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A comparison of growth rate and trace element accumulation in Atlantic salmon (Salmo salar) fry fed four different commerical diets
Aquaculture, 79 (1989) 267-273 Elsevier Science Publishers B.V., Amsterdam -
267 Printed in The Netherlands
A Comparison of Growth Rate and Trace Element Accumulation in Atlantic Salmon (Salmo salar) Fry Fed Four Different Commercial Diets AMUND MAAGEI, HARALD SVEIBR2 and KAARE JULSHAMNl ‘Institute of Nutrition, Directorate of Fisheries, P.O. Box 1900, N-5024 Bergen (Norway) 2A/S Saevareid Fiskeanlegg, N-5674 Saevareid (Norway)
ABSTRACT Maage, A., Sveier, H. and Julshamn, K., 1989. A comparison of growth rate and trace element accumulation in Atlantic salmon (Salmo sub) fry fed four different commercial diets. Aquaculture. 79: 267-273. “Swim up” Atlantic salmon (Salmo sah) fry were fed four commercial start diets for 4 weeks. Calcium, iron, copper, zinc, selenium as well as ascorbic acid, water, protein, fat and ash contents were determined in the feeds and the elements were determined in the fry. There were significant correlations between the levels of calcium, iron and zinc in the diets and in the whole fry. Significant positive correlations were also found between the weight of the developing fry and the concentrations of carcass zinc and selenium, thus showing an accumulation of these elements during early growth.
INTRODUCTION
All elements found essential in mammals (Underwood, 1977) are probably also essential for fish.. In addition to the dietary sources, minerals and trace elements can be absorbed directly from the water by salmonids (Phillips, 1959). The mineral and trace element requirements which cannot be covered by uptake from the water must be provided in the diets. Dietary deficiencies in fish are known for phosphorus, iodine, magnesium and zinc (NRC, 1981). The aim of the present study was to evaluate four commercial feeds as sources of the essential elements calcium, iron, copper, zinc and selenium for Atlantic salmon fry and, further to correlate the dietary element levels with whole body element concentrations and fish growth. MATERIALS
AND METHODS
Fish. Atlantic salmon fry of the “Bolaks” breed were distributed in 24 circular tanks (diameter 2 m) with approximately 80000fish in each. The tanks were situated in two different production halls, 12 in each, under constant light.
0044-8486/89/$03.50
0 1989 Elsevier Science Publishers B.V.
268
Water. The flow rate was 2 l/min per kg fish. Regular checks of outflowing water showed that the oxygen saturation was never less than 90%. The water temperature at hatching and onwards to the end of the yolk sac stage (after about four weeks) was 8” C. At the start of feeding the water temperature was raised and kept between 13’ C and 15’ C. Frequent water quality analyses carried out by commercial laboratories showed the following values in water from the hatchery: pH 6.0-6.6; conductivity 21-28 @/cm; Ca 1.45-1.8 mg/l; Mg 0.37-0.51 mg/l; Fe 0.3-0.5 mgfl; Zn 10 @g/l; and Cu 3 pg/l. Diets. Four commercial starter feeds with small grain size (no. 1) were used. Diets A, C and D were pelleted while diet B was extruded. Feeding and weighing. Each feed was fed in six replicate tanks, three in each production hall. The fish were fed ad lib. Fifty fry were randomly collected for mean weight measurements each week. This was repeated 2-4 times in each tank to secure optimum weight estimations. After 4 weeks of feeding, four fish from each tank were randomly caught, weighed, and prepared for carcass element analyses. Analytical methods. The samples were digested in 2 ml (9 : 1) HN03/HC104 overnight, boiled under pressure for 2 h and diluted to 10 ml. The contents of calcium, iron, copper and zinc were determined by flame