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Fish status in terms of 'good', 'sparse', 'lost', 'never had fish', has previously been reported for several thousands of lakes in southermost Norway. In more than a thousand of these lakes pH and conductivity have also been measured. These data have been used to establish a relationship between pH and fish status (brown trout). It is estimated that a uniform pH increase of 0.2 units will result in status changes from category 'lost' to 'sparse' in 12~o of the lakes (27% of the lakes which have lost the fish population). Altogether 21 To of the lakes are predicted to change to a better category. We have used a fixed pH shift in order to make the approach applicable. This is a rather drastic simplification since the lakes will respond very differently to a reduction in S deposition depending on the original acidity and a number of other factors. The limitations of the approach and an alternative method used by Chester (1982) are discussed in detail.Water, Air, and Soil Pollution 23 (1984) 97-113. 0049-6979/84/0231-0097502.55.

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