Abstract
Time-series of the species caught in a complex of four fyke nets that were monitored and recorded by a team of volunteer fishers provided the basis for identifying trends in the changes of fish assemblages during the 1995-2006 period. The fish assemblages exhibited strong seasonality with herring, Clupea harengus L. domination during spring. In the summer season, juvenile pikeperch, Sander lucioperca (L.) and ruffe, Gymnocephalus cernuus (L.), were dominant in number. The relative abundance of eel, Anguilla anguilla (L.) exhibited a clearly decreasing trend due to fishery exploitation and the lack of stocking. The decreased share of the largest pikeperch individuals suggested that this species is being overexploited. Ruffe exhibited a strong decline more probably due to strong impact from numerous black cormorants. Increased abundance was observed in the case of roach, Rutilus rutilus (L.), flounder, Platichthys flesus (L.) and round goby, Neogobius melanostomus (Pall.), a new species in ecosystem.