atomic absorption on a Perkin Elmer (P-E) 3030 AAS. Due to a high content of interfering phosphorus in most of the samples, selenium was analysed on a Perkin Elmer Zeeman 5000 AAS which was equipped with a P-E 400 graphite furnace (Maage et al., 1989). When using the graphite furnace-AA& the standard addition method was applied for the calculations, otherwise the standard curve procedure was used. Ascorbic acid was analysed according to a method described by Sandnes and Utne (1982). The fat content in the feeds was analysed gravimetrically after ethyl acetate extraction. Protein (N x 6.25) was determined according to a modified micro-Kjeldahl method (Crooke and Simpson, 1971) and ash after combustion at 550°C for 18. Carbohydrate was calculated by difference. Statistics. Statistical evaluation of the data was performed using a Luxor IDA800 calculation programme. RESULTS
Feed analyses. The water contents in the feeds A, B, C and D were 6.0%, 4.3%, 6.1%, and 7.3%, respectively. Proximate analyses of the feeds are presented in Table 1 and show only minor differences between the diets, except from a markedly higher ash content in feed C. The contents of calcium, iron, copper,
269 TABLE 1 Analyses of major feed components (g/kg dry weight) in four commercial start feeds for Atlantic salmon Component
Feed
Fat Protein Ash Carbohydrate
A
B
C
D
158 528 92 222
175 531 89 205
153 510 120 217
165 550 83 202
TABLE 2 Analyses of calcium, trace elements and ascorbic acid (AA) in four different commercial start feeds for salmon fry (mg/kg feed, iV= 4) Feed component Feed (mg/kg) A Calcium Zinc Iron Copper Selenium AA
B
C
D
16 100 _+1000 16 100 +900 25 200 f600 14 000 2400 201 + 4 197 + 10 211 f 18 292 I 26 64.8 f 4.4 51.2 + 4.1 107.3 + 7.2 104.8 I!I 7.5 4.8 + 0.1 3.3 + 0.1 5.4 f 0.9 17.0 f 0.9 1.98-10.04 2.13f 0.13 1.67+ 0.04 2.62+ 0.12 178 105 258 555
zinc and selenium and ascorbic acid in the feeds are shown in Table 2. The zinc concentrations in the feeds were high, showing values ten times the recommended dietary requirement for rats. Ascorbic acid in the feeds ranged from 105 mg/kg to 555 mg/kg (feed D). Growth. The weight of the fry was 0.17 g at the start of feeding and was more than doubled in all groups during the experimental period. The fish weights at the end of the experiment are presented in Table 3. In both halls, fish fed diets C and D showed the best growth and fish fed diet A the lowest. Element concentration in the fry. The carcass concentrations of the elements are shown in Table 4. No significant differences were found due to location, thus mean values based on all replicates are presented. The carcass calcium concentrations were significantly higher (PC 0.01) in groups A, B and C than in group D (Table 4). There was also a positive cor-
270 TABLE 3 Final weight (g) of salmon fry fed four different commercial feeds in two production halls (in brackets the numbers of weighings, each representing the mean of fifty fry) Diet
Day-degrees
A
B
C
D
(12)
0.432 f 0.07 (10)
0.466 + 0.09” (11)
0.475 + 0.05b (11)
Hall 2 0.376 +_0.03abcd
0.423 +_0.048bc
0.494 + 0.048b”d
0.445 +o.oYM
(12)
(12)
(12)
(12)
Hall 1 0.398 +_0.06ab
804
750
abcdCommonsuperscript letters in one line indicate significant differences (PC 0.05).
TABLE 4 The concentration of Ca, Zn, Fe, Cu and Se (mg/kg wet weight) in salmon fry fed four different commercial feeds Elements (mg/kg)
Diet A (n=24)
B (~23)
C (n=24)
D (n=24)
Ca Zn Fe cu Se
2863 +302’ 39.5 I!I 4.5*bd 15.5 + 3.4”s” 0.58 f 0.28” 0.28+ 0.02
2858 +274b 40.7 I!I 4.4” 12.1 + 1.7”b”d 0.40 * 0.13”b 0.27 + 0.02”
2957 +206” 36 5 + 3 4abcd 19:4 Yc 4:o”b”l 0.46rf: 0.16 0.28f 0.02
2513 + 177abc 42.9 + 5.0aM 14.3 I!I 3.3”bd 0.61+ 0.34b 0.29 f 0.03”
abcdCommon superscript letters in one line indicate significant differences (PC 0.01).
