JournalofFish Biology (1991) 38,905-919

Survival, growth and feeding of vendace, Coregonus albula (L.), larvae in net enclosures

J . KARJALAINEN Karelian Institute, Suimaa Research, University of Joensuu, P.O. Box I1 I ,

SF-80101 Joensuu, Finland

(Received I October 1990, Accepted 2 February 1991)

Vendace. Coregonus ulbula (L.), larvae reared in net enclosures anchored in the littoral zone of Lake Yla-Enonvesi, eastern Finland, were observed for a period of 44 days in order to determine the effects of density on mortality, growth and feeding of the larvae. The ratio of newly-hatched fish to their food was regulated by controllingthenumber offish stocked into theenclosures. Prey animals were expected to move freely in and out of the enclosures. The effect of nylon mesh (500 p n ) on the penetration of zooplankters into the enclosures was negligible. The influence of possible starvation was determined by analysing the growth o l t h e larvae, their Fulton condition factor, the instantaneous amount of food ingested, and the food composition in their guts. All thesc indicators were inversely correlated with larval density. Vendace larvae were found to be resistant to mortality from direct starvation. At all densities the survival rate was over 60%. At the time olcomplete yolk absorption, there was no increase in mortality.

Key words: Coregonus albulu; diet; growth; larvae; net enclosure; survival.

I. INTRODUCTION It is widely accepted that starvation and predation are the main causes of extensive mortality among fish larvae (e.g. Blaxter, 1969; Braum, 1978). The death of fish from starvation during the first weeks of life is thought to determine the strength of the year-class of several planktivorous fish. This critical period concept, which was first developed for pelagic marine fish, assumes that mortality is based on the availability of planktonic food after the larval yolk reserves have been exhausted (May, 1974). The density-dependent survival of larval vendace, Coregonus albula (L.), is considered by several authors to be the primary determinant of recruitment to commercial stocks of vendace (Auvinen, 1978; Salojarvi, 1987; Viljanen, 1988; Valtonen & Marjomaki, 1988) and other coregonids (Salojarvi, 1982; Braum, 1978; Taylor & Freeberg, 1984). These various assumptions are, however, not based on direct field evidence demonstrating that early larval mortality of vendace is related to recruitment failure.

In estimating larval vendace survival, the most difficult problems are quanti- tative sampling of populations and the use of adequate space-time sampling scales (Viljanen, 1987, 1988). Changes in behaviour and habitat requirements during early life stages, larval migration and the patchy distribution of the larvae demand time-intensive and space-extensive sampling efforts (Taggart & Leggett, 1987). Especially at the age of about 2 months, the changes in vendance ontogeny are reflected in the behaviour and distribution of the larvae, and sampling fish of that age is difficult with any type of sampling gear. Experimental studies are needed to adjust the sampling efforts to critical stages of vendace development, and also to help interpret field observations.

905 0022 - 1 I 1219 I /060905 + 15 $03.00/0 @ 1991 The Fisheries Society of the British Isles

906

7 -

J . KARJALAINEN

( a )

I I

5 30 ( b ) t-

25 -

20 -

15 -

1 10 - / ---’.’

I I I I L

FIG. I . Water temperature in (a) incubation jars containing vendace larvae and in (b) the enclosures located in Lake YIC-Enonvesi in 1988 (----) and 1989 (---).

The aim of these experiments in in situ enclosures was to test the critical period concept in natural food conditions where the availability of planktonic food is the primary factor affecting growth and mortality of larval vendace. The ratio of fish to their food was regulated by controlling the number of fish stocked into the enclosures. The influence of possible starvation was determined by analysing the growth of the larvae, their Fulton condition factor, the instantaneous amount of food ingested, and the composition of food in their guts.

11. MATERIALS AND METHODS In 1988, vendace eggs were obtained from Lake Puruvesi and in 1989 from Lake Enovesi,

both in eastern Finland. The artificially fertilized eggs were incubated in glass jars at the Central Fish Culture and Fisheries Research Station for Eastern Finland, Enonkoski. During the incubation period (November to early May) the water temperature was the same as that in Lake Yla-Enonvesi (Fig. 1) from which the water was pumped to the station.

