Aquaculture 144 ( 1996) 25 I-263

Weaning of metamorphosed winter flounder (Pleuronectes americanus) reared in the laboratory:

comparison of two commercial artificial diets on growth, survival and conversion efficiency.

Gillian W.Y. Lee a~1 , Matthew K. Litvak b7*

a Department of Biology, McGill University, I205 Avenue Docteur Penfield, Monrr&zl, Qu6. H3A IBI, CamUla

b Department c>f Biology, University of New Brunswick Saint John, Ganong Hall, P.O. Box 5050, Saint John, N.B. E2L 4L5, Canada

Accepted 23 February 1996

Abstract

In this study, the effects of two dry diets on growth, survival and conversion efficiency of juvenile winter flounder, Pfeuronectes americanus, were tested. Laboratory-reared juveniles were successfully weaned onto both artificial diets with approximately 70% survival by the end of the experiment (37 days). Conversion efficiency ranged between 0.52% and 17.50% between Days 7 and 37 of the experiment depending on the sampling period. Specific growth rates (length) over the experimental period were similar for juveniles fed the two starter diets (1.32% day-’ for the salmonid feed and 1.36% day _ ’ for the non-salmonid diet). Specific growth rates (wet weight) were also similar; 3.11% day-’ ( sa monid feed) and 2.65% day-’ (non-salmonid diet). There was I no significant difference between the non-salmonid diet compared to salmonid starter feed on growth and survival of juvenile winter flounder. This study demonstrated that juvenile winter flounder can be easily weaned, and that these results may improve with special diet formulation and feeding technique.

Keywords: Weaning; Growth; Fish; Survival; Artificial diet; Pleuronectes americunus; Juvenile

* Corresponding author. Tel.: (506) 648-5508; fax: (506) 648-5650; e-mail: LITVAK@UNBSJ.ca. ’ Present address: Huntsman Marine Science Centre, Brandy Cove Road, St Andrews, N.B. EOG 2X0,

Canada.

0044-8486/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. P/I SOO44-8486(96)0 1295- 1

252 G.W.Y. Lee, M.K. Litvak/Aquaculture 144 (1996) 251-263

1. Introduction

At present, there is considerable interest in using alternative finfish species for aquaculture (Waiwood et al., 1988; Tilseth, 1990). One potential candidate in Atlantic Canada is winter flounder (Brown, 1994; Litvak, 1994) because it possesses several important attributes. These include high economic value (Lee, 1995), existence of artificial spawning protocols (Smigielski and Arnold, 19721, and the ability to survive winter temperatures below - l.O”C (Pearcy, 1961; Duman and DeVries, 1974). These temperatures are lethal to other local aquaculture species such as Arctic char and

Atlantic salmon (King et al., 1989). A pilot attempt to rear winter flounder in New

Hampshire, USA was initiated over a decade ago with minimal success (Sawyer and Hoombeek, 1980). However, current rearing of winter flounder from egg through to metamorphosis has been successful (Litvak, 1994), and research can now address early weaning and diet development for laboratory-reared juveniles.

The most practical time to wean winter flounder is after metamorphosis when juveniles can be transferred into shallow, self-cleaning tanks. There are several different methods for weaning flatfish onto dry pellets (Bromley, 1977; MCtailler et al., 1981) and most studies advocate gradual adaptation to artificial feed. At present, there are a number of dry artificial diets on the market which differ in price and nutrient content.

Hoombeek et al. (1982) and Lee (1995) successfully weaned wild Year 0 and Year 1 winter flounder onto artificial diets. Mean sizes of the Year 0 fish were 63.9 mm (Hoombeek et al., 1982) and 39.3 mm (Lee, 1995). However, there have been no studies on weaning of smaller, recently-metamorphosed, laboratory-reared winter flounder. Thus the objectives of this study were two-fold. Firstly, an attempt was made to wean laboratory-reared winter flounder soon after metamorphosis (mean size 12.5 mm) onto one of two dry artificial diets. Secondly, these commercially-available diets were compared in terms of fish growth, survival and conversion efficiency.

2. Materials and methods

2.1. Hatchery rearing

Fertile eggs were collected from artificially-spawned adult winter flounder (Pleuronectes americanus) held at Huntsman Marine Science Centre, St Andrews, N.B., Canada. They hatched after 13 days of incubation at 7°C. Larvae were reared at a mean temperature (+ 1 S.E.) of 15.0 + O.l”C (range 14 to 17“(Z), under continuous light in 100 1 white polyethylene cylindro-conical tanks with sides covered with black plastic following Litvak’s (1994) protocol. Metamorphosis began on Day 26 post-hatch, and newly-metamorphosed flounder were transferred into weaning tanks on Day 33. Fish were fed brine shrimp, Artemiafianciscana, until the experiment began after a 2 week acclimation period.

