Perch production in forest lakes: the contribution of abiotic and biotic factors

The biomass, growth and production of perch (Perca fluviatilis (L.)) in five humic forest lakes in southern Finland were estimated and the abiotic and biotic factors contributing to the between-lake differences were explored. The average perch biomass in the study lakes varied from 13.1 to 45.6 kg center dot ha-1 and annual production from 6.0 to 16.5 kg center dot ha-1. The highest biomass and production were found in Lake Majajarvi with highest water colour and nutrient concentration. Variability in fish density rather than in individual growth rate seemed to be the main factor behind the between-lake differences in perch production. No dependence of perch production on water transparency was found, because transparency had a stronger effect on individual growth than on fish density. The length increment of perch in their second year increased significantly with water transparency but at older ages no such dependence was found. At age of 4 years, the growth rate of perch decreased with perch density, indicating intraspecific competition for benthic food resources. At age of 7 years, the growth rate increased with increasing roach (Rutilus rutilus (L.)) abundance, suggesting that roach served as a resource for piscivorous perch. The effects of predatory top-down control on perch populations were probably obscured due to the variable size and water quality of the study lakes.