Short Communication Sodium Balance in Fresh-run Atlantic Salmon

Preliminary experiments have been carried out to determine the main features of salt balance in adult salmon immediately following transfer to fresh waters, both neutral and acidic. The processes of adaptation from hypo-osmotic regulation in sea water to hyperosmotic regulation in fresh water must involve many rapid adaptations of the gills, kidneys and endocrine systems. From the practical point of view these changes may affect the time spent by returning salmon in estuaries, where in Britain they are vulnerable to commercial netting. In addition, in a few rivers, the entry of salmon into fresh water may coincide with the most acidic episodes. Mass kills of fish were one of the factors which drew attention to the acid rain problem and have been reported in both Norway and Britain (e.g. Leivestad & Muniz, 1976; Crawshaw, 1984). This unfortunate coincidence of upstream migration and acidic episodes arises because of the high correlation between increased river flow and lowered pH, on the one hand and river flow and fish movements on the other, Gee (1980). Adult salmon, Salnto salar L., were obtained from coastal nets in sea water at Carnoustie, Angus (fish nos 6, 7, 8, 9, 10, 11, 17 and 27). These were maintained in the DAFS laboratory at Almondbank, Perth in a tank of recirculating sea water at 28 %o salinity, at 12°C. Other adult salmon were caught in tidal fresh water in the Tay estuary at Inchyra, 2 km south of Perth. Some of these fish, which still had 'sea lice', Salmonicola species, on their bodies were transferred to the sea water tank for at least 5 days before use (fish 4 and 5), while others were transferred to running fresh water from the river Almond (fish 1, 2, 16, 22 and 28). The salmon ranged in weight from 1-73 to 3-71 kg. In addition 2-year-old parr approx. 150g, reared in Almond river water were also used.