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...

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