relation between the concentrations of calcium in the feeds and in the fry (RZ0.41, N=95). The zinc concentrations in the fry were significantly higher in group D than in groups A and C (P-c 0.01)) but not in group B. A significantly positive correlation was found between the dietary zinc and the carcass zinc concentration (Rz0.39, N=95). Group C fish, which were given the highest dietary iron level (107 mg/kg feed), showed a significantly higher carcass iron concentration (P < 0.01) than the other groups. A significant correlation was found between the dietary iron concentrations and the iron concentrations in the fry (R = 0.44, N= 95). The copper and selenium levels of the developing fry reared on the different test diets after 4 weeks were variable but showed no apparent trends in relation
271
to dietary copper. There was, however, a significant positive correlation between the fry weight and the carcass selenium concentration (R = 0.37, N= 94). DISCUSSION
Calcium. Nose and Arai (1976) reported that the calcium requirement of rainbow trout could be met by 16-20 mg/l dissolved calcium in the water. The carcass calcium levels in the present study (2.5-2.9 g/kg) correspond to those found by Satch et al. (1987) in rainbow trout fed commercial feeds. These authors reported a concentration of calcium in the water of 18-20 mg/l which is ten times higher than found in the present experiment. This indicates that Atlantic salmon fry compensate for a reduceduptake of calcium from the water by dietary calcium. However, the reduced calcium concentration in fish fed the lowest calcium level (group D) suggests that a high dietary calcium content may be important at this stage of development. Feed C apparently contained more calcium than could be utilized, thus indicating an upper limit for calcium absorption. In the developing rainbow trout there is a fast accumulation of calcium in the period from week 4 to week 12 after the start of feeding (Satch et a., 1987). In the present experiment we found no significant correlation between calcium accumulation in the carcass and the growth of the fry. Other reports have shown that dietary calcium is poorly utilized by salmonids (NRC, 1981) . A low calcium level in the water is a stress factor upon ion regulation in the fish (Evans, 1984). This work implies that low calcium in the water also could impair carcass calcification in the growing fry. Iron. Kawatsu (1972) found that dietary iron supplementation increased trout growth and prevented anaemia. La11and Hines (1987) suggested that the requirement for iron (as iron (II) citrate) of Atlantic salmon is about 60 mg/kg. The iron levels in the two commercial feeds A and B thus approximated the requirement (64.8 and 51.2 mg/kg, respectively). Studies at our institute have revealed a limited absorption of iron in rats given fish meal-based diets (Maage and Julshamn, 1989)) probably because a significant part of the iron is in the metallic form. The producers of feeds A and B should preferably increase the iron contents in these diets. Zinc. The zinc requirement in rainbow trout based on growth rate studies has been estimated at 15-30 mg/kg (Ogino and Yang, 1978). When white fish meal (WFM) was used as protein source, deficiency signs (eye cataract, growth retardation) were observed even with zinc concentrations up to 60 mg/kg (Ketola, 1979). The feeds tested in the present experiment contained high zinc concentrations (200-300 mg/kg) and the feeds were not formulated on a WFM basis.
