Biological characteristics of white (Hypophthalmichthys molitrix Valenciennes, 1844), bighead (Hypophthalmichthys nobilis Richardson, 1845) carps and their hybrids (Hypophthalmichthys sp.) as objects of pasture aquaculture in the Kremenchuk reservoir

Purpose: Determination and analysis of parameters, which characterize the biological and fishery aspects of the formation and exploitation of the commercial stock of silver and bighead carps of the Kremenchuk Reservoir. Methodology. As primary materials, the results of field studies at the Kremenchuk Reservoir carried out in 2019-21 and 2023 have been used. Data collection and processing was carried out in accordance with generally accepted methods. Ichthyological material was collected from catches of commercial fishing nets with mesh sizes a=38-140 mm; in total, during the study period, catches of 2,254 fishing nets were examined, of which 652 individuals of silver and bighead carps as well as their hybrids were analyzed.. Predicted survival rate was assessed based on the annual natural mortality rate, which was determined based on the the Bertalanffy equation parameters. Statistical data processing was carried in MS Excel. Findings. In recent years, the basis of the commercial catches of silver, bighead carps and their hybrids in the Kremenchuk Reservoir was composed of age-7-10 fish, which was due to their predominant catch due to specialized fishing with nets with mesh sizes of a=100-140 mm, which accounted for up to 96.7% of the total catch of these species; the share of native species in the catches of these nets did not exceed 10%. The average annual absolute linear growth of these species in modal age groups was 6.0...9.6 cm, relative - 8.7...14.5%; the weight gains were 1416...3829 g and 28.1...43.5%, respectively, which indicated satisfactory grazing conditions in the Kremenchuk reservoir. The parameters of the Bertalanffy equation, which characterize the growth rate of silver, bighead carps and their hybrids in the Kremenchuk Reservoir, were: K = 1.08 (year-1), L∞ = 115.7 (cm) to = 0.17 (years). Based on the calculated natural mortality rates and 30% removal starting from age-5 fish, the fish return rate from stocking age-1+ fish into the Kremenchuk Reservoir is 10.2%. The peak culmination of ichthyomass of silver, bighead carps and their hybrids falls on age-4-6 fish; the optimal legal length of 55 cm allows ensuring the catch rates of 751 kg per 1,000 stocked age-1+ juveniles. Originality. Original data on the current state of commercial stocks of the main objects of pasture aquaculture in the Kremenchuk Reservoir were obtained from the point of view of the theoretical foundations of the formation and exploitation of their commercial stock. New methodical approaches to the assessment of fish return rate are proposed, as the main criterion for the effectiveness of measures for the artificial reproduction of ichthyofauna. Practical value. It has been established that the conditions for the formation of the commercial stock of the Kremenchuk Reservoir can be considered as satisfactory, i.e. the main limiting factor limiting the growth of their catches is the insufficient stocking volume. The obtained actual coefficients, which characterize the survival of fish seeds, can be used to forecast the volume of stocking and catch of silver, bighead carps and their hybrids. The shortcomings of the organization of modern fishery from the point of view of ensuring the maximum catch per recruitment unit have been established. The advisability of increasing the legal length for silver, bighead carps and their hybrids to 55 cm has been confirmed. Key words: Kremenchuk reservoir, pasture aquaculture, silver carp, bighead carp, organization of fishery.