

m

.FISH COMMUNITY FORAGE FISH COMPONENTS

I.

II.

IV.

TYPE-Ir INNSJO-VASJ'EN AAR-66

100

s0 a M

y so0

it 40

y 600L

n 40

204

0 i0 I m n10-20 20-2e 29-3e 30-33 0 1 I196-20 20-23 20-30 3o-sa i9-20 20-28 25-30 30-35 D 20-30 3e-4010-20 ROUP

LENCTH GROUP LENCTH C_ LENGTH GROUP LENGTH GROUP

TYPE-F INNSJo-YJOOSJrE AAR-79 IND-9 INNSJO-pOPPOLLL AAR-82 MND-9 INNSJO -OOPPOLLL AAR-33 MND-9 NNSJO-SYNNFJOR AAR-79 YND-100 1 100 100 1 ^ 100 1

V0L

9

100,

so

so

4e

20 a

10-20 20-25p l 1 ^ I I I I

,a-zo :0-23 25_30

LENGTH GROUP_ LENGTH GROUP

TYPE-F INIIIJO-DOKKFLWY AAR-76 4ND-4 INN3JO-R(I33Jq^EN AAR-76

60

V 600L

9 40

20

01,-20 2^ 3a-3s

. _ LENGTH GROUP

TYPC-F INNSJO-STRANDEF AAR-77 MND-1o

1001 y

ER

V so^0L

6 40

208

^20-30 30-40 --

LENGTH GROUP

0

m

V 000L

6 40

20

to0

LE

V so.0L

s 40 1204 0

M

40

m

so

40

m

0

m

LE

40

m

IR

40

20 a

0

LENGTH GROUP

MND-9F INNSJO-VOLBUFJO AAR-77 MND-10

100

m

so

40

20 a

T

10-2a 20-28 23-30 30-]S ^ 10-20 20-30 30-40

LE]1GTH GROUP LENGTH GROUP

FISH COMMUNITY:

13-20 20-2a

LENGTH GROUP

1. TROUT , MINNOW

II. TROUT, MINNOW,WHITEFISH

III. TROUT , MINNOW,PERCH

IV. TROUT, MINNOW ,PERCH,WHITEFISH

Fig. 4. Percentageof forage fish of total gut contents of

brown trout during autumn in lakes with different

fish Communities. Regulated lakes areindicated with

an asterix.

MND-9.F INNSJO-DOKKVATN AAR-76 AIND-9 INNSJO-DJUPEN AAR-63 MND-9 INNIJO-ØANOEN AAR-77 MND-1100 1! rl n ,po, *. 100

M

FISH COMMUNITY TERRESTRIC FOOD COMPONENTS

TYPE-T INNSJO-DOKKVATN

100 ,

•04

I.

V so^OL

n 40

20 ,

0

II.

a16-20 x0-23 23-00 30-36

LENGTH GRØ

TYPE-T iNNSJO-MJOOSJ2IE AAR-79 MND-^. 100,

m

V 600L

It 40

204

0

III. V 000L

• 40

0

m

204

804

so,

404

20

0

m

604

40,

20

16-20 20-23 22-30 0 L10

LENGTH GRO

100

le 13-z0 20-xe 26-20 30-]S

. UP ------ ----- LENGTH GROUP

TYPE-T INNSJO-STRANDCF AAR-77 IAND- } INNSJO-STRANDEF AAR-77 MND-} F1001 .f^ IOO1 ^

IV.