272
The significant correlation between the zinc concentrations in the feeds and in the fry indicates that the absorption capacity was not exceeded within the concentration range studied. In rainbow trout fry fed various diets, the carcass zinc concentrations ranged from 22.5-30.0 mg Zn/kg after 4 weeks of feeding and thereafter decreased (Satch et al., 1987). Somewhat higher values (36.542.9 mg Zn/kg) were found in the present experiment with Atlantic salmon fry at the same stage. There was a steady accumulation of zinc in the weight range studied. This indicates that the fry may not have the ability to excrete surplus zinc. Jeng and Sun (1981) found growth retardation in the common carp (Cyprinus cc&o) at a later stage with a dietary level of 294 mg Zn/kg. Even though zinc generally is known to be non-toxic within a wide concentration range (Underwood, 1977)) the levels in the commercial feeds studied seem rather high. Copper. La11and Hines (1987) suggested that the requirement for copper in Atlantic salmon is in the order of 3-5 mg Cu/kg dry feed. The feeds tested in the present experiment fell within this range, except for feed D which contained 17 mg/kg. It should be noticed that the concentrations of copper in the fry were near the detection limit for flame-AAS analysis. Similarly, carcass analyses may have masked retention differences as copper accumulates mainly in the liver of salmonids (Julshamn et al., 1988). SeZenium. In this study dietary levels in the range of 1.67-2.62 mg Se/kg were found. The requirement for selenium of Atlantic salmon has been reported to be 0.1 mg Se/kg feed, provided there is sufficient supplementation of vitamin E (Poston et al., 1976). Hilton et al. (1980) suggested that even a dietary level of 3 mg Se/kg could be toxic over a longer feeding period. These findings and the observed selenium accumulation reported in the present study suggest that it should not be neccesary to fortify fish meal-based feeds with selenium as fish meal contains 1.5-2.5 mg Se/kg. Ascorbic acid is known to have an influence on the metabolism of trace elements in animals (Hornig et al., 1984). In the present study all the feeds contained more than 100 mg ascorbic acid/kg, thus covering the dietary requirement in this species. The differences could most probably be explained by different storage times as ascorbic acid has limited stability in dry fish feed during storage (Sandnes and Utne, 1982). The present knowledge of the interactions between ascorbic acid and trace elements does not support the hypothesis that the variations in the content of ascorbic acid in the diets tested in the present study would cause significant differences in the metabolism of the trace elements. The contents of the elements calcium, copper, zinc and selenium in the four
273
commercial starter feeds tested in the present experiment seem not to have any growth-limiting effects in Atlantic salmon fry. The iron requirement should be further investigated and the molecular speciation of the element should be taken into account. The levels of zinc and selenium seem rather high, as indicated by an accumulation of these elements in fish with a high growth rate.
REFERENCES
Crooke, W.M. and Simpson, W.E., 1971. Determination of ammonium in Kjeldahl digest of crops by an automated procedure. J. Sci. Food Agric., 22: 9-10. Evans, D.H., 1984. The roles of gill permeability and transport mechanisms in euryhalinity. In: W.S. Hoar and D.J. Randall (Editors), Fish Physiology, X. Ion and Water Transfer. Academic Press, London, pp. 239-283. Hilton, J.W., Hodson, P.V. and Slinger S.J., 1980. The requirements and toxicity of selenium in rainbow trout (Salmo guirdneri). J. Nutr., 110: 2527-2535. Hornig, D., Glatthaar, B. and Moser, U., 1984. general aspects of ascorbic acid function and metabolism. In: I. Wegger, F.J. Tagwerker and J. Morustgaard (Editors), Ascorbic Acid in Domestic animals. Royal Danish Agric. Sot., Copenhagen, pp. 3-24. Jeng, S.S. and Sun, L.T., 1981. Effects of dietary zinc levels on zinc concentrations in tissues of common carp. J. Nutr., 111: 134-140. Julshamn, K., Andersen, K.