Aspects of intermediary metabolism in salmonid fish

The basic pathways of intermediary metabolism that have been investigated so far in salmonid fish are very similar to those occurring in mammals and other animals, though important differences do exist with regards to the presence/absence and physiological importance of some pathways. Salmonids like many other fish, have relatively high dietary requirements for both protein and essential amino acids which in some cases are more than twice those of rat, chicken and pig (see reviews by Mertz, 1972; Cowey, 1975, 1979; Ogino, 1980). Carbohydrates are relatively poorly utilised by fish and the evidence suggests that proteins together with lipids are the major sources of energy (Atherton & Aitken, 1970; Pieper & Pfeffer, 1978; Cr6ach & Serfaty, 1974; Mommsen et al., 1980). This contrasts to the situation in omnivorous mammals where, under normal nutritional conditions, protein catabolism is of little significance in supplying energy whereas carbohydrates and lipids are important energy sources. Phillips (1969) has suggested that 70% of dietary calories in trout feed is from protein, thus a greater percentage of dietary protein is metabolised for energy, rather than utilized for body protein synthesis. Since the end produce of N-metabolism in teleosts is ammonia rather than the more energy costly urea or uric acid (as in mammals, birds and reptiles), fish do not derive the same amounts of energy from dietary constituents as do mammals and hence food protein may have a higher metabolisable energy than carbohydrates. The energy requirements of fish (being poikilothermic) are lower than mammals which expend much energy to maintain their body temperature. Since fish body temperature varies with the water temperature, then reaction rates and occurrence of some pathways within the fish are affected by the environmental temperature. This article is a somewhat brief attempt to describe aspects of the intermediary metabolism of amino acids, carbohydrates and lipids that have been reported for salmonids (predominantly).