In both years the vendace larvae hatched only 3 days before the ice on the lake melted (16 May 1988 and I May 1989). The larvae used in the experiments all hatched during the same

VENDACE SURVIVAL IN N E T ENCLOSURES 907

4200 mrn 3”

I/ 1

Wooden floating deck

FIG. 2. Specification of the experimental enclosures for monitoring growth and survival of vendace larvae.

2 days and were transported to the experimental area in humic Lake Yla-Enonvesi (62”05’6’N, 28’56’14’E) within 48 h of hatching. Before they were transported, the larvae were kept without food at the hatching temperature.

Six nylon net (500 pm mesh) enclosures in two wooden frames were anchored c. 20 m from the shoreline of Lake Yla-Enonvesi. The volume of each enclosure was 1.1 m3. The depth of the water in the area was about 2.5 m. To protect the enclosed fish from predation by birds, a white nylon net (1 5-mm mesh) was tightened 15 cm above the enclosures. All potential fish predators of vendace larvae [perch, PercaJluviatilis (L.), roach, Rutilus rutilus (L.), pike, Esox lucius L., smelt, Osmerus eperlanus (L.), minnow, Phoxinusphoxinus (L.)] were excluded from the enclosures.

In 1988, three enclosures (one frame) were stocked with 300 larvae (medium density), one with 100 larvae (low density) and one with 500 larvae (high density). The sixth enclosure (0-enclosure) contained no fish and was used as a control of the plankton density (food level) in the enclosures. In 1989 an experiment with the same design but with different numbers of larvae (0, 100,3 x 500, and 2000) was used. At the start of the experiment 30 larvae from the hatching group were measured individually for total length, dry mass and yolk volume. Prey animals were expected to move freely into and out of the enclosures.

The experiments started on 18 May 1988 and on 3 May 1989 and lasted for 44 days each year. During these experiments the enclosures were lifted every day and washed. The conical lower part, which contained the larvae, was always under water (Fig. 2). The nylon mesh was cleaned with a high pressure (90 bar) jet of water directed through the wall from the inside.

MORTALITY Water temperature, weather conditions and the number of dead larvae were recorded

each day for the first 25 days in 1988 and the first 30 days in 1989. The dead larvae sank to the bottom of the enclosure and were collected with a rubber valved bilge pump (Whale Gusher). The bottom and the collecting bottle was vacuumed with a 2.5 m long suction hose (diameter 7 cm) and the dead larvae were picked from the water. No replacements were made for mortality or sampling. After this period (the first 25 or 30 days) all enclosures were emptied and the larvae counted. These counts always differed less than 5% from those

908 J. KARJALAINEN

based on daily mortality, and the error was highest in the high density enclosures. The larvae were returned to the enclosure and counted again 2 weeks later when the experiment was over.

GROWTH A random sample of 12 larvae was taken every fifth day (eight times in 1988) or 16 larvae

every tenth day (four times in 1989) for analyses of growth and diet. The sample was taken at 11 .OO hours. After they had been anaesthetized with MS222 the larvae were preserved in 4% formaldehyde. Total length (T.L.) and dry mass (D.M., after 24 h at + 60" C) of the larva and length (I) and height ( A ) of the yolk sac were measured. The yolk volume ( V ) was calculated from the formula:

V = 0.5236.1.h2.

During the first stage of the experiment the instantaneous growth rate was calculated from the equation:

logc(y) = a + bl,

where y = body size (total length, mm or dry mass, mg), e = base of natural logarithms, t = time (days), b=instantaneous growth rate G and a the intercept of regression line. The Fulton condition factor ( ~ = D . M . mg.T.L.

DIET ANALYSIS For each sample the gut contents of six larvae were dissected out on glass slides. The food

items were counted, measured and identified. The length of each copepod was measured from the head to the end of the anal segment, and the total length of each cladoceran was measured. The average carbon mass of each size class and taxon, based on the data of M. Viljanen (unpublished) and Huusko et al. (1989), were used to reconstruct the food carbon mass (RCM). The significance of the differences in gut contents between the larval densities was tested both between means and in ranked manner with Kruskal-Wallis one-way analysis of variance.

