Lake Võrtsjärv is a large (area 270 km2), shallow (maximum depth 6.0 m) and turbid eutrophic lake in which eel Anguilla anguilla (L.), pikeperch Sander lucioperca (L.), northern pike Esox lucius L. and bream Abramis brama (L.) are the main commercial fishes, while perch Perca fluviatilis L. and burbot Lota lota (L.) are of secondary economic importance. High water level of the lake has a strong positive influence on the abundance and total catches of fish. Constant low water level and severe winters cause hypoxia and consequent fish kills. In winter, fish movements are negatively affected by oxygen content with fishes reducing their activity to economize on the use of oxygen. During March 1996, when the oxygen level was extremely low and stayed for a long time below 1 mg O2 dm3 throughout the water column, fishes had predicted this hard situation two months earlier. In this year the mean CPUE was 3 times lower than in winters of normal oxygen concentration. The present investigation shows that the CPUE decreases rapidly when the oxygen 1 m below the ice falls to below 4 mg O2 dm3. In winter, eel moves out of the bottom mud before the oxygen level falls below 0.5 mg O2 dm3 near the bottom. It either emigrates from the lake or else can be easily preyed upon by pike. Due to the extremely low water level in 1996-1997, the southern end of L. Võrtsjärv was full of macrophytes. Therefore, the normal migration of pikeperch, which prefers open water, was seriously hindered by dense vegetation and its conditions for spawning were disturbed.