Assessment of damage to fish populations in Norwegian lakes due to acidification

We assessed the current status of fish populations (N = 39 093) in 32 303 Norwegian lakes in relation to the effects of acidification. The number of lost and reduced populations of the 6 most common species of fish in Norwegian lakes greater than 3.0 ha ($0.03\ {\rm km}^{2}$) in acidic areas was estimated to be 9630 and 5405, respectively. Brown trout, which is the most widespread and common species of fish in Norwegian lakes, has suffered the most severe damage. A manual delineation method gave a total affected area of 1417 km2in 1940, which had risen to 51 530 km2in 1990. Acidification has affected fish populations in southernmost Norway most seriously. The damage was also estimated in grid cells of 12 × 12 km, which was based on an estimated damage index for each cell. This analysis yielded a total damaged area of 84 166 km2in 1990 or about 25% of the total land area of Norway.