WHAT CAN PERCH POPULATION-DATA TELL US ABOUT THE ACIDIFICATION HISTORY OF A LAKE

In this study an attempt was made to describe the acidification histor y of a lake through the changes in the population structure and the gr owth of perch (Perca flaviatilis L.). The lake was generally considere d fishless at the beginning of the 80s. Our etches in 1985 consisted m ostly of large, old perch but there was also a younger year-class pres ent born in 1983. The oldest perch of the etch were twenty years old T he slow growth of old perch suggests that the density of the populatio n was quite high in the late 60s and early 70s. We caught only a few p erch born in the 70s, suggesting that reproduction was more or less co mpletely unsuccessful then. The few that we did etch showed extremely high growth rates. This was interpreted as due to good feeding conditi ons, such as no competition for food in a sparse population. We conclu ded that the acidification-induced decrease in the perch population of Lake Orajarvi took place mainly during the 70s. This is supported by paleolimnological and modelling studies. In 1992, a higher number of 2 + perch were caught indicating successful reproduction in 1990. Since then, we have sampled the lake annually and recorded 0+ and 1+ fish ev ery year. Perch in these new year-classes are clearly growing slower t han those born in the last decade. This suggests that the population d ensity has essentially increased. Successful reproduction in perch in recent years has been possible because of improvements in water qualit y due to decreased acid deposition and/or exceptional hydrological con ditions during the 1990s.