Responses of plankton and fish communities to mass removal of planktivorous fish in a two-basin lake in southern Finland

Responses of plankton and fish communities to mass removal of cyprinid fishes were studied in a two-basin lake, Aimajarvi, in southern Finland during 1997-2001. Fish were removed from the lake with fyke nets and seining. By spring 2001, the cumulative catches were 276 kg ha(-1) in the northern and 226 kg ha(-1) in the southern basin. Roach dominated the catches of both basins. From summer 1997 to 2001 the total biomass of phytoplankton decreased from 10-30 to 5-13 mg l(-1) in the more eutrophic northern basin and from 5-19 to 2-4 mg l(-1) in the southern basin, respectively. The occurrence of maximum biomass of blue-green algae moved from July to August towards autumn during the study period. The mean biomass of cladoceran crustaceans in July to August showed a slightly increasing trend from 100-150 to 150-200 mug C l(-1). The biomasses of Daphnia spp. and Bosmina coregoni were higher in 2000 and 2001 compared to earlier years. The annual mean total catches of multimesh gillnets, dominated by roach, were 3.9-5.1 kg net(-1) in the northern and 1.8-2.6 kg net(-1) in the southern basin. A slight decrease took place in the gillnet catches in the southern basin but not in the northern. The increased growth of young perch and roach in the last years of the study period was attributed to the thermal conditions rather than to the fish removal.