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Twenty-one semipurified diets were formulated to determine the optimum protein:energy ratio (P/E) for channel catfish fingerlings. Seven crude protein levels and three energy levels at each protein level were utilized. The protein, lipid, and digestible carbohydrate sources were hexane-extracted whole egg powder, salmon-corn oil mixture, and white dextrin, respectively. After an initial 2 week conditioning period, the experimental diets were fed in triplicate to groups of 30 +/- 3 channel catfish fingerlings weighing 205 +/- 10 g and kept in 110 liter flow-through aquaria at 26.7 +/- 1.1 degrees. The diets were fed at a rate equalling 3% of the total wet body weight/day divided into three equal feedings and adjusted weekly for 9 weeks. An approximate fivefold increase in body weight was observed. These data indicate that the concept of P/E ratios must be restricted to diets containing adequate levels of protein and total energy. Based on average daily gain, diets containing energy levels between 275 and 341 kcal/100 g had an optimum P/E ratio of approximately 88 (24-36% crude protein). Diets containing 24% crude protein and 275 kcal/100 g diet appeared to be utilized more efficiently in terms of percent protein deposited than diets containing higher crude protein levels.

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Two experiments were conducted in aquaria to determine the minimum dietary selenium requirement of fingerling channel catfish (Ictalurus punctatus). Casein-gelatin diets containing graded levels of supplemental selenium (as Na2SeO3) ranging from 0 to 15 mg/kg were fed to catfish for 15 weeks in experiment 1 to broadly define their selenium requirement and toxicity levels. Although growth of catfish was affected by dietary selenium level, significant differences in weight gain were not easily discernible due to variability among the groups of fish. Weight gain data generally indicated that the basal diet containing 0.06 mg Se/kg diet caused growth depression, and a supplemental selenium level of 15 mg/kg also caused a reduced growth response, which indicated selenium toxicity. Selenium concentrations in edible muscle tissue increased almost linearly with increasing dietary selenium levels. Liver and plasma selenium-dependent glutathione peroxidase (Se GSH-Px) activities indicated the selenium requirement of fingerling channel catfish was between 0.1 and 0.5 mg Se/kg diet. In experiment 2, casein-gelatin diets containing incremental levels of supplemental selenium were fed to catfish for 14 weeks to more precisely determine their minimum dietary selenium requirement. Growth data and liver and plasma Se GSH-Px activities indicated that the minimum selenium requirement of fingerling channel catfish fed adequate vitamin E was 0.25 mg Se/kg dry diet. Based on these data, it appears that selenium supplementation of commercial catfish feeds is warranted.