FOOD RESOURCES Zooplankton was sampled with a tube sampler (500 mm long, 93 mm diameter) every

fourth day both inside and outside the enclosures and washed through a net with a mesh size of 50 pm. Inside all enclosures one composite sample of a &l m water column was taken. The composite samples outside the enclosures consisted of water columns of &l m in 1988 and Ck2 m in 1989. All samples were preserved in 4% formaldehyde. In the laboratory the 150 rnl composite sample was stirred and 5 ml subsamples were pipetted into plankton cuvettes. The total volume counted with an inverted microscope varied from 60 to 100 ml and usually 5C200 individuals ofeach dominant species were identified. For the dominant species and size classes the coefficient of variation (s.D./X) was 4 to 30%. The counts were converted to carbon mass (pgC) by using the average carbon values of the size classes in each taxon, as described above.

mm.100) was also calculated.

111. RESULTS MORTALITY

The mortality rates were less than 20% in both 1988 and 1989 at all larval densities (Fig. 3). In both years the highest mortality (1 1 %) after 10 days occurred in the low density enclosure. After the first weeks the mortality in the low density enclosures decreased to a very low (< 5 % ) level. At a density of 500 larvae the mortality was at the same level in both years. In 1989 the mortality in the high density enclosure increased slightly towards the end of the first rearing period. Yolk volume, growth and changes in the condition factor (Fig. 4, Table 11) indi- cated that in both years a gradual transition from yolk utilization to external feeding occurred after 10 days. This ontogenic change did not increase mortality.

VENDACE SURVIVAL I N NET ENCLOSURES

E

- .- ~~

No. of dead larvae 1988

6 23 6 4 13 15,310 05,610

( b )

15-

10

5 -

0

No. of dead larvae 1989

I I 15.3 58 2 22.3 76 2 21.0 165

I c

2:: .-. .... .. . ?. idd .. .

909

FIG. 3. Mortalityofvendacelarvae rearedat different densities(l00, 0; 300, Nl; 500, K3; 2000, 1.1 m ')in Lake Yla-Enonvesi in (a) 1988 and (b) 1989. The value for the medium density is given as the mean ( I I = ~ ) and vertical lines represent S.D. The mortality was calculated for the intervals between sampling.

In 1989, during the last 2 weeks of the experiment the mortality was 9.2% (s.D. = 2.8) at a density of 500 larvae and 29.6% at a density of 2000 larvae. At the lowest density there was no mortality. In 1988, owing to the sampling of the larvae during the second period, mortality was not estimated.

GROWTH AND CONDITION The total length of the newly-hatched larvae did not differ between 1988 and

1989 ( I = -0.995, d.f.=51, P>O.lO). On the other hand, dry mass (t=6.113, d.f.=51, P<O.OOl) and yolk volume (t=3-036, d.f.=51, P<O.Ol) differed sig- nificantly between the two years. Average yolk volume in 1988 was 0.143 mm3 (s.D. =0.05) and in 1989,0.109 mm3 (s.D. =0.03). In 1989, at the time of hatching the larvae had slightly more yolk reserves left, and in both years the amount of yolk was quite low (e.g. Viljanen & Koho, 1990). The difference in dry mass was due mainly to the larger yolk reserves in 1988.

In both years, the increase in total length and dry mass (Fig. 4) was most rapid in the low density enclosures and slowest at the high larval density. The growth rates (T.L. or D.M.) calculated as regression for the first 25 days or 30 days differed

910 1. KARJALAINEN

1988

12 12 12 12

28

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24

25

20 - v E

.3 15

I

c C

3 tn

0 E F 10 n

5

0

Z-KzCzq 6 6 6 6 25 20 I S

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32 -

28 -

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20 -

16 -

12 -

1989

16 16 21 IS 47 49 88 IW 16 16 28 59

____ 10 10 15

'-28 29 31 TO 10 10 22

10

5

I 0- '

10 20 30 40

FIG. 4. Total length (mm) and dry mass (mg) of vendace larvae reared at different densities (100, . . .; 300, _-_ ; 500, - - -; 2000, ~ 1.1 m-') in 1988 and 1989. Thecurves connect the means, and the vertical lines represent the 95% confidence limits. Number of larvae in the samples is shown in the upper part of each section.

between the densities (Table 1). When the mean value of T.L. or D.M. at the last sampling day for each enclosure were described as a function of larval density, the significant negative slopes was found (1988 T.L. b = - 2.194, r2 = 0.982; D.M. b = - 2-606, r2 = 0.961, n = 5 and 1989 T.L. b = - 2-244, r2 = 0.982; D.M. b = - 1 ~430, r 2 =0*911,n=5).

