INTERMEDIARY METABOLISM IN FISH WITH REFERENCE TO OUTPUT OF END-PRODUCTS OF NITROGEN AND PHOSPHORUS

The nutritional biochemistry of proteins and amino acids is briefly di scussed; 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 require ment for non-specific N, that of salmonids is for both essential and f or nonessential amino acids. Net retention of dietary N by fish is qua ntitatively similar to that of omnivorous birds and mammals (40-50%) s o 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 tissu e. Protein turnover in fish muscle is low, much of the protein synthes ized there being retained and direct oxidation accounts for much of th e loss of assimilated amino acids. The main P reservoir in fish tissue s is flesh; few data are available on P turnover in fish under intensi ve cultivation. P flux in fish in natural ecosystems is low - less tha n 1% body P per day. Ecological studies indicate higher rates of P out put in young, than in older, fish. However, under farming conditions a bout 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 solu ble P (comprising about 60% of the total egested) appears to be biolog ically available to plants.