The greatest development of bivalves in the Goczałkowice Reservoir occurred at the turn of the 1960s. In 1983 and 1992 detailed investigations of the exposed bottom of the reservoir were conducted. In 1983, in the north-eastern zone of the reservoir 2.23 individuals with a biomass of 26.9 per 1 m2 were recorded, while in the south-eastern zone there were 0.77 individuals with a biomass of 20.2 g per m2 . In 1992, in the north-eastern zone of the reservoir there were 1.68 individuals with a biomass of 37.6 g per m2. The population of bivalves, when compared with that in the year 1983, had diminished there by about 30%.