VENDACE SURVIVAL IN NET ENCLOSURES

TABLE I. Instantaneous growth rate (G per day) of vendace larvae reared in net enclosures at different densities (100, 300, 500, 2000 larvae 1.1 IX--~) in 1988 and 1989, standard error of G (s.E.), 2,

(d.f.), F-ratio and the associated probability level P

Enclosure G S.E. r2 d.f. F P<

(a) Total length (mm)

1988 100 0.037 300 0.032 500 0.029

1989 100 0.028 500 0.020

2000 0.0 14

(b) Drj, muss (mg)

1988 100 0. I 20 300 0.093 500 0.084

100 0.101 500 0.070

2000 0.043

1989

0.001 0.001 0.00 1

0.00 1 0.00 1 0.00 I

0-002 0-003 0.003

0-002 0-002 0.002

0.972 0.914 0.93 1

0.934 0.869 0-869

0.980 0.897 0.945

0.982 0.933 0.887

105 259 102

79 210 86

49 97 48

61 156 68

3667.8 2762.8 1381.4

1 198.9 1510.9 2729.4

2353.1 834.9 834.1

35 17.7 2 153.9

535.7

0.00 1 0.001 0.00 1

0.00 1 0.001 0.00 1

0.00 1 0.00 1 0.00 1

0.001 0.00 I 0.001

91 1

Values from the equation: log,(v) = a+ bt.

In 1988, the growth rate of dry mass was greater than in 1989 at densities of 100 larvae (t=5.407, d.f.= 110, P<O.OOl) and 500 larvae (t=3.048, d.f.=204, P<O.OOl). Since the average temperature of the surface water in the enclosures during the first 4 weeks was 13.7" C (s.D. = 3.4) in 1988 and 10.4" C (s.D. =2.6) in 1989, the influence of the water temperature on growth is thought to be the main reason for the discrepant growth rates. On the last day of the experiment, however, there were no significant differences between 1988 and 1989 in average length ( t = 1.909,d.f.=36.9, P>O.O5)oraveragedryrnass(t= 1.356,d.f.=22.8, P>O.lO)at a density of 500 larvae.

Fulton's condition factor (k) was used to describe the ratio of dry mass to total length (Table 11). The condition factor increased when the larvae grew, and this increase was most rapid at the lowest density. In 1989 the k values for the larvae kept at different densities were unequal after the first 10 days of the experiment (ANOVA, P< 0.05).

FEEDING AND FOOD The amount of food in the gut of the larvae was affected by the density of fish

(Fig. 5 ) . During the first 10-15 days the larvae had the same amount of food in their guts at all densities (Kruskal-Wallis, P>O.lO), but after that the amount of

912 J. KARJALAINEN

TABLE 11. Fulton's condition factor ( k = D . M . / T . L . ' * ~ ~ ~ ) for vendace larvae reared at differ- ent densities (100, 300, 500, 2000 larvae 1.1 m--3) in 1988 and in 1989, with standard deviation (s.D.) and number of larvae (n). Days are from the beginning of the experiment

Enclosure 100 Enclosure 500 Enclosure 300

k S.D. n k S.D. n k S.D. n Days

1988 0

25 30 37 44

0.074 0.008 ~ -

0.060 26 0.053 - 0.062 - 0.070 __ 0.072

0.01 1 0.005 0.007 0.0 16 0.01 1

25 25 20 15 20

0,057 0.009 74 0.076 0.005 20 0.087 0.009 20 0.089 0.012 20

1989 0

10 20 30 44

0.045 0.004 0.054 0.002 0.066 0.006 0.095 0.005

0.043 10 0.043 10 0.046 15 0.055 15 0.070

0.005 0.006 0.005 0.005 0.007

28 29 31 70 86

Enclosure 2000

0.042 0.004 10 0.044 0.003 10 0.044 0.003 22 0.056 0.008 53

food ingested was inversely related to the larval density. In 1989 at day 20 (Kruskal-Wallis T = 18-4, P < 0-001 and multiple comparison between densities with the method of Conover, 1980, MC P<O.O1) and at day 30 (T= 10.184, P<O.O1 and MC, Pc 0.05) the larvae in the low density enclosure had the largest amount of food (RCM) in their guts and the larvae in the high density enclosure had the smallest. Similarly, in 1988 at day 25 the largest amount of food was found at the low fish density and the smallest quantity at a density of 500 larvae ( T = 10.627, P<O-OI and MC, PcO-01) .

