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Effect of dietary fat level on weight gain, digestibility, and fillet composition of Atlantic halibut
Aquacuiflure, 99 ( 199 I ) 33 1-338
331
Elsevier Science Publishers B.V., Amsterdam
Effect of dietary fat level on weight gain, digestibility, and fillet composition of Atlantic halibut Gerd Marit Berge and Trond Storebakken AK VAFORSK (institute of Aquaculture Research), N-6600 Sunndalmra, Norway
(Accepted 2 I March 199 1)
ABSTRACT Berge, GM. arld Storebakken, T., 1991. Effect of dietary fat level on weight gain, digestibility and fillet composition of Atlantic halibut. Aquaculture, 99: 33 l-338.
Two experiments were carried out to obtain information on the effect of dietary fat levels on juvenile (6-12 g) and larger (0.6-1.5 kg) Atlantic halibut (Hippoglassru hippoglossus). The juvenile halibut were fed dry diets containing 12% fat and 54Ohprotein and 21% fat and 49% protein, respectively. The larger fish were fed moist diets formulated to be isonitrogenous ( 19-2 I% protein on a wetweight basis), to contain equal amounts of carbohydrate (9-IO%) and increasinp amounts of fat (8, I 2. I6 or 20%. respectively ). Dietary fat level did not result in significant differences in weight gain for any of the two size classes of halibut. No significant effects of the various diets were seen for feed efftciency, ch::mical composition of fillets and livers or digestibility of dietary dry matter, fat and protein for the larger tish. Large individual variation was observed in all recorded data.
INTRODUCTION
Information concerning the nutrition of halibut (Hypoglossushippoglossus) after metamorphosis is limited. However, some studies on diges!ion and activity of digestive enzymes have been published (Glass et al., 1987; Gronseth and Myhre, 1988; Stromsnes, 1989). Due to lack of information on the nutrient composition of the diet for Atlantic halibut in the wild, we formulated the diets of the present study on the basis of nutritional information obtained from other flatfish species. Increasing dietary fat content had a negative effect on weight gain of juvenile turbot (Scophthalmusmaximus) (Bromley, 1980; Caceres-Martinez et al., 1984), whereas Cowey et al. ( 1975) did not find any effect on growth of plaice (Pieuronectespfatessa).Bromley ( 1980) and Cowey et al. ( 1975,19?6) found that the addition of dietary lipid resulted in an increased lipid content of the carcass in both turbot and plaice. 1~ contrast, Cac~rssXartinez et al. ( 1984) OO44-8486/9 t/$03.50
0 I99 1 Elsevier Science Publishers B.V. All rights reserved.
GM BERGE AND T. STOREBAKKEN
332
reported that carcass protein and lipid content in turbot were not influenced by diet composition. The aim of the present experiments was to gain information on the effects of different fat content (dietary fat to protein ratios) in diets for juvenile and ongrowing halibut. The study included effects on weight gain, digestibility and chemical composition of the fillet and liver. MATERIALS AND METHOD!;
Experiment I
Of the halibut larvae hatched in April-May 1987 at AKVAFORSK at Sunndalsnra, 38 individuals survived until December 1987. The larvae were fed live feed until metamorphosis was completed. The food organisms were rotifers, artemia and collected plankton (mainly copepodes) (Holmefjord et al., 1988). From metamorphosis to the start of the experiment the juveniles were fed a commercial agglomerated starter diet for salmon (Tess Merle; T. Skretting A/S, Stavanger, Norway). The fish were divided into two size groups; 16 with a mean weight of 6.1 g ( 1-9 g), and 22 with a mean weight of 11.6 g (6-22 g). Each diet was fed in duplicate to fish from both size groups, four tanks in total, in 0.7-m’ fibreglass tanks with a saltwater supply. The water depth was 30 cm, and the tempcraTABLE I Composition (g/ 100 g) of the diets for Experiment 1 Diet I
--
Diet 2
Formulation” Fish mealb Capelin oil’ Carbohydrates Vitaminsd. minerals and binder
68.8 6.3 16.0 9.0
61.7 15.9 14.3 8.1
Analysis Dry matter Protein Fat Ash
88.6 54. I 12.3 9.4
89.5 48.6 20.5 8.6
“Composition of basal diet according to producer’s specification. bNorSeaMink LT (Norsildmel, Bergen, Norway). ‘NorSalmOil (Norwegian Herring Meal Industries). dGuaranteed minimum vitamin content per kg feed (producer’s specification): vitamin A, 5000 RJ; vitamin Ds. 4000 IU; vitamm Kj, I5 mg; vitamin E, 100 mg; vitamin B,, 20 mg; vitamin Bz, 25 mg; vitamin Bb. 20 mg; pantothenic acid, 40 mg; niacin, 150 mg: folic acid, 2 mg; vitamin Br2, 0.02 mg; biotin. 0.5 mg: vitamin C, 600 mg; choline, 1000 mg; inositol, 300 mg.
