Salt and water balance in salmon smolts.

INTRODUCTION Although the salmon is probably the best known euryhaline fish, for the first 2 or 3 years of its life it is stenohaline. The salmon becomes euryhaline only in the smolt stage, the process of 'smoltification' taking place in April or May in fish 2 or 3 years old, although a proportion of late parr can tolerate sea water. In recent years a more detailed picture has emerged of salt and water balance in both marine and fresh-water teleosts as the result of a number of studies with isotopic tracers. It is now known that the greater part of the salt influx in sea-water teleosts takes place through the body surface (Motais & Maetz, 1964; Potts & Evans, 1967; Evans, 1967) and not through the gut alone as had previously been assumed. This salt, together with salt drunk in sea water, is continuously excreted. These processes result in a large turnover of salt in marine teleosts, usually of the order of 10-20% of the total sodium and chloride per hour. In contrast, in fresh water the permeability to salt is low, and although some drinking may occur the salt turnover is low, usually |—1 % of the total body salt per hour. Studies of the process of adaptation of marine fish to fresh water and back again show that adaptation to different media is associated with both rapid and slow changes in permeability and indicate that although in some species exchange diffusion may be an important component of the salt fluxes in other species it is of minor importance (Motais, Romeu & Maetz, 1966). In the light of these developments the exchanges of sodium and chloride ions and water have been investigated in salmon smolts in both fresh water and sea water. For comparison similar measurements have been made on salmon parr where possible.