The optimum dietary protein to energy (P:E) ratio for rapid and efficient gain of juvenile Tilapia aurea was shown to fall with increasing size of fish. The optimum concentration of protein and energy also fell with growth. A diet providing roughly 56% protein at 4,600 kcal/kg digestible energy (gross energy adjusted for indigestible fiber) with a P:E ratio of 123 mg potential/kcal produced highest gains of fry (2.5 g). Larger fish (7.5 g) grew most rapidly when fed a 34% protein, 3,200 kcal/kg diet with a P:E ratio of 108. Apparent feed conversion (grams of feed offered/grams of fish weight gain) was superior on diets having lower P:E ratios and was best on the 34% protein, 3,200 kcal/kg diet. Linear regression analysis indicated highly significant differences in average fish weight, condition [10(5) x weight (g)/total length (mm)3], and feed conversion efficiency attributable to changes in either protein or energy concentration. Significant interaction between protein and energy was also demonstrated. Condition and level of carcass fat were high on all diets which produced good growth rates and were inversely related to P:E ratio. Moisture and ash were inversely related to carcass fat. No trend was established for carcass protein.

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We studied the effects of water temperature, density as measured by density index (DI), and food deprivation (alternating cycles of daily feeding and fasting) on dorsal fin erosion of steelhead Oncorhynchus mykiss through a factorial experiment lasting 22 weeks. Dorsal fin index (DFI) correlated inversely with age, length, and weight of fish and was significantly affected by crowding (i.e., DI). Increasing DI (from 0.0 to 0.5) also improved feed conversions and slightly depressed survival, but DI had no significant effects on weight gain, condition (K ), or carcass composition. Water temperature (10ЊC or 15ЊC) had a strong and significant effect on DFI; colder water improved DFI while slowing body growth. Low temperature treatments improved survival slightly and altered carcass composition by increasing moisture at the expense of fat and protein.The effects of density and temperature were also strongly interactive. Food deprivation slightly improved final DFI at low DIs, but greatly reduced the average size of fish affected by severe fin erosion. Food-deprived fish exhibited carcass composition trends similar to fish reared in 10ЊC water. Feed conversion improved significantly with alternating fasting and feeding cycles. Histological evaluation proved inconclusive for physical fin nipping evidence or for the presence of detrimental microorganisms.

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