DIETARY FAT LEVEL FOR ATLANT!? H4LIBUT
333
fur,: varied between 7 and 9°C during the experiment. The tanks were covered with plastic lids, with a circular 30-cm-diameter opening in the cerrtre. Diets were prepared from a commercial pelleted salmon diet (Tess Elite Pluss; T. Skretting A/S, Stavanger, Norway), drawn from the production line before fat was added. Fat was added by spraying, to obtain two diets with 12% and 2 1% fat (Table 1). Feed, in excess, was provided every 10 min, 24 h/ day, by automatic feeders. Individual weights of the fish were recorded at the start, at day 30, day 56 and day 85 (termination) of the experiment. Differences in growth between the two diets were tested by Student’s t-test. Experiment 2 Halibut, 0.6- 1.5 kg, were caught at the More-coast off Western Norway, during the summer of 1987. The halibut were initially held in 3-m’ circular tanks and fed sliced herring for 4-5 weeks until they ate properly. Then they were fed a moist pellet made from 50% herring, 25% coalfish-filleting offals and 25% binder meal (Tess Salmomix; T. Skretting A/S, Stavanger, NorTABLE 2 Composition (g/ 100 g) of the diets and apparent digestibility (O/b)of dry matter, fat and protein in halibut fed diets with different fat levels in Experiment 2 Diet 3 Formulation Mackerel Argentine Squid Capelin oil Binder meala
Diet 4
Diet 5
Diet 6
10
10
IO
35 30 25
30 30 5 25
25 30 10 25
10 20 30 15 25
Analysis Dry matter Protein Fat “Carbohydrates”b Ash
42.3 21.1 7.5 9.4 4.3
46.5 20.3 11.9 10.2 4.1
49.6 19.8 15.5 10.4 3.9
52.6 19.3 20.0 9.1 4.2
Apparent digestibility Dry matter Fat Protein
76.0 84.9 84.8
77.5 91.0 83.8
84.6 94.1 85.9
84.7 86.7 84.1
‘Tess Salmomix 25Oh,T. Skretting A/S, Stavanger, Norway: 50% fishmeal ( NorSeaMink, Nordsildmel, Bergen, Norway), 40% carbohydrate feedstuffs and 101 vitamins (as specified in Tabie i ), minerals and binder. ““Carbohydrates”= dry matter- (protein+fat+ash).
334
G.M BERGE AND T. STOREBAKKEN
way). Four weeks prior to the experiment the fish were individually tagged, using plastic tags (Floy tag and Manufacturing, Seattle, WA, USA) The fish were distributed into eight 2-m2 fibreglass tanks supplied with 1% I 5 1 saltwater (33-34 o/00)per min, 2 weeks before the experiment started, nine fish in each tank. The water depth and temperature were the same as described for Experiment 1. The tanks were located in an indoor hall and were partly covered ( l/4 of the surface area) by a wooden frame to give extra shade, and to prevent the fish from jumping out. Four moist diets were formulated to be isonitrogenous ( 19-20°h crude protein ), contain equal amounts of carbohydrate (9- 10%)) and 8, 12,16 or 20% fat (% wet weight ), and 1% (dry weight) Cr203 was added as indicator for determination of apparent digestibility (Table 2). The fish were hand-fed to satiation every second day, and feed consumption was recorded for each tank. Individual weights of the fish were recorded at the start of the experiment, and after 5, 10 and 14 weeks (termination). Weight gain was calculated for each period separately and for the entire experiment. At the end of the experiment lo-20 fish from each groups were anaesthetized and faeces were collected according to Gronseth and Myhre ( 1988). The samples were stored at - 20 OC prior to analysis. Three halibut from each tank were slaughtered for body composition analyses. Samples of the livers were immediately frozen in liquid nitrogen and stored at - 80 OC. A 2-cm-thick slice was obtained from behind the peritoneal cavity of the fish. Skin, bone and adipose tissue were removed prior to mincing the fillet samples. The feeds, fillet, liver and faeces samples were analysed for dry matter (lOS°C, 16-18 h), ash (flame-combustion followed by 3-4 h at 55O”C), fat (Folch et al., 1957; for liver: modified using chloroform : methanol : water = 8 : 3 : 4, for feeds: fat was extracted after HCl-hydrolysis (Stoldt, 1952 ) ) and protein (semi-micro-Kjeldahl, Kjeltec-Auto system ). Liver glycogen was determined as the difference between free glucose in the sample and total glucose after hydrolysis of the sample, multiplied by a factor of 0.9. Glucose was determined enzymatically (Glucose UV Method, Boehringer-Mannheim, Germany). Cr203 was analysed by atomic absorption modified according to Williams et al. ( 1962 ) . The results obtained were subjected to a two-way analysis of variance, including the effects of diets and replica. Calculations were based an mean values from each tank. RESULTS
Experiment I The mean weights of the fish in the four tanks during the experiment are shown in Fig, 1. The weight gain of the fish was 2.2% per day in the first
DIETARY FAT LEVEL FOR ATLANTIC HALIBUT
335
Fish weight (g) 50 45 40 35
20% fat in the diet ---- . !2!5 fat ik?t!!n diet
30 25 20 15 10 5
0'
0
I
I
I
I
20
40
60
60
Days
Fig. 1. Growth of juvenile halibut fed diets with two different fat levels.
