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 history of a lake through the changes in the population structure and the growth of perch (Perca fluviatilis L.). The lake was generally considered fishless at the beginning of the 80s. Our catches in 1985 consisted mostly of large, old perch but there was also a younger year-class present, born in 1983. The oldest perch of (he catch were twenty years old. The slow growth of old perch suggests that the density of the population was quite high in the late 60s and early 70s. We caught only a few perch bom in the 70s, suggesting that reproduction was more or less completely unsuccessful then. The few that we did catch showed extremely high growth rates. This was interpreted as due to good feeding conditions, such as no competition for food in a sparse population. We concluded that the acidification-induced decrease in the perch population of Lake Orajärvi 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 every year. Perch in these new year-classes are clearly growing slower than those born in the last decade.This suggests that the population density has essentially increased. Successful reproduction in perch in recent years has been possible because of improvements in water quality due to decreased acid deposition and/or exceptional hydrological conditions during the 1990s.