Ecological consequences of a manual reduction of roach and bream in a eutrophic, temperate lake

A biomanipulation experiment was carried out in the eutrophic lake, Frederiksborg Slotsso (Denmark). During 1987 and 1988, densities of roach (Rutilus rutilus) and bream (Abramis brama) were reduced, using seine and pounding nets, and large-sized perch (Perca fluviatilis) were added instead. Nutrients, oxygen, phytoplankton, zooplankton, fish and zoobenthos were measured two years after the manipulation and compared with results obtained two years before the manipulation. A total amount of 6524 kg wet weight of roach and bream was removed. Roach and bream constituted 45% of the total fish biomass after the reduction, compared with 78% before the manipulation. Recruitment of roach decreased, and mortality rates of young-of-the-year perch were lower after the fish reduction. After the manipulation, decreases in phytoplankton biomass coincided with increases in zooplankton biomass during spring and autumn periods, although, the mid-summer level of the biomass of cyanobacteria did not change. Inorganic nutrients generally increased, but no significant changes were found, either in the oxygen budget or in the community structure or quantitative distribution of zoobenthos after the fish manipulation. Although the effects of the fish manipulations were not as pronounced as those found in lakes with lower nutrient regimes, the results indicate positive changes in the water quality. Nevertheless, it is probably necessary to continue a fish reduction programme to maintain or further improve the water quality. (DBO)