Examination of average gut contents both as the number of food items and as RCM was needed to obtain an overall view of the food composition (Fig. 5). Dominant prey species were: Rotatoria: Keratella cochlearis, Kellicottia longispina, Polyarthra remata and Asplanchna spp.; Cladocera: Bosmina coregoni s.l., Chydor- us sphaericus and Ceriodaphnia spp.; Copepoda < 600 pm: nauplii of cyclopoids, the calanoid Eudiaptomus gracilis, large nauplii (stages N5-N6) and young cope- podids of species Thermocyclops oithonoides and Mesocyclops leuckarti; Copepoda > 600 pm: copepodids of Thermocyclops, Mesocyclops, Cyclops kolensis and adults of cyclopoids.

Compared with the O-enclosure, the densities of zooplankters inside the fish enclosures decreased (Fig. 6) . Owing to the good penetration of zooplankters into the enclosures, larval predation did not depress much the number of zooplankton in the low density enclosures. In the high density enclosures, however, the number of cladocerans and copepods decreased. The density of rotifers was the same inside and outside all enclosures.

IV. DISCUSSION Various kinds of net enclosures have been used successfully in feeding exper-

iments with the larvae and young of several fish species (Laurence et al., 1979;

VENDACE SURVIVAL IN NET ENCLOSURES

H

V W

a 30

& 20

- f 25 rn

0 x, l 5 - 10 'c

O 5 5 a 0 5 10 15 20 25

f, 50

40

5 30 u)

+ ._ "0 20

10

0

L M-

1989 T

L I

0, 3 20 + = W

L 15 0

"0 10 2 z 5

a 0 I,

10 20 30 44

913

Day

FIG. 5 . Number of food items (upper sections) and the reconstructed carbon mass (RCM) of food items (lower sections) in the guts of vendace larvae reared at different densities (100,300 and 500 in 1988, 100.500and2000 1.1 m 'in 1989). Verticallinesrepresent standarderrorsofthemeans. Thenumber of larvae (n) was 18 for densities of 300 and 500 larvae and six for the other densities. 0, Copepoda >600pm; R, Copcpoda <600pm; D, Cladocera; B, Rotatoria. H, high denaty; M. medium density; L. low density.

Marciak, 1979; Mills, 1982; English, 1983; Cryer et a/., 1986; Lafontaine & Legget, 1987; Mamcarz & Nowak, 1987). However, growth and survival experiments have been carried out mostly in tanks with high densities of both fish and prey. Some limitations of tank experiments, such as limited searching volume, rapid changes in the number of prey and high prey densities, can be avoided by using net enclosures in growth and survival experiments (e.g. English, 1983). In net enclosures, the fish

914 J . KARJALAINEN

200 I 1988

L

FIG. 6. Densities [no. m ’ of copepods (above), and cladocerans (below)], inside the rearing enclosures of vendace larvae (100, - - -; 300, -. -. -: 500, - - -; 2000, . larvae) and outsideenclosures--..-..- in 1988 and 1989in LakeYla-Enonvesi. Lines fordensitiesof 300 and 500 are the mean of three samples, and the vertical lines represent S.D.

. . 1.1 m-’) and 0-enclosure. ~- (no

density can be adjusted to a natural level in field feeding conditions and so the growth of the enclosed fish is affected more naturally by energy expended in the feeding efforts.

