The effects of food-web management on fish assemblage dynamics in a north temperate lake

There was an intensive trawl fishery in the Enonselka basin in Lake Vesijar vi (southern Finland) in 1989-1993 and thereafter the fishing intensity dec lined. The fish assemblage dynamics were studied both during the years of i ntensive trawling and for 3 years afterwards. Roach Rutilus rutilus dominat ed the fish assemblage before the mass removal. The intensive trawling effe ctively diminished the roach stock and the present fishing intensity has be en sufficient to prevent its recovery. There were significant decreasing tr ends in the gillnet cpue (catch per unit effort) of bream Abramis brama (L. ) and bleak Alburnus alburnus (L.) while increasing trend was observed in c pue of ruffe Gymnocephalus cernuus (L.) and vendace Coregonus albula (L.). Perch Perca fluviatilis L. and pikeperch Stizostedion lucioperca (L.) showe d variations in cpue but no trends existed. The proportion of cyprinids dec reased while that of percids and coregonids increased in the catches. The c hanges in the fish assemblage were induced by the intensive fishing and by the concomitant decline in the ecosystem productivity. (C) 1999 The Fisheri es Society of the British Isles.