period, 1.3% in the second period, and 1.2% per day in the last period of the experiment. Three fish died during the experiment. This was cawxi by an accident during the last period, in which most of the water in one of the tanks drained out, and gave a reduced growth in this tank in the last period (Fig. 1). The t-test did not reveal significant difference in weight gain of fish fed the diets during any of the test periods.
Experiment 2 No differences in digestibility of dry matter, fat and protein were found (Table 2 ) . Negative values for digestibility of ash were found, ranging from - 15% to - 20%. Weight gain varied among individuals from 0 to 0.6% per day; however, no significant difference in weight gain of the fish fed the different diets during any of the periods or during the total experiment were found. Mean growth rate was 0.27% per day. Best feed efficiency was recorded for the halibut fed the diet with 20% fat (0.49 kg gain per kg feed ) and the lowest for the diet with 8Oh fat (0.40), but the difference was not significant. The mean values of carcass and liver to whole-body weight percentages were 93.5% and 2.1%, respectivell. Chemical analyses of livers gave mean vaiues of 44.6% dry matter, 9.0% protein, 32.9% fat, 4.7% glycogen and 0.9% ash, and the values for fillets were 22.8% ‘dry matter, 14.8% protein, 2.4% fat and 1.4% ash. Diet 3 provided fillets with a significantly higher ash content. This was the only parameter giving a significant difference among dietaq groups. DISCUSSION
Weight gain per day (%) for the juvenile halibut was about the same as expected for Atlantic salmon of the same size and at the same temperature,
336
G.M BERGEAND T. STOREBAKKEN
but slightly poorer than for rainbow trout (Austreng et al., 1987 )* This SUPports the view that halibut has potential as a species for aquaculture in cold waters. The weight gain of the larger halibut caught in the wild was poorer than that of salmon and rainbow trout (Austreng et al., 198’7 )- Furthermore, the results revealed a large variation in weight gain within groups, indicating that there m,ay be a difference in the ability to adapt to the conditions offered in captivity. This appears to be a problem when using halibut caught in the wild in experiments. Efforts to reduce variation due to different stages of adaptation to captivity are neccessary. One such effort will be to modify the experimental facilities in order to make the conditions mofe optimal for the fish. Individuals should also be observed before the start of a new experiment and stunted fish removed. It is also desirable to increase the number of fish and replicas in future experiments. In the present experiments we did not observe any effect of increasing the fat content in the diet from 8% to 20% on the weight gain of Atlantic halibut. This is in keeping with results obtained by Cowey et al. ( 1975 ) who did not find any effect on weight gain of dietary fat levels from 5.6 to 17.6% in an experiment with plaice. The findings are in contrast to results obtained with juvenile turbot (Bromley, 1980; Caceres-Martinez et al., 1984). The feed intake and, thus, the feed efficiency of Experiment 2, was of the same magnitude in all dietary groups, regardless of dietary composition. This is in agree? ment with the findings of Caceres-Martinez et al. (1984) for turbot. When the dietary fat to protein ratio is increased from a low to a moderate level in Atlantic salmon and rainbow trout, the result is increased weight gain, survival, and feed conversion eff?ciency, and a slight increase in body fat content ( Austreng, 1976a, b, Austreng, 1979). With a further increase in dietary fat content, growth rate does not neccesarily increase, but the fish get fatter ( Austreng, 1986). The amount of fat in the halibut fillet samples ranged from 6.5 to 12.9% (dry weight), and was not