2.2. Weaning tanks

Winter flounder juveniles were reared in ten square tanks (46 cm long, 20 cm deep; volume 21.1 1). The sides of these tanks were constructed of white corrugated plastic

G.W.Y. Lee, M.K. Lituak/Aquaculture 144 (1996) 251-263 253

Table 1 Six-day schedule employed to wean recently-metamorphosed winter flounder a

Day Amount dry diet (g tank- ’ ) Artemia density (number ml- ‘)

1100 1700

1 0.5 2.0 3.0 2 0.5 1.6 2.4

3 0.5 1.2 1.8 4 0.5 0.8 1.2

5 0.5 0.4 0.6

6 0.5 0 0

a Initial density was 79 fish tank- ’

(Coroplast, C a 1 d’ll ac Plastics, Halifax, N. S., Canada), and siliconed to the bottom of a rectangular fibreglass raceway tank (3.1 m long X 1.25 m wide). The bottom of this raceway was used as a common floor for all ten tanks. Seawater (salinity 28-30 p.p.t.), filtered to 5 km and UV-treated, was supplied to each tank at a rate of 200 ml min-‘. Temperature ranged between 14 and 15°C. Studies by Alhossaini and Pitcher (1988), and Karakiri and von Westernhagen (1989) reported slightly higher growth rates for juvenile plaice (Pleuronectes plutessa) reared under 24 h light. Litvak (unpublished data) obtained higher growth rates in laboratory-reared larval winter flounder grown under continuous light compared to 16 h light. Therefore, juvenile winter flounder were exposed to a 24 h photoperiod.

2.3. Feeding

There were 79 fish tank-’ at the start of the experiment. Juvenile winter flounder were fed to satiation twice daily until the end of weaning to ensure sufficient food remained in the tanks throughout the day. Juveniles in each tank were fed approximately 45000 (2 ml) newly-hatched Arlernia in the morning (1 I:00 h) and 65000 (3 ml) Artemia in the afternoon (17:OO h). Fish were fed more Artemia in the afternoon as there was a longer time interval until feeding the following morning.

A 6 day schedule was employed in weaning juveniles onto each of the two dry diets (Table 1). Fish in five randomly-selected tanks received a salmonid starter diet (Hi-Pro, Corey Feed Mills, Fredericton, N.B., Canada) while fish in the remaining five tanks were fed non-salmonid dry pellets (Nippai SFI-3, CATVIS, Hertogenbosch, The Nether- lands). The nutrient composition of the two diets used is shown in Table 2. Juveniles were fed once daily using the dry diets.

Manufacturers’ reported food particle size for both diets was 500 pm. Juveniles were fed in the morning, and tanks cleaned 24 h later prior to the next feeding. Both excess food and feces were collected from each tank during cleaning, and all feces removed from food samples using a pipette. The excess food samples were rinsed with freshwater and weighed after air-drying for 10 min on a 200 p,rn sieve. These samples were dried to a constant weight (24 h at 60°C) prior to re-weighing. All samples were weighed using a Mettler AE 240 analytical balance.

254 G.W.Y. Lee, M.K. Limuk/Aquaculture

2.4. Data collection and statistical analysis

144 (1996) 251-263

2.4.1. Survival Daily mortalities were removed and preserved in 5% buffered formalin. Repeated

measures analysis of variance (ANOVAR) was used to determine the effect of food type on the number of fish in all 10 tanks at the end of each sampling period (SAS proc GLM, Statistical Analysis Systems Institute Inc., 1988). Up to ten juveniles were removed from each tank on Day 29 of the experiment as these fish had fin-erosion. Removal was necessary to prevent spread of disease, and eliminate the confounding effect of stressed fish which did not feed. There was no replacement of either sick or dead fish.

2.4.2. Growth

Five juveniles from each tank were sampled on five occasions during the experiment: at the start of weaning (Day O), at the end of the weaning period (Day 7), and at three other times 10 days apart (Days 17, 27 and 37). Standard length was measured from videotaped images using image analysis software (BioScan Optimas, BioScan Inc., Edmonds, WA, USA). Individual juveniles were weighed once after blotting dry on a paper towel (wet weight), and a second time after being dried to a constant weight (24 h at 60°C; dry weight).

