Lake Restoration by Fish Removal: Short- and Long-Term Effects in 36 Danish Lakes

During the past 10-15 years removal of plankti- and benthivorous fish (mainly roach, Rutilus rutilus, and bream, Abramis brama) has commonly been used as a method to improve the ecological quality of Danish lakes. Here, we examine the general and long-term effects obtained after the removal of 41-1360 kg fish ha(-1) in 36 mainly shallow and eutrophic lakes. In lakes in which less than 200 kg fish ha(-1) were removed within a 3-year period only minor effects were observed, but at higher removal rates both chemical and biological variables were markedly affected. The concentrations of chlorophyll a (Chla), total phosphorus (TP), total nitrogen (TN), and suspended solids (SS) decreased to 50-70% of the level prior to removal. The most significant and long-lasting effects were found for SS and Secchi depth, whereas the most modest effects were seen for Chla. This probably reflects an efficient and persistent reduction of the bream stock which reduced resuspension and SS, while the biomass of roach returned to former levels, decreasing the zooplankton grazing with less control on Chla. Total algal biomass also declined after fish removal, particularly that of cyanobacteria, whereas the biomass of cryptophytes increased, indicating enhanced grazing pressure by zooplankton. The abundance and species number of submerged macrophytes increased in the majority of the lakes. For most variables the effects of the fish removal were significant for 6-10 years, after which many lakes tended to return to pre-restoration conditions, probably mainly because of consistently high external and internal phosphorus loading. We conclude that a sufficiently extensive removal of plankti- and benthivorous fish is an efficient tool to create clear water; however, repeated fish removal is presumably required to obtain long-term effects in the most nutrient rich lakes.