different for the various dietary treatments. These values overlap the levels of fat in white musc1.e in halibut reported by Andreasen et al. ( 1989 ), and were slightly lower than the values reported by Haug et al. ( 1988). The halibut has fat depots close to the bones and fins. They were trimmed away before analysis, thus the effects on these depots were not included in our study. In rainbow trout the fat content of the dorsal depots responds rapidly to different dietary treatments (tiessling et al., 1989). The fat content of the livers was high in all the halibut sampled (2 l-5 1% wet weight, 579% dry weight), in keeping with results obtained by Andreasen et al. ( 1989 ) and Haug et al. ( 1988 ) . The fish for slaughter were randumly sampled. Since we only sampled three fish from each tank, casual sampling of slow-growing fish might have had an effect on the results of the chemical analyses. Mean weight gain of sampled
DIETARY FAT LEVEL FOR ATLANTIC HALIBUT
337
fish proved to be lower than mean weight gain for the entire experiment, but there was no systematic difference among the dietary groups., The apparent digestibility of fat was within a range of 85-95%, and that of protein within 84-86%, in agreement with previous findings (Gronssth and Myhre, 1988; Stromsnes, 1989). The absolute values for digestibility and the lacking effects of dietary fat level were also consistent with digestibilities reported for rainbow trout (Austreng, 1979). In contrast, Lie et al. ( 1988 ) found reduced digestibililty of both fat and protein in cod (Gadus morhua) with increasing fat contr:nts.The negative digestibility of ash can be explained by the drinking of salt water by marine fish. There is absorption of Na+, Cl- and water in the intestne, and an increase in the concentration of divalent ions (Kirsch et al., 1985). ACKNOWLEDGEMENTS
We are grateful to A. Linseth for skillful technical assistance, to E. Austreng and A. Rrogdahl for useful discussions and to T. Skretting A/S for providing the basal diet for Experiment 1. The experiments were financed by the Norwegian Fisheries Research Council. G.M. Berge was supported by a grant from the Norwegian Agricultural Research Council.
REFERENCES Andreasen, T.V., Haug, T. and Rings, E., 1989. Food, condition, and the lipid and protein content of young Atlantic haiibut (Hippoglossushippogfossus,L. ) captured in the autumn in north Norway. Int. Count. Explor. Sea, Maricult. Comm., C.M. 1989/F:3, 17 pp. Austreng, E., 1976a. Fat and protein in diets for salmonid fishes. I. Fat content in dry diets for salmon parr (Sufmosular, L.). Meld. Nor. Landbrukshogsk., 55(S), 16 pp. Austreng, E., 1976b. Fat and protein in diets for salmonid fishes. II. Fat content in dry diets for rainbow trout (Sufmoguirdneri, Richardson). Meld. Nor. Landbrukshogsk., 55(6), 14 pp. Austreng, E., i 979. Fat and protein in diets for salmonid fishes. VI. Digestibility and feed utilization by rainbow trout (Sufmoguirdneri, Richardson) fed diets containing different levels of fat. Meld. Nor. Landbrukshragsk., 58(6), 12 pp. Austreng, E., 1986. Fett i f&et til slakteflsk. Norsk Fiskeoppdrett, 11(6): 30-3 1. Austreng, E., Storebakken, T. and Asgird, T., 1987. Growth rate estimates for cultured Atlantic salmon and rainbow trout. Aquaculture, 60: 157- 160. Bromley, P.J., 1980. Effect of dietary protein, lipid and energy content on the growth of turbot (Scophthalmus maximus L. ). Aquaculture, 19: 359-369. Caceres-Martinez, C., Cadena-Roa, M. and Metailier, R., 1984. Nutritional requirements of turbot (Scophthalmus maximus). I. A preliminary study of protein and lipid utilization. J. World Maricult. Sot., 15: 19 l-202. Cowey, C.B., Adron, J.W., Brown, D.A. and Shanks, A.M., 1975. Studies on the nutrition of marine flatfish. The metabolism of glucose by plaice (Pleuronectespfutessu) and the effect of dietary energy source on protein utiliza?ion in plaice. Br. J. Nutr., 33: 2 19-23 1. Cowey, C.B., Owen, J.M., Adron, J.W. and Middleton, C., 1976. Studies on the nutrition of