Data were tested for normality (SAS proc Univariate, Statistical Analysis Systems Institute Inc., 1988) and homogeneity of variance (F,,,,, test, Sokal and Rohlf, 1981). As variance was linked to the mean with increasing variation with size, data were transformed according to Taylor’s Power Law (Elliot, 19771, and retested.

A three-factor nested analysis of variance (ANOVA) was used to test the effects of time and food type on length and weight (SAS proc GLM, Statistical Analysis Systems Institute Inc., 1988). Time was not a repeated effect as measures were performed on different fish at each sampling occasion. Tank effect was nested within food type (Winer, 1971). Tukey’s a posteriori studentized range (HSD) test was used to detect for significant differences between levels of time. Power analysis on the t-test between final

Table 2 Nutrient composition of the two commercial starter feeds evaluated in this study

Salmonid starter diet Non-salmonid starter diet

Food particle size (pm) 500 500

Crude protein (%o) 54 58.6

Crude fat (%> 16 6.8 Crude tibre (%) 2.5 1.1

Crude ash (W) 9 14.3

Moisture (W) 10 5.3 Vitamin C 165 mg%

Vitamin E 300 IU kg- ’ 30 mg%

Calcium (%) 1.5 > 2.5

Phosphorous (%) I.1 > 1.5

Target species Atlantic salmon Sea bream, seabass, elvers, hoi, tetra, sturgeon

G.W.Y. Lee, M.K. Lituak/Aquuculture 144 (1996) 251-263 25.5

mean wet weights of fish reared on both diets was performed to determine the probability of committing a Type II error (Zar, 1984).

Instantaneous growth rate CC) was calculated using the formula

G = (lnYi+, - lnY;)/( ti+, - r;)

where Yi is length or weight at time ti (Ricker, 1979). Specific growth rate is instantaneous growth expressed as a percentage.

2.4.3. Food consumption and conversion Gross food conversion efficiency K calculated as

K= total weight gained

total feed fed x 100%

may be indicative of the effects of environment, ration level and diet within an experiment (Brett and Groves, 1979). Total weight gained or growth was calculated as absolute gain in dry weight and total feed fed was the dry weight of food consumed, over the experimental period. The amount of food consumed daily was the difference between weight of food fed, and weight of excess food collected 24 h later, after removal of fecal matter. Feed consumption data were collected daily and later summed to obtain total consumption over the entire experimental period. Food which underwent 24 h submersion and oven-drying lost weight due to leaching of nutrients and water loss. A correction factor for this weight loss was obtained by subjecting food samples to the same procedure in the absence of fish. This correction factor was the ratio between dry weight of food fed, and dry weight after submersion and oven-drying.

3. Results

3.1. Survival

Survival at the end of weaning (Day 7) was 92.2% for juveniles fed the non-salmonid diet, and 90.7% for those fed the salmonid diet. Daily mortality was highest during weaning, but remained low and constant after the end of weaning (Fig. 1). Instantaneous daily mortality M was calculated as

M = (In4 - lnNf) ttf- ‘i)

where Ni and Nf are initial and final number of fish, and (t,-ti> is the experimental time period in days. There was no significant effect of food type on the number of surviving fish at each sampling time. However, there was a significant effect of time (P = 0.0008, F = 34.69, d.f. = 5; ANOVAR). Survival at the end of the experiment was 68.2% (non-salmonid diet) and 71.1% (salmonid diet). There was a slight increase in daily mortality on Day 29 when juveniles with fin-erosion were culled (Fig. I>. Mean length

256 G.W.Y. Lee, M.K. Litvak/Aquuculture 144 (1996) 251-263

O-l I

0 10 20 30 40

Time (days)

Fig. 1. Daily mortality of recently-metamorphosed winter flounder over the experimental period. Weaning

occurred between Days 0 to 6. The arrow indicates increase in mortality on Day 29 due to removal of juveniles

with tin-erosion.

and weight of these fish were significantly lower than means of healthy juveniles (length: P < 0.0001, t = 8.27, d.f. = 104; weight: P < 0.0001, f = 8.78, d.f. = 104).

3.2. Growth and food conversion

Transformed data were normally-distributed and homoscedastic. There was no signif- icant effect of food type on length, wet and dry weights, although length and weight increased significantly over time (Table 3). The ANOVA results failed to reject a true null hypothesis at the 5% significance level. A power analysis was then employed to ensure that a false null hypothesis was not rejected (Type II error). There was an 80-90% chance a true null hypothesis was not rejected.

