Structural parameters of pikeperch (Sander lucioperca) population of the Kyiv reservoir

Purpose. Analysis of the dynamics of structural parameters as an integral property of the processes of recruitment and elimination of the reproductive and commercial population nuclei of pikeperch of the Kyiv reservoir. Methodology. Ichthyological material was collected during spring-summer period of 2015-2016 from survey catches done with gill nets with mesh sizes of 30-120 mm. Collection and processing of the field materials were carried out using conventional methods. In total, catches of 3.5 thousand net-days were processed during the study period and 1031 individual pikeperch were analyzed. Total mortality coefficient (Z) was determined by graphic method based on the dynamics of the natural logarithms of the abundances of different age groups. Statistical processing of data was done in MS Excel. Findings. Commercial landings of pikeperch during last 10 years has a clear trend for a growth: from 20-25 to 80-100 tons. The catch of pikeperch per unit effort of survey gill nets in an interannual aspect is also characterized by a growth. At the same time, there is a certain improvement of the structural parameters of pikeperch population, e.g. an increase in the maximum age and extension of the modal frequency distribution. Age-3 and age-4 fish represented the major part of the population (71.6%) in the catches of 2015, while the share of age-5-6 fish was also relatively high (10.8%). It resulted in an increase in the weighed mean age of the pikeperch population to 4.5 years. For 2016, there is a significant shift of the frequency distribution to its left wing ? the major part of the population (64.7%) was composed of age-2 to age-4 fish. The weighed mean age in 2016 decreased to 2.6 years that was due first of all to abundant recruitment and intensive elimination of middle age groups. Total mortality (Z) of pikeperch in 2015-2016 was 0.53-0.56 that corresponded to normal conditions of the formation of the commercial stock of this species and moderate level of its exploitation. The major part of pikeperch catches in 2016, both by number (61.5% of the total catch) and biomass (54.9%) was done by gill nets of 40-60 mm mesh size (in 2015 ? a=60-70 mm). The distribution of pikeperch catches according to the mesh sizes indicate on the necessity for the implementation of a special regime for the exploitation of this species. Originality. We obtained new data for the theoretical justification of the principles of the maintenance of optimum structural parameters of pikeperch population in the condition of a commercially fished water body. Practical value. The study results will be used for the preparation of the procedure of the fisheries exploitation of ichthyofauna in the Kyiv reservoir, which include, in particular, the regulation of mesh sizes in commercial fishing gears.