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G.M BERGE AND T. STOREBAKKEN
marine flat&h. The effect of different dietary fatty acids on the growth and fatty acid composition of turbot (Scophthalmus maxims). Br. J. Nutr., 36: 479. Folch, J., Lees, M. and Sloane Stanley, S.J., 1957. A simple method for the isolation and purification of total lipids from animal tissues. J. Biol. Chem., 226: 497-509. Glass, H.J., MacDonald, N.L. and Stark, J.R., 1987. Metabolism in marine flat&h. IV. Carbohydrate and protein digestion in Atlantic halibut (Hippoglossus hippog/ossusL. ). Camp. Biochem. Physiol., 86B( 2): 281-289. Gronseth, F.A. and Myhre, P., 1988. Innledende fbringsforsrak pi flatfisk. Cand. Agric. Thesis, Norw. Univ. Agric., AS-NLH, 82 pp. Haug, T., Ring@, E. and Pettersen, G.W., 1988. Total lipid and fatty acid composition of polar and neutral lipids in different tissues of Atlantic halibut, Hippogfossushippoglossus (L.). Sarsia, 73: 163- 168. Holmefiord, I., Bolla, S. and Reitan, MI., 1988. Startfeeding of Atlantic halibut (Hippogfossus hippogiows) on enriched rotifers and artemia compared to collected plankton. Int. Count. Explor. Sea, 1988 ELHS. Paper no. 98. Abstract, 1 p. Kiessling, A., Johansson, L. and Storebakken, T., 1989. Effects of reduced feed ration levels on fat content and fatty acid composition in white and red muscle from rainbow trout. Aquaculture, 79: 169-l 75. Kirsch, R., Humbert, W. and Simonneaux, V., 1985. The gut as an osmoregulatory organ: Comparative aspects and special references to fishes. In: R. Gilles and M. Gilles-Baillien (Editors). Transport Processes, Iono- and Qsmoregulation: Current Comparative Approaches. 1st Int. Congress of Comparative Physiology and Biochemistry I. Liege, Belgium. Springer, New York, NY, pp. 265-278. Lie, 0., Lied, E. and Lambertsen, G., 1988. Feed optimization in Atlantic cod (Gadus morhua): fat versus protein content in the feed. Aquaculture, 69: 333-341. Stoldt, W., 1952. Vorschlag zur Vereinheitlichung der Fettbestimmung in Lebensmitteln. Fette, Seifen, Anstichm., 54: 206-207. Stremsnes, 0.. 1989. Fordoyelighetsforsnk med kveite. Cand. Agric. Thesis, Norw. Univ. Agric., AS-W-~, 89 pp. Williams, C.H., Davis, D.J., and Iismaa, O., 1962. The determination of chromic oxide in faeces samples by atomic absorption spectrophotometry. J. Agric. Sci., 59: 381-386.
331
Elsevier Science Publishers B.V., Amsterdam
Effect of dietary fat level on weight gain, digestibility, and fillet composition of Atlantic halibut Gerd Marit Berge and Trond Storebakken AK VAFORSK (institute of Aquaculture Research), N-6600 Sunndalmra, Norway
(Accepted 2 I March 199 1)
ABSTRACT Berge, GM. arld Storebakken, T., 1991. Effect of dietary fat level on weight gain, digestibility and fillet composition of Atlantic halibut. Aquaculture, 99: 33 l-338.
Two experiments were carried out to obtain information on the effect of dietary fat levels on juvenile (6-12 g) and larger (0.6-1.5 kg) Atlantic halibut (Hippoglassru hippoglossus). The juvenile halibut were fed dry diets containing 12% fat and 54Ohprotein and 21% fat and 49% protein, respectively. The larger fish were fed moist diets formulated to be isonitrogenous ( 19-2 I% protein on a wetweight basis), to contain equal amounts of carbohydrate (9-IO%) and increasinp amounts of fat (8, I 2. I6 or 20%. respectively ). Dietary fat level did not result in significant differences in weight gain for any of the two size classes of halibut. No significant effects of the various diets were seen for feed efftciency, ch::mical composition of fillets and livers or digestibility of dietary dry matter, fat and protein for the larger tish. Large individual variation was observed in all recorded data.