Mean length and weights, and specific growth rates for both groups of juveniles are summarized in Fig. 2, Fig. 3 and Table 4. Specific growth rate was highest during weaning (between Days 0 and 7) regardless of food type.

Lengths were significantly different between Days 0 and 7, between Days 7 and 17, and between Days 7 and 37 (P < 0.05, Tukey’s studentized range test). Mean length was lower on Day 27 by chance from random sampling (Fig. 2). However, this decrease was not significantly different from mean length on Day 17 (P < 0.05, Tukey’s studentized range test).

G.W.Y. Lee, M.K. Lituak/Aquaculture I44 (19961251-263 257

Table 3

Three-factor analysis of variance (ANOVA) to test effects of diet, tank and time on standard length, wet

weight and dry weight of recently-metamorphosed winter flounder

Variable Effect Denominator

(Mean square)

d.f. F prob > F

(numerator,

denominator)

Length Food

Tank (Food)

Time

Food X Time

Tank (Food) X Time

Wet weight Food

Tank (Food)

Time

Food X Time

Tank (Food) X Time

Dry weight Food Tank (Food)

Time

Food X Time

Tank (Food) X Time

Tankcfood)

Error MS

Tank (food) X time

Tank (food) X time

Error MS

Tank (food)

Error MS

Tank (food) X time

Tank (food) X time

Error MS

Tank (food)

Error MS

Tank (food)X time

Tank (food) X time

Error MS

138 0.01 n.s.

8, 200 1.75 n.s.

4, 32 68.33 < O.oool

4, 32 0.52 n.s.

32,200 1.20 n.s.

198 0.30 n.s.

8, 200 1.83 n.s.

4, 32 30.95 < O.GOol

4, 32 0.79 n.s.

32,200 1.35 ns.

I.8 0.48 n.s.

8, 200 1.76 n.s.

4, 32 28.13 < O.oool

4, 32 0.94 n.s.

32,200 1.26 n.s.

Tank effect is nested under food type (Tank (Food)). interaction between two effects is shown by a

multiplication sign.

- Nonsalmonid diet 4 - Salmonid diet

12J.........,........., cl 10 20 30 40

lime (days)

Fig. 2. Comparison of mean standard length of juvenile winter flounder reared on two different artificial diets. Vertical bars indicate f 1 S.E. and n = 2.5 for each diet.

258 G.W.Y. Lee, M.K. Litvak/Aquaculture 144 (1996) 251-263

150 -Nonsalmonid diet -- Salmonid diet

1

30-1 . . . . I . . . . I . . . . I . . . . ‘

0 10 20 30 40

Time (days)

Fig. 3. Comparison of mean wet weight of juvenile winter flounder reared on two different artificial diets.

Vertical bars indicate f 1 S.E. and n = 25 for each diet.

Mean weight at the end of weaning (Day 7) differed significantly from the initial weight (Fig. 3). Mean weight 10 days after weaning (Day 17) was not different from weight at the end of weaning, but mean weight by the end of the experiment (Day 37) was significantly different (P < 0.05, Tukey’s studentized range test).

Table 4

Food consumption, gross conversion efficiency a and growth of juvenile winter flounder reared on two

different diets

Diet Day O-7 Day 7-17 Day 17-27 Days 27-37 (weaning)

Salmonid diet Weight gain fish- ’ (mg) 7.3 0.1 5.2 Food eaten fish- ’ (mg) 15.9 22.7 29.9 Conversion efficiency a (%) 45.9 0.5 17.5 Growth (% length day- ’ ) 3.8 1.1 -0.2 Growth (% wet weight day- ’ ) 9.8 0.8 2.9

Non-salmonid diet Weight gain fish- ’ (mg) 5.5 - 0.3 0.7 Food eaten fish- ’ (mg) 9.9 19.1 23.2 Conversion efficiency ’ (o/o) 55.1 - 1.5 2.9 Growth (% length day- ’ ) 2.9 1.6 - 0.6 Growth (% wet weight day- ’ ) 6.3 1.1 0.5

2.0

37.3

5.5 1.3 1.0

7.6 62.7

12.1

2.0

3.7

a Conversion efficiency is weight gained divided by food consumed, expressed as a percentage (dry matter

basis).

