Sexual cycle of white bream, Blicca bjoerkna (Actinopterygii, Cypriniformes, Cyprinidae), from three sites of the lower Oder River (NW Poland) differing in temperature regimes

Background. One of the largest European populations of white bream, Blicca bjoerkna  (Linnaeus, 1758), can be found in the estuary of the Oder River. This fish is not only very abundant in this area but also attains sizes that have no match in other areas of central Europe. In search for the clues behind such reproductive success we decided to study the annual development cycle of gonads of white bream from three sites in the lower Oder River, north-western Poland, differing in temperature regimes (depending on their position and the distance from the discharge outlet of the Dolna Odra Power Plant). 
Materials and methods. White bream individuals were obtained from the three sites as bycatch of commercial fishing in 2009 and 2010. Three sites were sampled: (1) the Oder River above the power plant, (2) the warm-water canal with post-cooling water discharged from the power plant, and (3) Lake Dąbie, 20 km below the warm-water canal. The fish age was determined as 2+ through 9+. In total, 506 females and 190 males were designated for histological analyses. The analysis of the annual cycle of gonad development was performed in both sexes using histological methods. A standard paraffi n technique and Heidenhain’s iron hematoxylin staining were used.
Results. In the Oder River, spawning of white bream lasted from early May to late June. In Lake Dąbie, it extended until the beginning of July. The spawn was laid in 2 or 3 portions. In the warm-water canal spawning began one month earlier, in April. The bream males from the thermally unaffected Oder River were ready for reproduction approximately one month earlier than the females and maintained their reproductive potential similarly to the females. Males from the warm-water canal became sexually mature two months earlier (February) than those from the river above the power plant. 
Conclusion. In waters with elevated temperature, gametogenesis of white bream occurs without problems and the   fish exhibit a typical pace of growth depending on the temperature. In the perspective of climate warming, white bream will be able to maintain its status of a common species in the natural waters of the region.