INTRODUCTION
Information concerning the nutrition of halibut (Hypoglossushippoglossus) after metamorphosis is limited. However, some studies on diges!ion and activity of digestive enzymes have been published (Glass et al., 1987; Gronseth and Myhre, 1988; Stromsnes, 1989). Due to lack of information on the nutrient composition of the diet for Atlantic halibut in the wild, we formulated the diets of the present study on the basis of nutritional information obtained from other flatfish species. Increasing dietary fat content had a negative effect on weight gain of juvenile turbot (Scophthalmusmaximus) (Bromley, 1980; Caceres-Martinez et al., 1984), whereas Cowey et al. ( 1975) did not find any effect on growth of plaice (Pieuronectespfatessa).Bromley ( 1980) and Cowey et al. ( 1975,19?6) found that the addition of dietary lipid resulted in an increased lipid content of the carcass in both turbot and plaice. 1~ contrast, Cac~rssXartinez et al. ( 1984) OO44-8486/9 t/$03.50
0 I99 1 Elsevier Science Publishers B.V. All rights reserved.
GM BERGE AND T. STOREBAKKEN
332
reported that carcass protein and lipid content in turbot were not influenced by diet composition. The aim of the present experiments was to gain information on the effects of different fat content (dietary fat to protein ratios) in diets for juvenile and ongrowing halibut. The study included effects on weight gain, digestibility and chemical composition of the fillet and liver. MATERIALS AND METHOD!;
Experiment I
Of the halibut larvae hatched in April-May 1987 at AKVAFORSK at Sunndalsnra, 38 individuals survived until December 1987. The larvae were fed live feed until metamorphosis was completed. The food organisms were rotifers, artemia and collected plankton (mainly copepodes) (Holmefjord et al., 1988). From metamorphosis to the start of the experiment the juveniles were fed a commercial agglomerated starter diet for salmon (Tess Merle; T. Skretting A/S, Stavanger, Norway). The fish were divided into two size groups; 16 with a mean weight of 6.1 g ( 1-9 g), and 22 with a mean weight of 11.6 g (6-22 g). Each diet was fed in duplicate to fish from both size groups, four tanks in total, in 0.7-m’ fibreglass tanks with a saltwater supply. The water depth was 30 cm, and the tempcraTABLE I Composition (g/ 100 g) of the diets for Experiment 1 Diet I
--
Diet 2
Formulation” Fish mealb Capelin oil’ Carbohydrates Vitaminsd. minerals and binder
68.8 6.3 16.0 9.0
61.7 15.9 14.3 8.1
Analysis Dry matter Protein Fat Ash
88.6 54. I 12.3 9.4
89.5 48.6 20.5 8.6
“Composition of basal diet according to producer’s specification. bNorSeaMink LT (Norsildmel, Bergen, Norway). ‘NorSalmOil (Norwegian Herring Meal Industries). dGuaranteed minimum vitamin content per kg feed (producer’s specification): vitamin A, 5000 RJ; vitamin Ds. 4000 IU; vitamm Kj, I5 mg; vitamin E, 100 mg; vitamin B,, 20 mg; vitamin Bz, 25 mg; vitamin Bb. 20 mg; pantothenic acid, 40 mg; niacin, 150 mg: folic acid, 2 mg; vitamin Br2, 0.02 mg; biotin. 0.5 mg: vitamin C, 600 mg; choline, 1000 mg; inositol, 300 mg.
DIETARY FAT LEVEL FOR ATLANT!? H4LIBUT
333
fur,: varied between 7 and 9°C during the experiment. The tanks were covered with plastic lids, with a circular 30-cm-diameter opening in the cerrtre. Diets were prepared from a commercial pelleted salmon diet (Tess Elite Pluss; T. Skretting A/S, Stavanger, Norway), drawn from the production line before fat was added. Fat was added by spraying, to obtain two diets with 12% and 2 1% fat (Table 1). Feed, in excess, was provided every 10 min, 24 h/ day, by automatic feeders. Individual weights of the fish were recorded at the start, at day 30, day 56 and day 85 (termination) of the experiment. Differences in growth between the two diets were tested by Student’s t-test. Experiment 2 Halibut, 0.6- 1.5 kg, were caught at the More-coast off Western Norway, during the summer of 1987. The halibut were initially held in 3-m’ circular tanks and fed sliced herring for 4-5 weeks until they ate properly. Then they were fed a moist pellet made from 50% herring, 25% coalfish-filleting offals and 25% binder meal (Tess Salmomix; T. Skretting A/S, Stavanger, NorTABLE 2 Composition (g/ 100 g) of the diets and apparent digestibility (O/b)of dry matter, fat and protein in halibut fed diets with different fat levels in Experiment 2 Diet 3 Formulation Mackerel Argentine Squid Capelin oil Binder meala
Diet 4
Diet 5
Diet 6
10
10
IO
35 30 25
30 30 5 25
25 30 10 25
10 20 30 15 25
Analysis Dry matter Protein Fat “Carbohydrates”b Ash
42.3 21.1 7.5 9.4 4.3
46.5 20.3 11.9 10.2 4.1
49.6 19.8 15.5 10.4 3.9
52.6 19.3 20.0 9.1 4.2
Apparent digestibility Dry matter Fat Protein
76.0 84.9 84.8
77.5 91.0 83.8
84.6 94.1 85.9
84.7 86.7 84.1
‘Tess Salmomix 25Oh,T. Skretting A/S, Stavanger, Norway: 50% fishmeal ( NorSeaMink, Nordsildmel, Bergen, Norway), 40% carbohydrate feedstuffs and 101 vitamins (as specified in Tabie i ), minerals and binder. ““Carbohydrates”= dry matter- (protein+fat+ash).