-J., Ringdal, 0. and Brenna, J., 1988. Effect of dietary copper on the hepatic concentration and subcellular distribution of copper and zinc in the rainbow trout (Salmo gairdneri) . Aquaculture, 73: 1433155. Kawatsu, H., 1972. Studies on anemia in fish. V. Dietary iron deficient anemia in brook trout, Salvelinus fontinalis. Bull. Freshwater Fish. Res. Lab., 22: 59-67. Ketola, H.G., 1979. Influence of dietary zinc on cataracts in rainbow trout (Salmo guirdneri). J. Nutr., 109: 965-969. Lall, S.P. and Hines, J.A., 1987. Iron and copper requirements of Atlantic salmon (Salmo s&r). In: Abstr. Int. Symp. Feeding and Nutrition in Fish. Bergen, Norway, p. 48. Maage, A. and Julshamn, K., 1989. Absorption and organ retention of calcium, iron, zinc and selenium from fishmeal diets in rat with different growth rates induced by exchanging carbohydrate with fat (in prep.). Maage, A., Andersen, K.-J. and Julshamn, K., 1989. Determination of selenium in marine samples with graphite furnace atom absorption spectroscopy. J. Anal. At. Spectrom. (submitted). National Research Council, 1981. Nutrient Requirements of Coldwater Fishes. National Academy Press, Washington DC, 63 pp. Nose, T. and Arai, S., 1976. Recent advances in studies on mineral nutrition of fish in Japan. FAO Tech. Conf. Aquaculture, 12 pp. Ogino, C. and Yang, G., 1978. Requirement of rainbow trout for dietary zinc. Bull. Jpn. Sot. Sci. Fish., 44 1105-1108. Phillips, A.M., Jr., 1959. The known and possible roles of minerals in trout nutrition and physiology. Trans. Am. Fish. Sot., 88: 133-135. Poston, H.A., Combs, G.F., Jr. and Leibovitz, L., 1976. Vitamin E and selenium interrelations in the diet of Atlantic salmon (Salmo salar): gross, histological and biochemical deficiency signs. J. Nutr., 106: 892-904. Sandnes, K. and Utne, F., 1982. Processing loss and storage stability of ascorbic acid in dry fish feed. Fiskeridir. Skr., Ser. Ernmring, 2: 39-44. Satoh, S., Takeuchi, T. and Watanabe, T., 1987. Changes of mineral compositions in whole body of rainbow trout during growing stages. Nippon Saisan Gakkaishi, 53: 273-279. Underwood, E.J., 1977. Trace Elements in Human and Animal Nutrition. Academic Press, New York, NY, 545 pp.
267 Printed in The Netherlands
A Comparison of Growth Rate and Trace Element Accumulation in Atlantic Salmon (Salmo salar) Fry Fed Four Different Commercial Diets AMUND MAAGEI, HARALD SVEIBR2 and KAARE JULSHAMNl ‘Institute of Nutrition, Directorate of Fisheries, P.O. Box 1900, N-5024 Bergen (Norway) 2A/S Saevareid Fiskeanlegg, N-5674 Saevareid (Norway)
ABSTRACT Maage, A., Sveier, H. and Julshamn, K., 1989. A comparison of growth rate and trace element accumulation in Atlantic salmon (Salmo sub) fry fed four different commercial diets. Aquaculture. 79: 267-273. “Swim up” Atlantic salmon (Salmo sah) fry were fed four commercial start diets for 4 weeks. Calcium, iron, copper, zinc, selenium as well as ascorbic acid, water, protein, fat and ash contents were determined in the feeds and the elements were determined in the fry. There were significant correlations between the levels of calcium, iron and zinc in the diets and in the whole fry. Significant positive correlations were also found between the weight of the developing fry and the concentrations of carcass zinc and selenium, thus showing an accumulation of these elements during early growth.
INTRODUCTION
All elements found essential in mammals (Underwood, 1977) are probably also essential for fish.. In addition to the dietary sources, minerals and trace elements can be absorbed directly from the water by salmonids (Phillips, 1959). The mineral and trace element requirements which cannot be covered by uptake from the water must be provided in the diets. Dietary deficiencies in fish are known for phosphorus, iodine, magnesium and zinc (NRC, 1981). The aim of the present study was to evaluate four commercial feeds as sources of the essential elements calcium, iron, copper, zinc and selenium for Atlantic salmon fry and, further to correlate the dietary element levels with whole body element concentrations and fish growth. MATERIALS
AND METHODS
Fish. Atlantic salmon fry of the “Bolaks” breed were distributed in 24 circular tanks (diameter 2 m) with approximately 80000fish in each. The tanks were situated in two different production halls, 12 in each, under constant light.
