HYDROACOUSTIC MONITORING OF THE DISTRIBUTION, DENSITY AND THE MASS-REMOVAL OF PELAGIC FISH IN A EUTROPHIC LAKE

A large scale biomanipulation of pelagic fish by trawling was started in the eutrophic Enonselka basin (26 km(2)) of Lake Vesijarvi to impro ve the water quality which had remained poor in spite of the terminati on of nutrient loading. The distribution and density of the fish were studied by hydroacoustics before and during the removal. The initial a nnual fish density varied between 13 000-21 000 fish ha(-1) in the stu dy area in August 1984-89. The mass-removal of the fish by pelagic tra wling took place in 1989-1992. The catch varied annually between 64 an d 92 kg ha(-1). Roach (Rutilus rutilus (L.)) and smelt (Osmerus eperla nus (L.)) accounted for c. 85% of the weight of the catch. The mass-re moval decreased the pelagic fish density in the Enonselka basin during the trawling. An increase in the density was observed after the trawl ing ceased in these years, and the initial density level was reached w ithin one month. The density level after mass-removal remained high co mpared with oligotrophic lakes. The pelagic fish had a diurnal ascendi ng trend with the decreasing light intensity in August, and the fish w ere significantly (p < 0.01) higher in the water mass in temperature n on-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 num erically dominant throughout the whole study period.