334
G.M BERGE AND T. STOREBAKKEN
way). Four weeks prior to the experiment the fish were individually tagged, using plastic tags (Floy tag and Manufacturing, Seattle, WA, USA) The fish were distributed into eight 2-m2 fibreglass tanks supplied with 1% I 5 1 saltwater (33-34 o/00)per min, 2 weeks before the experiment started, nine fish in each tank. The water depth and temperature were the same as described for Experiment 1. The tanks were located in an indoor hall and were partly covered ( l/4 of the surface area) by a wooden frame to give extra shade, and to prevent the fish from jumping out. Four moist diets were formulated to be isonitrogenous ( 19-20°h crude protein ), contain equal amounts of carbohydrate (9- 10%)) and 8, 12,16 or 20% fat (% wet weight ), and 1% (dry weight) Cr203 was added as indicator for determination of apparent digestibility (Table 2). The fish were hand-fed to satiation every second day, and feed consumption was recorded for each tank. Individual weights of the fish were recorded at the start of the experiment, and after 5, 10 and 14 weeks (termination). Weight gain was calculated for each period separately and for the entire experiment. At the end of the experiment lo-20 fish from each groups were anaesthetized and faeces were collected according to Gronseth and Myhre ( 1988). The samples were stored at - 20 OC prior to analysis. Three halibut from each tank were slaughtered for body composition analyses. Samples of the livers were immediately frozen in liquid nitrogen and stored at - 80 OC. A 2-cm-thick slice was obtained from behind the peritoneal cavity of the fish. Skin, bone and adipose tissue were removed prior to mincing the fillet samples. The feeds, fillet, liver and faeces samples were analysed for dry matter (lOS°C, 16-18 h), ash (flame-combustion followed by 3-4 h at 55O”C), fat (Folch et al., 1957; for liver: modified using chloroform : methanol : water = 8 : 3 : 4, for feeds: fat was extracted after HCl-hydrolysis (Stoldt, 1952 ) ) and protein (semi-micro-Kjeldahl, Kjeltec-Auto system ). Liver glycogen was determined as the difference between free glucose in the sample and total glucose after hydrolysis of the sample, multiplied by a factor of 0.9. Glucose was determined enzymatically (Glucose UV Method, Boehringer-Mannheim, Germany). Cr203 was analysed by atomic absorption modified according to Williams et al. ( 1962 ) . The results obtained were subjected to a two-way analysis of variance, including the effects of diets and replica. Calculations were based an mean values from each tank. RESULTS
Experiment I The mean weights of the fish in the four tanks during the experiment are shown in Fig, 1. The weight gain of the fish was 2.2% per day in the first
DIETARY FAT LEVEL FOR ATLANTIC HALIBUT
335
Fish weight (g) 50 45 40 35
20% fat in the diet ---- . !2!5 fat ik?t!!n diet
30 25 20 15 10 5
0'
0
I
I
I
I
20
40
60
60
Days
Fig. 1. Growth of juvenile halibut fed diets with two different fat levels.
period, 1.3% in the second period, and 1.2% per day in the last period of the experiment. Three fish died during the experiment. This was cawxi by an accident during the last period, in which most of the water in one of the tanks drained out, and gave a reduced growth in this tank in the last period (Fig. 1). The t-test did not reveal significant difference in weight gain of fish fed the diets during any of the test periods.
