The aim of this study was to detect the most important factors structuring the fish communities of not or extensively exploited mesotrophic lakes in Central Europe (Brandenburg, Germany). The basic hypotheses underlying the study were as follows: i) perch (Perca fluviatilis) has a higher proportion of fish biomass compared to cyprinids, ii) age-0 perch use zooplankton as a food resource until the end of summer, show comparatively good growth and thus are not forced into a competitive juvenile bottleneck, iii) perch switch to piscivory early during their ontogeny and iv) the number of cyprinids - the main competitors for food resources - is significantly reduced by these piscivorous perch. Further issues of this study were to estimate the influence of overwinter mortality on age-0 fish, larval mortality and to test whether the biomass of perch is positively related to structural complexity.
The trophic state of a lake was shown not to be the most important factor structuring the fish community. The expected dominance of perch was only found in the largest (8.04 kmē) of all five lakes studied. The fish biomass in the four smaller lakes (0.09 - 0.53 kmē) was dominated by cyprinids (i.e. roach, Rutilus rutilus).
Age-0 perch were not forced to compete for benthic food resources with older conspecifics in the course of summer as they continued to feed on zooplankton until September. At the same time the food niche overlap with age-0 roach was relatively low.
Age-1 to age-3 perch showed low growth rates accelerating after reaching a size of 10-15 cm. Juvenile perch from the small lakes grew best in lakes with a high proportion of submerse macrophytes. However, macrophyte density was negatively correlated with perch biomass.
As predicted the majority of adult perch were piscivorous, but they selectively consumed smaller perch and not cyprinids. The amount of benthos consumed by adult perch was highest in the largest lake. Adult roach had mainly consumed plant material (macrophytes and algea) and bream (Abramis brama) had mainly consumed chironomids.
Overwinter mortality seems to be of minor importance given the climatic conditions of Central Europe. The estimated mortality rate for perch larvae was high in one of the smaller lakes (08.05. - 10.06.1996; z = 0.197) but could not be estimated in the others. The mortality rate of cyprinid larvae could not be estimated.
The results of this study and published data reveal a positive relationship between lake area and perch biomass. Two hypotheses are discussed in this context.