PURIFIED DIET DEVELOPMENT AND REEVALUATION OF THE DIETARY-PROTEIN REQUIREMENT OF FINGERLING RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Two experiments were carried out to develop a purified diet, and to examine the protein requirement of fingerling rainbow trout. Four replicate tanks (50 fish each) of trout were assigned to a commercial salmon diet or a purified diet containing 30, 35, 40 or 45% protein; and three replicate tanks (40 fish each) to a diet containing 10, 15, 20, 25 or 35% protein (not counting crystalline dispensable amino acids which replaced casein protein to vary the protein level). Trout were fed three times a day for 6 weeks, and weight gain and feed/gain ratio were monitored. No significant differences were found in weight gain or feed/gain ratio among 10.5-g fish fed diets containing 30, 35, 40 and 45% protein. Fish fed these diets gained about 80% as much weight as fish fed a commercial salmon diet which produced gain of 19.5 g/fish per 6 weeks. Weight gain increased with the increasing levels of casein up to 25% and the breakpoint was found at 24% intact protein, indicating that the levels of indispensable amino acids (IDAA) supplied by the 24% intact protein were sufficient to meet the requirements of trout for IDAA. Results suggested that energy sources used for trout diets play an important role in the determination of the protein requirements, and that the conventionally established protein requirement (40%) includes protein required to meet IDAA requirements (24%) and that required to meet energy needs (16%).