0044-8486/89/$03.50
0 1989 Elsevier Science Publishers B.V.
268
Water. The flow rate was 2 l/min per kg fish. Regular checks of outflowing water showed that the oxygen saturation was never less than 90%. The water temperature at hatching and onwards to the end of the yolk sac stage (after about four weeks) was 8” C. At the start of feeding the water temperature was raised and kept between 13’ C and 15’ C. Frequent water quality analyses carried out by commercial laboratories showed the following values in water from the hatchery: pH 6.0-6.6; conductivity 21-28 @/cm; Ca 1.45-1.8 mg/l; Mg 0.37-0.51 mg/l; Fe 0.3-0.5 mgfl; Zn 10 @g/l; and Cu 3 pg/l. Diets. Four commercial starter feeds with small grain size (no. 1) were used. Diets A, C and D were pelleted while diet B was extruded. Feeding and weighing. Each feed was fed in six replicate tanks, three in each production hall. The fish were fed ad lib. Fifty fry were randomly collected for mean weight measurements each week. This was repeated 2-4 times in each tank to secure optimum weight estimations. After 4 weeks of feeding, four fish from each tank were randomly caught, weighed, and prepared for carcass element analyses. Analytical methods. The samples were digested in 2 ml (9 : 1) HN03/HC104 overnight, boiled under pressure for 2 h and diluted to 10 ml. The contents of calcium, iron, copper and zinc were determined by flame atomic absorption on a Perkin Elmer (P-E) 3030 AAS. Due to a high content of interfering phosphorus in most of the samples, selenium was analysed on a Perkin Elmer Zeeman 5000 AAS which was equipped with a P-E 400 graphite furnace (Maage et al., 1989). When using the graphite furnace-AA& the standard addition method was applied for the calculations, otherwise the standard curve procedure was used. Ascorbic acid was analysed according to a method described by Sandnes and Utne (1982). The fat content in the feeds was analysed gravimetrically after ethyl acetate extraction. Protein (N x 6.25) was determined according to a modified micro-Kjeldahl method (Crooke and Simpson, 1971) and ash after combustion at 550°C for 18. Carbohydrate was calculated by difference. Statistics. Statistical evaluation of the data was performed using a Luxor IDA800 calculation programme. RESULTS
Feed analyses. The water contents in the feeds A, B, C and D were 6.0%, 4.3%, 6.1%, and 7.3%, respectively. Proximate analyses of the feeds are presented in Table 1 and show only minor differences between the diets, except from a markedly higher ash content in feed C. The contents of calcium, iron, copper,
269 TABLE 1 Analyses of major feed components (g/kg dry weight) in four commercial start feeds for Atlantic salmon Component
Feed
Fat Protein Ash Carbohydrate
A
B
C
D
158 528 92 222
175 531 89 205
153 510 120 217
165 550 83 202
TABLE 2 Analyses of calcium, trace elements and ascorbic acid (AA) in four different commercial start feeds for salmon fry (mg/kg feed, iV= 4) Feed component Feed (mg/kg) A Calcium Zinc Iron Copper Selenium AA
B
C
D
16 100 _+1000 16 100 +900 25 200 f600 14 000 2400 201 + 4 197 + 10 211 f 18 292 I 26 64.8 f 4.4 51.2 + 4.1 107.3 + 7.2 104.8 I!I 7.5 4.8 + 0.1 3.3 + 0.1 5.4 f 0.9 17.0 f 0.9 1.98-10.04 2.13f 0.13 1.67+ 0.04 2.62+ 0.12 178 105 258 555
zinc and selenium and ascorbic acid in the feeds are shown in Table 2. The zinc concentrations in the feeds were high, showing values ten times the recommended dietary requirement for rats. Ascorbic acid in the feeds ranged from 105 mg/kg to 555 mg/kg (feed D). Growth. The weight of the fry was 0.17 g at the start of feeding and was more than doubled in all groups during the experimental period. The fish weights at the end of the experiment are presented in Table 3. In both halls, fish fed diets C and D showed the best growth and fish fed diet A the lowest. Element concentration in the fry. The carcass concentrations of the elements are shown in Table 4. No significant differences were found due to location, thus mean values based on all replicates are presented. The carcass calcium concentrations were significantly higher (PC 0.01) in groups A, B and C than in group D (Table 4). There was also a positive cor-
270 TABLE 3 Final weight (g) of salmon fry fed four different commercial feeds in two production halls (in brackets the numbers of weighings, each representing the mean of fifty fry) Diet
Day-degrees
A
B
C
D
(12)
0.432 f 0.07 (10)
0.466 + 0.09” (11)
0.475 + 0.05b (11)
Hall 2 0.376 +_0.03abcd
0.423 +_0.048bc
0.494 + 0.048b”d
0.445 +o.oYM
(12)
(12)
(12)
(12)
Hall 1 0.398 +_0.06ab
804
750
abcdCommonsuperscript letters in one line indicate significant differences (PC 0.05).