In Finnish lakes, the abundance of vendace larvae during the first 3 weeks after hatching fluctuates greatly, both from lake to lake and within one lake. The maximum abundance generally occurs in the littoral zone of lakes immediately after hatching (Viljanen, 1988; Huusko et al., 1989). In the pelagic zone the den- sities are usually <0.2 larvae mp3 (Viljanen, 1987; Sarvala et al., 1988; Huusko et af., 1989), but in the littoral zone the highest densities may exceed 100 larvae mW3 (Viljanen, 1986, 1987, 1988) or even 1600 larvae mP3 (Huusko et al., 1989). The numbers of larvae stocked in the enclosures in this study (91,273,455,1818 larvae mP3) were high compared with the densities in Finnish lakes, but high densities of larvae create the intense competition necessary for possible starvation. On the other hand, the densities found in lakes, based on the sampling of the larvae with different tow nets (e.g. Viljanen, 1987), underestimate the actual density of the larvae in their schools.

MORTALITY Despite the high densities in the enclosures, no catastrophic mortality of the

larvae from starvation was observed during the 6 week rearing period. In both years the highest relative mortality occurred at the lowest larval density but in any case the mortality was low at all densities. Two possible explanations for the higher mortality at a density of 100 larvae are: (1) a small population of larvae is more sensitive to accidental death, e.g. caused by sampling of zooplankton, and (2) there could be some component of the zooplankton, perhaps large cyclopoids (Eckmann, 1985; Eckmann et af., 1986; Hartig & Jude, 1988), which is in some way

VENDACE SURVIVAL IN NET ENCLOSURES 915

injurious to vendace larvae and which are captured more frequently at a low fish density. The second suggestion is not supported by the analysis of gut contents.

GROWTH AND CONDITION While the vendace larvae were not directly susceptible to mortality from

starvation, their density in the enclosures clearly affected their growth (cf. Rice rt al., 1987; Fox, 1989). Fish growth was inversely related to larval density. The condition factor was also greater in the low density enclosures.

During the first 20 days of the experiment the amount of food in the guts of the larvae was constant and was about the same in all enclosures, but the growth rates were unequal at the different fish densities. The number of prey animals inside the enclosures was supposed to be adequate for the larvae and the higher energy costs required for the feeding would be the reason for the different growth rates. As the larvae grew their energy and food demands increased; this was reflected in the amount and composition of food in the guts. The ' scramble ' type of competition, which reduces the availability of food (Engen et al., 1988), began to have an effect and the densities of favoured prey animals inside the high density enclosures decreased. The different predation intensities in the low density and in the high density enclosures led to an increased difference in the growth rates.

Sarvala et al. (1988) reported the growth rates for vendace during 30 days after the break-up of ice in Lake Pyhajarvi (SW Finland) where the annual catch and growth of vendace are at very high levels in Finnish lakes (Sarvala & Jumppanen, 1988). The growth rates in the present experiments at the low fish density were comparable to those in Lake Pyhajarvi in 1986 and in 1987, but considerably lower than the rates in 1984 and in 1985.

FOOD AND FEEDING All larvae began exogenous feeding during the first 5 days of the experiment.

The nearly unchanged condition factor and slow growth of the larvae indicated that, before the yolk was completely utilized, external food was not very important for metabolism, although it was not insignificant (cf. Gunkel, 1979; Hofer & Burkle, 1986). Loewe & Eckmann (1988) stated that the alimentary tract of Coregonusfera Jurine was morphologically well formed in the first few days after hatching, but the mechanisms for absorption of fat and protein did not seem to be completely developed.

The succession of the food composition during larval growth was similar to that observed by Huusko et al. (1988), Sarvala et al. (1988) and Auvinen (1988) in some Finnish lakes. The first prey of the larvae were copepod nauplii, young copepodids and rotifers. The frequency of copepodids and especially the cladocerans in the guts began to increase at the beginning of June. The changes in larval diet reflect the life cycle succession of their prey. For the first weeks of life the physical dimensions of the mouth and the escape behaviours of different prey organisms regulate the upper limit ofprey size (Hartmann, 1986, Ponton & Muller, 1990). At 30 days of age, as 16-25 mm long larvae, vendace can capture small rotifers and even cladocerans over 1 mm.

The amount of food in the gut and variation in the diet are thought to indicate the feeding status of coregonid larvae (Auvinen, 1988; Huusko et al., 1988; Ponton & Muller, 1988; Sarvala et al., 1988). In the enclosures with high densities of fish,

916 J. KARJALAINEN

the amount of food per larvae remained constant, but at low densities the amount of ingested food increased rapidly. The larger larvae ate more and more and the difference in the amount of food between the fish densities increased. A large number of rotifers was observed in the diet at the high density. Optimal foraging models predict an increasingly varied diet with decreasing prey (e.g. Werner & Hall, 1974; Confer & Blades, 1975; O’Brien et al., 1976). The feeding behaviour of larval vendace support this general hypothesis.

