AbstractThree size groups ofColossoma macropomumwere submitted to a 4‐week growth trial. Five nearly isocaloric (18.8‐21.0 kJ g−1) diets with protein concentrations ranging between 17 and 64 were administrated at a fixed, near satiation level. Maximum growth was 6.6, 3.6 and 1.9 g protein kg−0.8day−1for 5, 50 and 125 g fish, respectively. The protein requirement to achieve maximum growth decreased from 28.9 g protein kg−0.8day−1for 5 g fish to 11.7 g kg−0.8day−1for 125 g fish. Possibly because of its high growth rate.C. macropomumneeds a slightly higher dietary P/E ration (25.4‐27.9 mg protein kJ−1) to obtain maximal growth than most other fishes. The relation between protein ration and protein gain was studied by a quadratic regression model. In fish receiving protein rations equal or below rations resulting in maximal growth, protein ration and protein gain were almost linearly related. The model showed that the portion of the dietary protein which is digested decreases with increasing protein ration. Body protein content increased and body lipid content decreased with feed protein level. Fish fed a 17 protein diet deposited
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