MILLIKIN: NUTRIENT REQUIREMENTS OF FISHES 



ton et al. 1977). However, after 16 wk, 100% of 

 lake trout fed 0.36% dietary methionine devel- 

 oped bilateral lenticular cataracts, while none of 

 the individuals fed the control diet containing 

 1.2% dietary methionine (3.27% of the dietary 

 protein) had cataracts. 



Attempts to supplement suboptimal dietary 

 concentrations of methionine plus cystine with 

 dietary taurine or inorganic sulfate have proven 

 unsuccessful with channel catfish fingerlings 

 and rainbow trout fingerlings. Channel catfish 

 fingerlings fed dietary taurine or inorganic sul- 

 fate as a partial replacement for methionine had 

 reduced growth rates (Robinson et al. 1978), 

 whereas rainbow trout fingerlings had reduced 

 growth and developed cataractogenesis (Page et 

 al. 1978). The absence of cataractogenesis in me- 

 thionine-deficient channel catfish may be the 

 result of the relatively shorter duration of this 

 study (8 wk) compared with a 16-wk study on 

 lake trout (Poston et al. 1977). Another explana- 

 tion for the absence of cataract formation in 

 channel catfish may be due to the slightly larger 

 initial size of channel catfish (mean weight = 7.7 

 g) compared with lake trout (mean weight = 5 g). 

 Either of these factors may have produced a 

 slower turnover rate of amino acids in channel 

 catfish. 



According to a summary by Poston et al. (1977), 

 insufficient methionine often results in reduc- 

 tions in sulfhydryl group concentrations, and 

 lens glutathione synthesis also decreases rapidly 

 during formation of most cataracts. The authors 

 speculated that lens glutathione possibly pro- 

 tects the lens sulfhydryl groups from oxida- 

 tion. 



Tryptophan 



Tryptophan deficiency symptoms have been 

 described in rainbow trout; sockeye salmon; 

 brook trout, S. fontinalis; and channel catfish. 

 Tryptophan deficiency in rainbow trout and sock- 

 eye salmon has resulted in scoliosis (Shanks etal. 

 1962; Halver and Shanks 1960, respectively). In 

 a separate study with rainbow trout, hyperemia, 

 scoliosis, and abnormal deposition of calcium 

 occurred in kidney and bony plates surrounding 

 the notochord and sheath of fish fed tryptophan- 

 deficient diets (Kloppel and Post 1975). The 

 authors suggested that hyperemia might be at- 

 tributed to a lack of serotonin resulting from a 

 deficiency of its precursor, tryptophan. In dis- 

 cussing scoliosis, Kloppel and Post (1975) noted 



that tryptophan is a major component of proto- 

 collagen, a supposed precursor of collagen. How- 

 ever, the absence of scoliosis in channel catfish 

 fingerlings fed tryptophan-deficient diets (Wil- 

 son et al. 1978), compared with studies on sal- 

 monids, probably resulted from a slower growth 

 rate of channel catfish because of a larger initial 

 mean weight (12.5 g). 



Quantitative tryptophan requirements have 

 been shown to be similar for three different sal- 

 monid species and channel catfish. Almquist- 

 type plots of growth responses indicated dietary 

 tryptophan requirements of 0.15 to 0.25% of the 

 diet (0.4 to 0.6% of dietary protein) for chinook 

 salmon and 0.20 to 0.25% of the diet (0.5 to 0.6% of 

 dietary protein) for sockeye salmon and coho 

 salmon (Halver 1965). Channel catfish finger- 

 lings fed a tryptophan-deficient diet (0.05%) for 8 

 wk had significantly poorer weight gain and feed 

 efficiency than individuals fed diets containing 

 as low as 0.12% tryptophan (0.5% of dietary pro- 

 tein) (Wilson et al. 1978). Wilson et al. (1978) sug- 

 gested that the lower dietary tryptophan require- 

 ment of fishes compared with that of terrestrial 

 animals may be due to an inability of fish to con- 

 vert tryptophan to niacin, thus reducing the 

 metabolic need of tryptophan in fishes as com- 

 pared with terrestrial species. Growth rate of 

 brook trout and the low ratio of two enzyme 

 activities (3-hydroxyanthranilic acid oxygenase 

 to picolinic acid carboxylase) concerned with an 

 intermediate of the conversion pathway of tryp- 

 tophan to niacin indicated that dietary trypto- 

 phan is not an efficient niacin precursor for this 

 species (Poston and DiLorenzo 1973). Addition- 

 ally, a low ratio of the two liver enzyme activities 

 exists in lake trout, rainbow trout, Atlantic salm- 

 on, and coho salmon compared with terrestrial 

 vertebrates (Poston and Combs 1980). 



LIPIDS 



Optimal Dietary Lipid Concentrations and 

 Protein-to-Energy Ratios 



Optimal dietary lipid concentrations for inclu- 

 sion in formulated feeds for fishes involve con- 

 sideration of several factors. The minimal die- 

 tary lipid concentration that maximizes dietary 

 protein available for growth rather than energy 

 (i.e., protein sparing) may be excessive for diet 

 incorporation if fish are being cultured as a lean 

 product for human consumption, or if freezer 

 storage space is unavailable to retard develop- 



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