The effects of cyprinid reduction on the fish community

Lake Ringsjon in southern Sweden was restored by biomanipulation by means of a reduction in zooplanktivorous cyprinids. The fish community in the three different Basins of Lake Ringsjon were followed between 1987 and 1994. In 1990, c. 100 tons (60% of the community) were removed from Satofta Basin, in 1988, c. 500 tons (80%) were removed from Eastern Basin and in 1992, c. 120 tons (50%) of cyprinids were removed from Western Basin. In all basins, the fish biomass was much lower after fish reduction than before, but only in Eastern Basin has the proportion of piscivores increased substantially (from 10% to 25-40%). Due to very high abundance of young-of-the-year following the fish reductions, the fish abundance has increased in all basins. The abundance of the young-of-the-year was negatively correlated to the abundance of piscivore perch at the 3 m depth. Although the young-of-the-year abundance was very high after the fish reduction, there are no clear evidences that this affects recruitment to the adult populations. The results are discussed in relation to critical fish biomasses and critical proportions of planktivory and piscivory to ensure a long lasting effect of fish reduction.