0

204

10-20 20-30 36-40

LENGTH GROUP

IR

m

404

204

010-20 20-30 30-0

LENGTH GROUP

so

m

404

EN

13-20 20-23 2'3-3636-391

LENGTH GROUP

MND-4i INNSJO-VOLBUFJO AAR.77 MN

100

so

20,

0

FISH COMMUNITY:

so 4

40

to-xo :0-30 300=40

LENGTH GROUP

I. TROUT, MINNOW

II. TnOUT, MINNOW,WHITEFISH

III. TROUT, MINNOW ,PEnCti

IV. TROUT, MINNOW,PERCH,WHIITEFISH

Fig. 5. Percentageof terrestric food components of total gut

contentsin brown trout in early summer in lakes with

different fish Communities.Regulated lakes are

indicated with an asterix.

n16-20 20-23 23-30

LENGTH GROUP

so,

so4

40 ,

20^

T I -11 F1 F113-20 20-21 :3-30 30 3e 0 +o-ze :0-30 3e-40

LENGTH GROUP LENGTH GROUP

TYPE-T INNSJO-001CKFLdY AAR-70 tAND-^. INNSJO-R4SSJOEN AAR_,• ...... :; .. ....._ : _.

004

V 600L

R 40

AM-7E MND- } INNSJO-VISJOEN AAR-86 MND-7 INNSJO-flYANOEN AAR-77 MND-7

^ n 100 1 ^ 100 1 ^

FISH COMMUNITY

I

I.

II.

TERRESTRIC FOOD COMPONENTS

INNSJO-DOKKVATN AAR-78 MND-9 INNSJO-DJUyPEN AAR-03

1001 1001 ^

$0 4

V so ^

L

2t 40

20 4

0

Boa

004

40

2O

0 16..2e 20a3 •w-ee •w-•w 0 0

UND-9

LENGTH CROUP

INNSJO-UJOCSJøE AAR-70 MND-4

f00,

IR

V B0^

L

s 40 ^

204

13-20 20-29

LENGTH cRDUP

INN3J0-DOKKFØY AAR-70 MND-9

100,

0

804

III. oL

BDI

R 40

FE

0

LENGTH GROUP

r .

+V-uw -iLENGTN GROUP LENGTN GROUP LENGTH GROUPINNSJO-DOPPOLLL AAR-82 sMD-q INNSJO-OOPPOLLL AAR-07 MND-9 INNSJO-SYNNFJOR AAR-70 UND-9

100 ^ .^ . 1D01 ^ 100 ^

m

IR

40

PR

INNSJO-VRIJISEN AAR-96 UND-9 INNSJO -fYANOEN AAR-77 UND-10

100

1^ 100

1^

80 4

V B01

L

S 40,

m

m

40

20`,

D^ n ,.I I I I I13-20 20-29 23-30 20-23 23-30

LENGTH GROUP _ LENGTH GROUP

INNSJO-R43sJøEN AAR_

-7B YNO-9 ]S INN3J0-VOLBUFJO AAR-77 MND-11)tO0 1 100

m

IR

40

LIE

Bo4

40

20

- L=19-20 20-23 23-30 30-33 0 lo-te 30-43

m

013-20 20-23 30-3e -

LENGTH GROUP

iNNSJO-sTRANDEF AAR-77 MND-Ip F

1001 ^

Iv,

20 201

ø-te 2o-23 :eae ^e=ew

M

r-"20-30 30-40

LENGTH OR"

LENGTH GROUP

80 4

Bo 4

40

ER

IS]

40

m

0 n13-20 20-23

LENGTH GROUP

FISH COMMUNITY:

I. TROUT, MINNOW

iI. TROUT, MINNOW ,WHITEFISH

III. TROUT , MINNOW,PERCH

IV. TROUT , MINNOW ,PFRCH,WHITEFISH

Fig. 5. Percentage of terrestric food components of total gut

contents in brown trout during autumn in lakes with

different fish communities. Regulated lakes are

indicated with an asterix.

m

V 000L

r 40

m

trout

-planktivorousfish species

regulation

1

.

^ Trout® Whitefish

. El Perch-minnows

+ planktivorousfish species

regulation

1Um

Fig. 6. Main pattern of habitat available for brown trout

(Salmo trutta) in absence and presence of littoraland pelagic competitors. Regulation is indicated byan t .