The Roach Population in the Hypertrophic Bautzen Reservoir: Structure, Diet and Impact on Daphnia galeata

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.