STRUCTURAL INDICES OF THE PRUSSIAN CARP (CARASSIUS GIBELIO B.) POPULATION IN THE KKREMENCHUK RESERVOIR

Abstract

Ukraine ranks first in Europe in the number of natural water bodies. These abundant water resources are the source of significant fish resources, which are effectively exploited by the fishing industry. Rationalization and equal distribution of the commercial load, the reduction of the commercial pressure on the main fish species has always been an urgent issue. This can be achieved by shifting the focus of fishing to other types of fish, which will allow rational use of the entire complex of commercial species. The Prussian carp Carassius gibelio (Bloch, 1782) is among such promising objects of fishing - a massive, small-particle species, which in certain water bodies of Ukraine has displaced other minor commercial fish species and formed stable numerical populations.
A decisive role in the age structure formation of the Prussian carp population was played by the increase in the specific number of middle and older age groups, which is associated with an insufficient level of commercial load. Thus, the variation series of this species in the commercial catches of 2021, unlike other species, had the form of a double-peaked curve, which was formed due to the loss of size classes of 25-28 cm. According to the Fishing Rules and the Regime of Fishery Operation of the Dnipro Reservoirs in force today, nets with a step are allowed hole a=38-49 mm and 70 mm and more. This distribution of commercial load selectivity reduces pressure on bream and bream populations. However, it also causes a significant increase in the stock of Prussian carp. And this alien species has formed abundant populations in the Kremenchuk Reservoir. Thus, its average annual catch in the period 2000-2020 increased from 44.6 to 462.4 tons or 10 times in 20 years.
We should note that Prussian carp occupied an insignificant segment of the catch - no more than 0.1% by weight in the traditional commercial set of gill nets (a=75-90 mm) at the Kremenchuk Reservoir, while, in the 50-60 mm mesh size nets this species is dominant with a share in catches of up to 60% by weight. Thus, the main indicators that characterize the fisheries and environmental protection components of gill net fishing are a mesh size that deals with part of the main target fishing objects, size and weight indices, and bycatch of immature commercially valuable species. The gill nets with 50-60 mm generally meet the requirements for the current legislation and might be used for targeted harvest of the species in the littoral zones. Considering the need to intensify fishing for Prussian carp and its high actual specific mass in catches of nets with a mesh size of 50-60 mm, the implementation of specialized fishing for this species in the Dnieper reservoirs can be considered as a means of optimizing the use of the formed bioresource for the commercial fishery. In order to minimize the negative impact of this fishing on the structural and functional indicators of the bream population, specialized fishing should be focused on the areas of accumulation of Prussian carp, i.e. it is a specialized fishery with a share of Prussian at least 50%. The necessity of the meliorative capture introduction aimed to downgrade the reproductive core of the population of this alien species to prevent its uncontrolled population growth.