Sodium uptake rates were measured in wild and hatchery reared Norwegian brown trout (Salmo trutta) exposed to media of a range of pH. Sodium uptake was strongly dependent on external acidity, being reduced in media of low pH. Wild fish from the naturally acid R. Tovdal (S. Norway) were more tolerant of acid media than hatchery reared fish. The effects of increasing external sodium concentrations were strongly influenced by low external pH. The results are discussed with respect to brown trout population decline in certain areas of S. Norway.

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