The study focuses on the changes of fish community and population parameters (abundance, biomass, growth rate) in Lake Drūkšiai as the cooler of the Ignalina Nuclear Power Plant. Results of investigations before launching the INPP and in different stages of its exploitation as well as the impact on ecological processes of the ecosystem of the cooler, including biological productivity, are generalised. Ichthyological investigations in different aquatic areas of the lake in 2005–2007 revealed rather significant changes in fish species diversity and community structure as a result of changes in its thermal regime and intensive anthropogenic eutrophication. The tendencies of changes in fish diversity and community structure are revealed. Fish growth rates in different aquatic areas of the lake are estimated. Roach and perch populations served as an example to demonstrate higher growth rates in the “warm water” zone of the lake as compared with the “cold water” zone. As regards fishery, Lake Drūkšiai continues to be a high-productivity water body with intensive angling, but insufficient commercial fishing. In 2007, fish stocks in the lake averaged 671.78 t and the commercial fish catching limit was ca. 67.180 t, or 18.5 kg/ha. Our investigations show that fish stocks in Lake Drūkšiai have decreased by 9% compared with the results of earlier investigations.

Keywords: Lake Drūkšiai – cooler of the Ignalina Nuclear Power Plant, fish community, population parameters and their changes