Lakes Gosławskie and Ślesińskie are two out of five Konin lakes which receive a discharge of heated effluents from “Pątnów” and “Konin” power plants. In these two lakes bream females are characterized by single portion spawning. 25% of the females tend to batch spawning. Lake Gosławskie in warmer than Lake Ślesińskie: in about 46% of bream females from this lake percentage of maturing oocytes in the ovaries was higher than in Lake Ślesińskie (and than in natural conditions), 20-38.8%. Increased temperature in Lake Ślesińskie caused intensive development of eggs in the ovaries of young females, 3-7 years old, so that they passed from the class of moderately growing to the class of rapidly growing. In Lake Gosławskie maturation of eggs in the ovaries of 4-6 year old females became even more intensive, so that they passed from the class of slowly growing to the class of more than moderately or rapidly growing ones.