G.W.Y. Lee, M.K. Lituak/Ayuaculture 144 (1996) 251-263 259

Food consumption and gross conversion efficiency for both groups of fish are summarized in Table 4. Consumption of dry feed increased throughout the experiment. Conversion efficiencies for both groups of fish were higher during the weaning period.

4. Discussion

4.1. Survival

High survival after the weaning period indicated that the gradual weaning protocol developed in this study was successful. Survival was not influenced by diet, and daily mortality after weaning remained low and consistent. It appears that laboratory-reared juveniles can be successfully weaned onto a dry artificial diet.

Instantaneous daily mortality at the end of the experiment, 84 days post-hatching was 0.006-0.007 day-‘. This is lower than mortality reported by Dinis (1992) who weaned Solea senegalensis for 100 days with instantaneous daily mortality between 0.008 and 0.012 day-‘. Results from the present experiment are encouraging as they are similar to other successful early weaning experiments with sole and turbot. Me’tailler et al. (1983) reported an instantaneous daily mortality of 0.005 day-’ for Solea uulgaris weaned from Day 35 for 40 days. Two studies on juvenile turbot reported very low mortality over approximately 50 days. Segner and Witt (1990) obtained instantaneous daily mortality of 0.0002 day-’ while Kuhlmann et al. (198 1) reported no mortality through- out weaning.

The increase in daily mortality on Day 29 of the experiment was due to removal of juveniles infected with fin-erosion. Smaller fish appeared to be more susceptible to this disease than their larger counterparts.

4.2. Growth

This study demonstrated that there was no difference due to feed type on fish growth at the completion of the experiment. Growth trends for both feed types paralleled one another with the most significant gain in length occurring between Days 7 and 17. However, greatest gain in weight occurred after Day 17. Juvenile winter flounder were observed to grow length-wise initially, but once a certain length was attained, to increase in width and thickness. Further study, however, is required to support the speculation that energy may be directed alternately between horizontal and lateral growth in winter flounder.

Juvenile winter flounder in this experiment were weaned three weeks after metamor- phosis, which began on Day 26. This delay was due to the time involved in removing juveniles from larval rearing tanks, and in allowing for a period of acclimatization to weaning tanks. Other studies which delayed weaning until approximately 3 weeks after metamorphosis include Kuhlmann et al. (198 I) and Dinis (1992). Kuhlmann et al. (198 1) weaned juvenile turbot 16-22 days post-metamorphosis. Growth rates during their 5 1 -day experiment were 1.33% day- ’ (length) and 3.47% day- ’ (weight). These growth rates are comparable to those obtained for juvenile winter flounder in this study,

Tab

le

5 Sp

ecif

ic

grow

th

rate

s a

in j

uven

ile

flat

fish

sp

ecie

s fr

om

the

liter

atur

e

Spec

ies

Sour

ce

Tim

e w

eani

ng

initi

ated

Sp

ecif

ic

grow

th

(len

gth)

Sp

ecif

ic

grow

th

(wet

w

eigh

t)

Sco

pkth

alm

us

max

imus

Pur

ophr

ys

vetu

lus

Sde

u se

nepl

ensi

s

Ple

uron

ecte

s ph

tess

cl

Liop

srttu

pu

tnam

i

Ple

uron

ecte

s am

eric

anus

Den

iel,

1976

Bro

mle

y,

1980

b

Kuh

lman

n et

al.,

19

8 1 b

Se

gner

an

d W

itt,

1990

b

Bro

mle

y,

1977

b

Met

aille

r et

al.,

19

81 ’

Cad

ena-

Roa

et

al.,

19

82 b

Met

aille

r et

al.,

19

83

Will

iam

s an

d C

aldw

ell,

1978

b

Din

is,

1992

(met

. D

ay

19)

Kir

k an

d H

owel

l, 19

72

Hoo

mbe

ek

et a

l.,

1982

Hoo

rnbe

ek

et a

l.,

1982

Lee

, 19

95 b

Thi

s st

udy

Yea

r 0

(78

mm

, 3.

5 g)

3 m

onth

s (2

-4

g)

Day

52

(m

et.

Day

30

-36)

Day

24

(at

met

.)

15 m

m,

53 m

g

Day

35

(I 2

0 m

g)

Day

60

(230

m

g>

Day

30

(95

mg)

Yea

r 0

(5 g

)

Day

40

6g

Yea

r 0

and

I

Yea

r 0

and

I

Yea

rO(3

9mm

. 1.

