The current state of the natural population of the white Dniеstеr crayfish (Pontastacus eichwaldi bessarabicus Brodsky, 1967) in the Dniester estuary

Purpose. To carry out a retrospective analysis of changes in the state of the white Dniester crayfish population (Pontastacus eichwaldi bessarabicus Brodsky, 1967) in the Dniester estuary. To provide a description of its abundance and range in this water body. To analyze changes in the main biological characteristics of this species at the population level in the estuary. Methodology. The material for the study was collected from the catches of commerical fishing gear during the crayfish fishery in the Dniester estuary in the period from 2017 to 2021. A complete biological analysis of the obtained material was carried out in the laboratory of the Department of Aquatic Bioresources and Aquaculture of the Odessa State Ecological University (OSEU). The total length of crayfish, their weight, absolute and working fecundity of females, egg diameter, size and weight characteristics of larvae and juveniles were determined. The feeding of crayfish in ontogeny was studied. To assess the reproductive capacity of the crayfish population in the Dniester estuary, we used the coefficient of fecundity preservation Ki calculated on the basis of the obtained data. Statistical data processing was carried out using MS Excel spreadsheets. Findings. In 1950-2021, the range of ​​the population of the Dniester white crayfish in the Dniester estuary decreased from 13 to 7‒8 thousand hectares, and its total stock — up to 200±50 thousand individuals. At the same time, the average density of this crayfish was 17-18 ind. per hectare of the estuary. In recent years, more than 60% of the water area, in which the population of white Dniester crayfish is found in the estuary, is located within the Lower Dniester National Nature Park. In addition, the size and weight of crayfish in the estuary have decreased in recent years. For example, in the 1960s and 1970s, up to 40-45% of individuals were characterized by a length of 13.5-16.5 cm. In 2017, the modal group consisted of individuals with a length of 9-9.5 cm, and in 2019-2021 — 9.6-10.0 cm. The maximum size of crayfish in 2019-2021 was 14.5-15.0 cm (19‒20 cm in 1960-1970s). The reproductive capacity of the population also deteriorated. Absolute and working fecundity decreased. The Ki coefficient for the population of this species in the Dniester estuary in 2017–2018 was 0.63, and in 2020–2021 it was 0.53. This indicates a progressive decrease in the abundance of its population. Originality. For the first time in the last 25‒30 years, an assessment of the current state of the Dniester white crayfish population is given. The analysis of the available data indicates the progressive degradation of the population. A decrease in the range of distribution of this species in the estuary, a decrease in its size, the share of females in the population, and indicators of their fecundity were recorded. A decrease in the coefficient of preservation of fecundity Ki indicates a depressed state of the Dniester white crayfish population in the estuary, which threatens its further degradation. In recent years, catches of white Dniester crayfish in the Dniester estuary have decreased significantly. For example, the catch per fyke net does not exceed 1.0-2.0 kg, and the share of individuals of non-commercial size (up to 10 cm) has increased significantly. Practical Value. The research results allowed: providing an ecological and biological characterization of the state of the Dniester white crayfish population in the Dniester estuary; characterizing changes in the abundance and spatial distribution of crayfish in this water body; highlighting the peculiarities of its biology; determining the strategy of restoration and rational use of the natural population of crayfish, becoming an opportunity during the development of effective methods of its artificial reproduction.