The nutritional biochemistry of proteins and amino acids is briefly discussed; the dietary protein requirement (expressed as g/MJ digestible energy) is higher than that of a mammalian carnivore (the cat) which, in turn is higher than that of mammalian omnivores. The high protein requirement of the cat, relative to warm blooded omnivores is a requirement for non-specific N, that of salmonids is for both essential and for non-essential amino acids. Net retention of dietary N by fish is quantitatively similar to that of omnivorous birds and mammals (40-50%) so that up to 60% of assimilated N is excreted, largely in soluble form, and is available for eutrophication. It is difficult to see how this figure can be reduced by conventional dietary means. In mammals much of the loss of assimilated amino acids may be associated with protein turnover especially in muscle, quantitatively the most important tissue. Protein turnover in fish muscle is low, much of the protein synthesized there being retained and direct oxidation accounts for much of the loss of assimilated amino acids. The main P reservoir in fish tissues is flesh; few data are available on P turnover in fish under intensive cultivation. P flux in fish in natural ecosystems is low - less than 1% body P per day. Ecological studies indicate higher rates of P output in young, than in older, fish. However, under farming conditions about 90% of the feed is given to large fish (>100g). Of the P egested by salmonids under farming conditions about 30% is in particulate form and is probably not immediately available to plants: most of the soluble P (comprising about 60% of the total egested) appears to be biologically available to plants.

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