WATER-QUALITY REQUIREMENT OF ATLANTIC SALMON (SALMO-SALAR) IN WATER UNDERGOING ACIDIFICATION OR LIMING IN NORWAY

Atlantic salmon are severely affected by acidification in Norway. Wate r quality criteria for the salmon have to be based on the most sensiti ve stage, the smelt stage. The sensitivity to acidic water increases e normously during smelting, the seawater tolerance being especially vul nerable. Even moderately acidic water (pH about 6) with low inorganic monomeric aluminium (LAI) concentrations (<20 mu g L(-1)) and shea-ter m episodes may be harmful. Mixing zones in limed or unlimed rivers may also represent a problem for seaward migrating smelts. In limed salmo n rivers, the national liming goal has been increased to pH 6.5 during smelting (1 February to 1 July) and to 6.2 the rest of the year as a result of our experiments. In contrast to what has been found for brow n trout, salmon strains originating from watercources undergoing acidi fication were not more tolerant than those from non-acidic watercourse s. At the moment no such ''tolerant'' strains are available for restoc king limed rivers in Norway.