TABLE 4 The concentration of Ca, Zn, Fe, Cu and Se (mg/kg wet weight) in salmon fry fed four different commercial feeds Elements (mg/kg)
Diet A (n=24)
B (~23)
C (n=24)
D (n=24)
Ca Zn Fe cu Se
2863 +302’ 39.5 I!I 4.5*bd 15.5 + 3.4”s” 0.58 f 0.28” 0.28+ 0.02
2858 +274b 40.7 I!I 4.4” 12.1 + 1.7”b”d 0.40 * 0.13”b 0.27 + 0.02”
2957 +206” 36 5 + 3 4abcd 19:4 Yc 4:o”b”l 0.46rf: 0.16 0.28f 0.02
2513 + 177abc 42.9 + 5.0aM 14.3 I!I 3.3”bd 0.61+ 0.34b 0.29 f 0.03”
abcdCommon superscript letters in one line indicate significant differences (PC 0.01).
relation between the concentrations of calcium in the feeds and in the fry (RZ0.41, N=95). The zinc concentrations in the fry were significantly higher in group D than in groups A and C (P-c 0.01)) but not in group B. A significantly positive correlation was found between the dietary zinc and the carcass zinc concentration (Rz0.39, N=95). Group C fish, which were given the highest dietary iron level (107 mg/kg feed), showed a significantly higher carcass iron concentration (P < 0.01) than the other groups. A significant correlation was found between the dietary iron concentrations and the iron concentrations in the fry (R = 0.44, N= 95). The copper and selenium levels of the developing fry reared on the different test diets after 4 weeks were variable but showed no apparent trends in relation
271
to dietary copper. There was, however, a significant positive correlation between the fry weight and the carcass selenium concentration (R = 0.37, N= 94). DISCUSSION
Calcium. Nose and Arai (1976) reported that the calcium requirement of rainbow trout could be met by 16-20 mg/l dissolved calcium in the water. The carcass calcium levels in the present study (2.5-2.9 g/kg) correspond to those found by Satch et al. (1987) in rainbow trout fed commercial feeds. These authors reported a concentration of calcium in the water of 18-20 mg/l which is ten times higher than found in the present experiment. This indicates that Atlantic salmon fry compensate for a reduceduptake of calcium from the water by dietary calcium. However, the reduced calcium concentration in fish fed the lowest calcium level (group D) suggests that a high dietary calcium content may be important at this stage of development. Feed C apparently contained more calcium than could be utilized, thus indicating an upper limit for calcium absorption. In the developing rainbow trout there is a fast accumulation of calcium in the period from week 4 to week 12 after the start of feeding (Satch et a., 1987). In the present experiment we found no significant correlation between calcium accumulation in the carcass and the growth of the fry. Other reports have shown that dietary calcium is poorly utilized by salmonids (NRC, 1981) . A low calcium level in the water is a stress factor upon ion regulation in the fish (Evans, 1984). This work implies that low calcium in the water also could impair carcass calcification in the growing fry. Iron. Kawatsu (1972) found that dietary iron supplementation increased trout growth and prevented anaemia. La11and Hines (1987) suggested that the requirement for iron (as iron (II) citrate) of Atlantic salmon is about 60 mg/kg. The iron levels in the two commercial feeds A and B thus approximated the requirement (64.8 and 51.2 mg/kg, respectively). Studies at our institute have revealed a limited absorption of iron in rats given fish meal-based diets (Maage and Julshamn, 1989)) probably because a significant part of the iron is in the metallic form. The producers of feeds A and B should preferably increase the iron contents in these diets. Zinc. The zinc requirement in rainbow trout based on growth rate studies has been estimated at 15-30 mg/kg (Ogino and Yang, 1978). When white fish meal (WFM) was used as protein source, deficiency signs (eye cataract, growth retardation) were observed even with zinc concentrations up to 60 mg/kg (Ketola, 1979). The feeds tested in the present experiment contained high zinc concentrations (200-300 mg/kg) and the feeds were not formulated on a WFM basis.