Immediately after hatching vendace larvae accumulate in patches near the water surface, but true schools are not formed until some weeks later. In general, school- ing fish are better protected against predators than are solitary fish, which is perhaps the major selective advantage of the schooling. This defence behaviour may have some disadvantages, because schooling affects the feeding behaviour and success of individual larva. Owing to competition between individual members of the school, the amount of food ingested would theoretically be less for schooling planktivores (Eggers, 1976). At the age of 15 days the vendace larvae (12-15 mm) in the enclosures swam in a distinct school. Visual observation showed that the location of a larva in the school was affected by its size. The largest larvae were in the front of the school and obviously had the best feeding position. In the low and medium density enclosures the distance between larvae inside the school seemed to be constant, but at a density of 2000 individuals 1.1 mp3 were clearly dispersed. Eggers (1976) stated that during food deprivation, schooling fish may disperse to minimize the effects of schooling on the rate of feeding.

The differences in the density of zooplankters between fish enclosures showed that larval predation could decrease the number of zooplankters, but no collapse was observed. Food for the larvae in the enclosures was produced in a large volume of water outside the enclosure. The density of zooplankton did not indi- cate the level of food available for the fish. To estimate the minimum amount of food needed for survival, the total food production area, the available number of prey animals and the total quantity of the larvae in the area must be known.

The average biomass of crustaceans inside the empty enclosure during the 4- week period was 15-70 mgC m-3 in 1988 and 5-37 mgC m-3 in 1989. The larger biomass in 1988 was due to the increasing number of cladocerans (mainly Bosmina) at the beginning of June. In the littoral zone of Lake Lentua in northern Finland, the corresponding biomass was about 90 mgC mp3 and in the pelagic zone about 10mgCm-3 in 1986; in the littoral zone of Lake Ontojarvi it was about 25 mgC m- and in the pelagic zone about 14 mgC mP3 (Huusko et al. 1989). In Lake Pyhajarvi (SW Finland) the average crustacean biomass in the pelagic zone was 40-65 mgC m-3 in late May-early June 1984 (Sarvala et al., 1988).

Sarvala et al. (1988) suggested that in Lake Pyhajarvi (SW Finland) there was no shortage of food for vendace or whitefish during the larval period. Equally, the present experiments suggest that the shortage of prey might not be directly respon- sible for the high mortality of vendace larvae during the first 6 weeks after melting of the ice. Increased mortality in the high density enclosure in 1989 during the last days of the experiment may indicate the starvation of older larvae or juvenile fish (cf. Mills, 1982; Cushing, 1983). After these enclosure experiments, all larvae except those reared at the highest density completed larval morphogenesis.

In addition to the availability of food, predation is a factor that regulates the mortality of larval fish. Slow growth increases the time spent at vulnerable stages

VENDACE SURVIVAL I N N E T ENCLOSURES 917

and tends to lower the rate of survival (e.g. Lindstrom, 1962; Mills, 1982; Werner etal., 1983; Rice et al., 1987). Predation may affect larval populations directly and indirectly (Werner et al., 1983). Escaping and avoiding predators decreases the growth and the feeding rate of young fish, especially if predation is intensive. On the other hand, predation can improve the growth conditions by decreasing the density of the population, which in turn improves feeding conditions for the surviving fish.

Owing to comparisons between the densities of enclosed fish, the densities in natural populations and the average crustacean biomass, the food production after melting of the ice can support very dense larval vendace population. However, the complete density-dependent concept of survival must include the interactions between hatching time, feeding conditions, food availability, growth of the larvae and predation as well as possible feedback mechanisms such as improved feeding conditions after predation.

This study was funded by the Academy of Finland. I thank J. Koho, M. Rahkola, S. Lahdevirta and A. Lavola and the employees of the Karelian Institute for their assistance during the experiments and Drs I. Holopainen, J. Sarvala and M. Viljanen for critical evaluation of the manuscript. Dr Viljanen supported and directed the work at all phases of the study and Dr Joann von Weissenberg checked the English.

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