Climate warming and the introduction of a new mass species of clam Dreissena bugensis can potentially lead to changes in the reproductive capacity of benthic fish species living in the upper Volga reservoirs. The age of puberty, the structure of the spawning stock, and fecundity are the main indicators of reproduction that determine, together with indicators such as the survival of young fish and the degree of fishing load, the relative number of species in the ichthyofauna. The purpose of this work was to study the size and age of the onset of puberty and the dynamics of fecundity of roach Rutilus rutilus in the Volga ples of the Rybinsk reservoir. Determination of individual absolute fecundity was performed by counting eggs in fresh gonads, without the use of fixation. The age of fish was determined by scales, and vertebrae and operculum were used as additional recording structures. As a result, it is shown that the beginning of sexual maturation of female roach occurs when reaching the age of 3+ and a length of more than 9 cm. Late maturing females were 4+ years old and over 18 cm long. The beginning of sexual maturation of males could not be determined. The smallest males in the catches, 7–8 cm long, were all sexually Mature. The maximum age of fish reached 12+ years with a length of no more than 32 cm. Individual absolute fertility ranged from 1,75 to 178,46 thousand eggs. The value of relative fertility ranged from 79 to 308, with an average of 171 eggs/g. The result concluded that high absolute and relative fecundity of roach and dynamic changes of fertility in terms of increase in average water temperature and absolute improvement of fodder after the invasion of Dreissena bugensis, may indicate the attainment of specific limit reproductive capacity of this species.
