Studies on the nutrition of marine flatfish. Utilization of various dietary proteins by plaice (Pleuronectes platessa)

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Abstract
1. Five proteins, freeze-dried cod muscle, a white fish meal, a yeast-protein concentrate, a soya protein and a fish-protein concentrate (a solvent-extracted fish meal) were given separately, as single sources of protein in diets containing 500g crude protein/kg, to small plaice (Pleuronectes platessa) of mean initial weight 12–15g. Weight gain and net protein utilization (NPU) were examined.2. Even at these high protein intakes, significant differences in growth rate between fish given the different proteins were evident. These findings were reflected in different NPU values between the proteins. This position contrasts with that in the rat where proteins which differ markedly in NPU at low protein intakes have similar NPU values when given at high dietary levels.3. Coefficients of apparent digestibility measured on larger plaice (150–200g) were high for all the proteins used except soya-bean meal, which had a digestibility coefficient of 0.68.4. At high protein intakes there was no clear relationship between NPU values and essential amino acid content (as measured chemically) of the proteins tested.5. The preparation of protein concentrates from fatty fish by the use of detergents is described. One of the preparations obtained gave weight gains and NPU values similar to those obtained with commercially available feedstuff proteins.