LOSSES AND RECOVERIES OF FISH POPULATIONS IN ACIDIFIED LAKES OF SOUTHERN FINLAND IN THE LAST DECADE

Acid-induced fish damage in small lakes in southern Finland was studie d in a fish status survey of eighty lakes from 1985-1987. Later, twent y of these lakes were selected for further monitoring. A sampling of t hese lakes from 1988-1989 showed that the decrease in some perch (Perc a fluviatilis L.) and roach (Rutilus rutilus L.) populations still con tinued. The results from the same lakes in 1992 showed that successful reproduction had taken place with many of the perch populations that had been close to extinction in 1985. in contrast, no signs of recover y in the roach populations were detected The explanation for the appea rance of new cohorts of perch could have been the decrease in acid dep osition but the exceptional hydrological conditions of winters in the early 1990s may also have affected them The different responses of the perch and roach populations were interpreted as a consequence of the different sensitivity of these two species to acidification. Even a sl ight improvement in the water quality has resulted in the appearance o f strong new year-classes of perch, but not of roach. Therefore, more improvement in water quality is needed until a sensitive species like roach can reproduce again.