Sexual Cycle of the Blue Bream (Ballerus ballerus) from the Lower Oder River and Dabie Lake (NW Poland)

Histological studies of the blue bream (Ballerus ballerus L., 1758) (Actinopterygii, Cypriniformes, Cyprinidae) gonad maturation have not been conducted to date. The aim of this study was to analyse the annual gonad development cycle of the blue bream from the Oder River and Lake Dabie in north-western Poland. The spawning period of the blue bream, a single-portion spawner, was short, lasted approximately two weeks and occurred in the first half of April in Lake Dabie, and in the second half of April in the Oder River. The blue bream male gonads reached maturity stage IV before the winter and remained at this stage until early spring. The gonads contained cells at all spermatogenetic stages and the tubule lumen was filled with numerous spermatozoa. The males from Lake Dabie finalized their spermatogenesis in April slightly faster than those from the Oder River. Some individuals had gonad filled with large amounts of spermatozoa even in May and