The research was held in terms of Zaporozhian Reservoir (Dnipropetrovsk, Ukraine) from 2010 till 2014 year. For the first time in the last 50 years complex histological studies of reproductive system of bream from Zaporozhian Reservoir was conducted. The regularities of oogenesis and gonadogenesis, passing of sexual cycles and ecology of spawning of bream in the conditions of ecological transformation of Zaporozhian Reservoir were researched. Bream spawning population included 10 age classes. Absolute fertility of individuals of different ages ranged from 54.31 to 856.0 thousand eggs, and the average fertility was within 148-161 thousand eggs. Histological study of the ovaries showed asynchronous oocyte development in period of vitellogenesis. In conditions of Zaporozhian Reservoir bream generate only one portion of spawn, and 6-10 % of oocytes in phase of vacuolization were resorpted. The current environmental conditions of reservoir bream populations are subjected to natural and forced intra-differentiation, which is the adaptive mechanism of its reproduction.
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