A large scale biomanipulation of pelagic fish by trawling was started in the eutrophic Enonselka basin (26 km 2 ) of Lake Vesijdrvi to improve the water quality which had remained poor in spite of the termination of nutrient loading . The distribution and density of the fish were studied by hydroacoustics before and during the removal . The initial annual fish density varied between 13 000-21 000 fish ha -' in the study area in August 1984-89 . The mass-removal of the fish by pelagic trawling took place in 1989-1992 . The catch varied annually between 64 and 92 kg ha -1 . Roach (Rutilus rutilus (L.)) and smelt (Osmerus eperlanus (L .)) accounted for c . 85% of the weight of the catch . The mass-removal decreased the pelagic fish density in the Enonselka basin during the trawling . An increase in the density was observed after the trawling ceased in these years, and the initial density level was reached within one month . The density level after mass-removal remained high compared with oligotrophic lakes . The pelagic fish had a diurnal ascending trend with the decreasing light intensity in August, and the fish were significantly (p < 0 .01) higher in the water mass in temperature non-stratified water in autumn than in stratified water in summer. The fish were somewhat deeper in the years of mass-removal than before it . Mass-removal did not affect the individual length of the echosurveyed fish . Fish smaller than c . 15 cm (TS < -44 dB ; mainly smelt) were numerically dominant throughout the whole study period .