TEMPORAL VARIATION IN LAKE BALATON AND ITS FISH POPULATIONS

Dramatic changes have occurred in Lake Balaton in the last 40-45 years , Variations in water quality have resulted in a decrease in fish and invertebrate diversity. Fish assemblages have changed in response to h uman-induced eutrophication. environmental perturbations, introduction of exotic fish species, over-fishing and species-habitat interactions . Fish biomass and productivity first increased during the early 1970s with enhanced primary production, then decreased with the reduction o f exploitation since the late 1970s and induced intra- and interspecif ic competition. Previous stable populations began to oscillate, and th e majority of stocks became fragile by the mid-1980s. Extinction of na tive fish and invasion of new alien species alternate. Alien species h ave transformed the food webs, causing long-lasting ecological changes . Reduced and non-selective exploitation has resulted in higher fish d ensity and lower production rates. Recent studies have revealed differ ences in the carrying capacity of fish between the littoral zone and t he open water areas.