Experiment 2 No differences in digestibility of dry matter, fat and protein were found (Table 2 ) . Negative values for digestibility of ash were found, ranging from - 15% to - 20%. Weight gain varied among individuals from 0 to 0.6% per day; however, no significant difference in weight gain of the fish fed the different diets during any of the periods or during the total experiment were found. Mean growth rate was 0.27% per day. Best feed efficiency was recorded for the halibut fed the diet with 20% fat (0.49 kg gain per kg feed ) and the lowest for the diet with 8Oh fat (0.40), but the difference was not significant. The mean values of carcass and liver to whole-body weight percentages were 93.5% and 2.1%, respectivell. Chemical analyses of livers gave mean vaiues of 44.6% dry matter, 9.0% protein, 32.9% fat, 4.7% glycogen and 0.9% ash, and the values for fillets were 22.8% ‘dry matter, 14.8% protein, 2.4% fat and 1.4% ash. Diet 3 provided fillets with a significantly higher ash content. This was the only parameter giving a significant difference among dietaq groups. DISCUSSION
Weight gain per day (%) for the juvenile halibut was about the same as expected for Atlantic salmon of the same size and at the same temperature,
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but slightly poorer than for rainbow trout (Austreng et al., 1987 )* This SUPports the view that halibut has potential as a species for aquaculture in cold waters. The weight gain of the larger halibut caught in the wild was poorer than that of salmon and rainbow trout (Austreng et al., 198’7 )- Furthermore, the results revealed a large variation in weight gain within groups, indicating that there m,ay be a difference in the ability to adapt to the conditions offered in captivity. This appears to be a problem when using halibut caught in the wild in experiments. Efforts to reduce variation due to different stages of adaptation to captivity are neccessary. One such effort will be to modify the experimental facilities in order to make the conditions mofe optimal for the fish. Individuals should also be observed before the start of a new experiment and stunted fish removed. It is also desirable to increase the number of fish and replicas in future experiments. In the present experiments we did not observe any effect of increasing the fat content in the diet from 8% to 20% on the weight gain of Atlantic halibut. This is in keeping with results obtained by Cowey et al. ( 1975 ) who did not find any effect on weight gain of dietary fat levels from 5.6 to 17.6% in an experiment with plaice. The findings are in contrast to results obtained with juvenile turbot (Bromley, 1980; Caceres-Martinez et al., 1984). The feed intake and, thus, the feed efficiency of Experiment 2, was of the same magnitude in all dietary groups, regardless of dietary composition. This is in agree? ment with the findings of Caceres-Martinez et al. (1984) for turbot. When the dietary fat to protein ratio is increased from a low to a moderate level in Atlantic salmon and rainbow trout, the result is increased weight gain, survival, and feed conversion eff?ciency, and a slight increase in body fat content ( Austreng, 1976a, b, Austreng, 1979). With a further increase in dietary fat content, growth rate does not neccesarily increase, but the fish get fatter ( Austreng, 1986). The amount of fat in the halibut fillet samples ranged from 6.5 to 12.9% (dry weight), and was not different for the various dietary treatments. These values overlap the levels of fat in white musc1.e in halibut reported by Andreasen et al. ( 1989 ), and were slightly lower than the values reported by Haug et al. ( 1988). The halibut has fat depots close to the bones and fins. They were trimmed away before analysis, thus the effects on these depots were not included in our study. In rainbow trout the fat content of the dorsal depots responds rapidly to different dietary treatments (tiessling et al., 1989). The fat content of the livers was high in all the halibut sampled (2 l-5 1% wet weight, 579% dry weight), in keeping with results obtained by Andreasen et al. ( 1989 ) and Haug et al. ( 1988 ) . The fish for slaughter were randumly sampled. Since we only sampled three fish from each tank, casual sampling of slow-growing fish might have had an effect on the results of the chemical analyses. Mean weight gain of sampled
DIETARY FAT LEVEL FOR ATLANTIC HALIBUT
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fish proved to be lower than mean weight gain for the entire experiment, but there was no systematic difference among the dietary groups., The apparent digestibility of fat was within a range of 85-95%, and that of protein within 84-86%, in agreement with previous findings (Gronssth and Myhre, 1988; Stromsnes, 1989). The absolute values for digestibility and the lacking effects of dietary fat level were also consistent with digestibilities reported for rainbow trout (Austreng, 1979). In contrast, Lie et al. ( 1988 ) found reduced digestibililty of both fat and protein in cod (Gadus morhua) with increasing fat contr:nts.The negative digestibility of ash can be explained by the drinking of salt water by marine fish. There is absorption of Na+, Cl- and water in the intestne, and an increase in the concentration of divalent ions (Kirsch et al., 1985). ACKNOWLEDGEMENTS
We are grateful to A. Linseth for skillful technical assistance, to E. Austreng and A. Rrogdahl for useful discussions and to T. Skretting A/S for providing the basal diet for Experiment 1. The experiments were financed by the Norwegian Fisheries Research Council. G.M. Berge was supported by a grant from the Norwegian Agricultural Research Council.
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