13g)

Day

47

(12

.5

mm

, 42

mg)

0.23

0.97

1.33

0.79

3.25

3.47

8.06

I .04

4.90

0.90

1.95

-3.7

6

0.72

1.

97

0. I

9 ’

0.08

d

0.63

‘0

.10

d

0.24

’ 0

.09

d 0.

71

co.2

l d

0.12

0.

53

1.32

(salm

onid

) 3.

11

(sal

mon

id)

I .36

(no

n-sa

lmon

id)

2.65

(n

on-s

alm

onid

)

0.95

0.54

a A

ll fi

sh

wer

e re

ared

on

ar

tific

ial

feed

s;

b Ju

veni

les

wer

e re

ared

fr

om

hatc

hing

in

th

e la

bora

tory

; ’

Fish

re

ared

in

he

ated

w

ater

; d

Fish

re

ared

in

am

bien

t w

ater

te

mpe

ratu

res.

Met

. is

the

age

at

whi

ch

met

amor

phos

is

occu

rred

.

G.W.Y. Lee, M.K. Lituak/Ayuacubure 144 (1996) 251-263 261

calculated over a 37-day period (1.36% day-‘, length; 3.11% day- ‘, weight). Dinis (1992) began to feed Solea senegalensis frozen Artemia in addition to live nauplii 12 days after metamorphosis, for approximately 1 week. These juvenile sole did not receive rehydratable pellets until Day 40 post-hatch, 3 weeks after metamorphosis. Juvenile sole specific growth rates over 100 days were much lower than those obtained for turbot and winter flounder (0.72% day- ‘, length; 1.97% day- ‘, weight). Values obtained from the literature indicate that laboratory-reared fish appear to have higher specific growth rates than wild fish (Table 5).

In this study, growth rates were much higher during the weaning period compared to post-weaning growth. It appears that juveniles fed preferentially on Artemia during

weaning because dry feed consumption was lowest during weaning, and mean weight did not increase significantly 10 days after weaning. In contrast, Segner and Witt (1990) weaned juvenile turbot immediately from the start of metamorphosis, 24 days post-hatch. They reported extremely high and similar pre and post-weaning growth rates (8.17% day-’ during the weaning process when fish received both Artemia and dry feed, and 8.06% day-’ when the diet was comprised entirely of artificial feed). This suggests that growth in winter flounder may be improved by earlier weaning on specially formulated starter diets.

Although growth rates may be higher with an Artemia-supplemented diet, the cost of rearing live feed must be an important consideration. The cost of rearing enough Artemia to feed 800 juveniles over 100 days is approximately US$ 26. This estimate does not include the expense of heat, light and labour. In comparison, the cost of the salmonid starter diet over 100 days of rearing is much lower at approximately US$ 3. Future research should be directed toward timing and duration of weaning, and live food supplementation.

4.3. Food consumption and conversion

Average dry feed consumption was approximately 5% of fish body weight per day. Dry feed consumption increased steadily throughout the experiment. This suggests that some juvenile winter flounder began to feed readily on the artificial diets. The highest conversions were obtained during weaning (Days 1 to 6) when Artemia was present in the diet. Low conversion values obtained for both diets 10 days after weaning suggest two things. Firstly, high conversion values obtained during weaning may be due to preferential feeding on the Artemia, and secondly, not all juveniles had completely adapted to the artificial feed by Day 17 of the experiment.

5. Conclusion

In this study, laboratory-reared juvenile winter flounder were successfully weaned onto both artificial diets tested. Juveniles fed preferentially on Artemia during weaning, but adapted quickly to artificial feed once Artemia was discontinued. There was no difference between diets with respect to fish growth, food conversion and survival. This study demonstrates that there is excellent potential for culture of juvenile winter

262 G.W.Y. Lee. M.K. Litunk/Aquaculture 144 (1996) 251-263

flounder. They are easily weaned and grow well on commercially-available diets. As the artificial feeds used in this experiment were originally formulated for other fish species, there is a need for further improvement in feed formulation and feeding technique which will promote higher growth and survival.

Acknowledgements

We would like to thank everybody who assisted in the larval culture (S. Barbeau, G. Downing, S. Frederiksen, S. Warrington) and who critically reviewed earlier versions this manuscript (G. Downing, D. Hamilton, R. Rangeley). This study was supported by a N.B. Co-operative Agreement grant (no. 361.107) and NSERC Research and Equipment grants to M. Litvak, and was conducted at the Huntsman Marine Science Centre, St Andrews, N.B., Canada.

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