272
The significant correlation between the zinc concentrations in the feeds and in the fry indicates that the absorption capacity was not exceeded within the concentration range studied. In rainbow trout fry fed various diets, the carcass zinc concentrations ranged from 22.5-30.0 mg Zn/kg after 4 weeks of feeding and thereafter decreased (Satch et al., 1987). Somewhat higher values (36.542.9 mg Zn/kg) were found in the present experiment with Atlantic salmon fry at the same stage. There was a steady accumulation of zinc in the weight range studied. This indicates that the fry may not have the ability to excrete surplus zinc. Jeng and Sun (1981) found growth retardation in the common carp (Cyprinus cc&o) at a later stage with a dietary level of 294 mg Zn/kg. Even though zinc generally is known to be non-toxic within a wide concentration range (Underwood, 1977)) the levels in the commercial feeds studied seem rather high. Copper. La11and Hines (1987) suggested that the requirement for copper in Atlantic salmon is in the order of 3-5 mg Cu/kg dry feed. The feeds tested in the present experiment fell within this range, except for feed D which contained 17 mg/kg. It should be noticed that the concentrations of copper in the fry were near the detection limit for flame-AAS analysis. Similarly, carcass analyses may have masked retention differences as copper accumulates mainly in the liver of salmonids (Julshamn et al., 1988). SeZenium. In this study dietary levels in the range of 1.67-2.62 mg Se/kg were found. The requirement for selenium of Atlantic salmon has been reported to be 0.1 mg Se/kg feed, provided there is sufficient supplementation of vitamin E (Poston et al., 1976). Hilton et al. (1980) suggested that even a dietary level of 3 mg Se/kg could be toxic over a longer feeding period. These findings and the observed selenium accumulation reported in the present study suggest that it should not be neccesary to fortify fish meal-based feeds with selenium as fish meal contains 1.5-2.5 mg Se/kg. Ascorbic acid is known to have an influence on the metabolism of trace elements in animals (Hornig et al., 1984). In the present study all the feeds contained more than 100 mg ascorbic acid/kg, thus covering the dietary requirement in this species. The differences could most probably be explained by different storage times as ascorbic acid has limited stability in dry fish feed during storage (Sandnes and Utne, 1982). The present knowledge of the interactions between ascorbic acid and trace elements does not support the hypothesis that the variations in the content of ascorbic acid in the diets tested in the present study would cause significant differences in the metabolism of the trace elements. The contents of the elements calcium, copper, zinc and selenium in the four
273
commercial starter feeds tested in the present experiment seem not to have any growth-limiting effects in Atlantic salmon fry. The iron requirement should be further investigated and the molecular speciation of the element should be taken into account. The levels of zinc and selenium seem rather high, as indicated by an accumulation of these elements in fish with a high growth rate.
REFERENCES
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