The roach population in the hypertrophic Bautzen Reservoir: Structure, diet and impact on Daphnia galeata

Abstract

The structure and diet of the roach ( Rutilus rutilus) population in the hypertrophic Bautzen Reservoir was examined from April to November 1998. Under the long-term impact of high predation pressure by piscivorous fish, a very heterogeneous population structure of roach had developed. Only a few age classes were dominant while other age classes were nearly absent. The proportion of males decreased with increasing age to 4% of the total abundance of one age class, which nevertheless seemed to have no negative effect on reproductive success. Food analysis revealed that the diet consisted of a high proportion of algae and macrophytes. The collapse of the Daphnia galeata population in early summer 1998 forced the roach to switch to benthic food resources [macroinvertebrates and fish: chironomids, molluscs and ruffe ( Gymnocephalus cernuus)] in early June. Total consumption of age-2 and age-4 roach, the two most dominant year classes, was calculated by a bioenergetics model. Additionally, consumption of age-0 roach was estimated by assuming a fixed daily food consumption rate. The results suggest that daily consumption by these age groups, which never exceeded 0.2% of total biomass of the D. galeata population, had a negligible impact on the population of daphnids in Bautzen Reservoir during the period studied.