Annual development cycle of gonads of bream (Abramis brama L. 1758) females from the lower Oder River sections influenced differently by the heated effluents from a power station

Gonads of female bream from three locations with varied temperature-influenced water from power plant cooling water effluents were analysed: (i) from the Oder River (above Gryfino, with no influence of the post-cooling water effluents from the Dolna Odra' power plant, NW Poland), (ii) from Lake Dbie, below the canal receiving the heated effluents from the power station, and (iii) from downstream the effluent canal. Histological analysis indicated that in ambient conditions, the spawning season began at the study sites at the end of April and lasted through May and part of June. The initiation of gonad restoration (beginning of vitellogenesis, stage 3) was observed at the end of July, both in the Oder River and Lake Dbie, and lasted throughout September. The gonads were at stage 4 in males from October to April and, in a few females, to May. The annual cycle of gonad development was similar in the female bream from both sites. The microscopic images of the oocytes were in the previtellogenesis and vitellogenesis at the two sites. The maximum diameter of the oocytes from the Oder was 909.7m, while oocyte diameter of fish from Lake Dbie was 1018.4m. Oocytes not expelled during spawning season (May to August) were seen to undergo atresia. Female gonads from the heated effluent canal did not show any anomalies and the oocytes from the few fish caught in the winter were similar to those from the breams in other locations. Adult bream avoided excessive temperatures in the power plant effluent canal in the spring and summer, whereas juveniles were found in great numbers. In the spring, no female bream with mature gonads were caught in the cooling water effluent canal.