On the question of the presence of natural reproduction in the “factory” population of the sterlet of the Oka river

Abstract

The increase in the number of sterlets in the Oka in recent years has been associated with long-term re-climatization measures, the basis of which was mass releases of resilient juveniles grown in conditions of full-cycle industrial-type fish farms. Despite the high growth rate and relatively early puberty of the Oka sterlet, the completion of the naturalization process of this representative of sturgeon due to the lack of spawning behavior and replenishment of the population through natural reproduction probably does not occur. According to the results of accounting surveys of early juvenile fish with ichthyoplankton traps and net fishing gear for mature sterlet in different parts of the Oka River over a long period of observations, no signs indicating the spawning of sterlet producers of “factory” origin have been established. A complete biological analysis of a significant amount of sterlet samples showed a sharp predominance of males in the sexual structure of the population and the absence of post-spawning gonadal changes in mature fish of both sexes, characteristic of the VI–II stage of gonad maturity in May and June catches. In the structure of catches of ichthyoplankton traps for a long-term period of observations on different hydrological regime and depths of the upper and middle reaches of the Oka, despite the rich (up to 21 species per survey) species diversity and a significant volume (5.66 thousand copies) of identified early juvenile fish caught in the Ruslov stream during the rolling migrations, it was not recorded not a single instance of sterlet in the early stages of development. At the same time, the number and ichthyomass of mature sterlet in the river areas adjacent to the places of setting traps was relatively high.