THE DIET AND GROWTH OF ROACH (RUTILUS-RUTILUS (L)) IN LAKE VESIJARVI AND POSSIBLE CHANGES IN THE COURSE OF BIOMANIPULATION

Previous studies have suggested that the roach Rutilus rutilus (L.) stock of Lake Vesijarvi is one of the main factors delaying the recovery of the lake after sewage diversion. This study is concerned with the documentation of the diet of roach in the lake. In total, 531 roach were examined. Both in the pelagial and in the littoral the roach had mixed diets in May and in September-October. The importance of zooplankton decreased and the importance of benthos and plants increased with increasing size of roach. In July, in the pelagic zone all sizes of roach fed exclusively on zooplankton (Bosmina spp.), while in the littoral zooplankton had the highest volume proportions only in the smallest (< 130 mm) roach. The frequent use of plant food and slow growth rate of large roach indicate a low availability of animal prey. As the fish densities decrease due to the mass removal taking place in the lake, the percentage of plant food in the diets of roach will probably decrease and the growth of roach will increase. Additionally, the tendency of the roach to migrate into the pelagic zone in early summer may be reduced, which would decrease their predation on the zooplankton.