Piscivorous ichthyocomplex of the Velykoburluts’ke reservoir as a factor affecting the survivability of fish seeds of Chinese carps
Purpose. To determine and analyze structural and quantitative parameters of piscivorous fishes of the Velykoburluts’ke reservoir with the assessment of trophic load on multiple-age juveniles of the silver and bighead carps and their hybrids. Methodology. Data of the analysis of commercial catches, which were done seasonally during 2017-19, were used as primary materials. The ichthyological material was collected from gill nets catches (mesh size a=30-100 mm). In total, 265 fishes of different species were collected for the biological analysis. The weight of fish preys in stomachs was reconstructed using the empirical equations of the length-weight relationships. Sampling and processing of the materials were carried out according to generally accepted methods. Fish stocks were calculated on the basis of fish mortality rates. The data of official commercial statistics were used in the work. Findings. The most abundant species of the current piscivorous ichthyofauna in the Velykoburluts’ke Reservoir are pikeperch (estimated stock for 2019 - 26.5 kg/ha), pike (11.7 kg/ha) and perch (kg/ha). The modal range of the pike and pikeperch in the catches was formed by younger and middle age groups, the average length of pikeperch was 35.1 cm, weight - 0.71 kg; pike - 43.6 cm and 0.91 kg, respectively. The most important prey objects of the pikeperch of up to 50 cm in length were bleak (40.7% by the frequency of occurrence and 20.6% by weight of the stomach content), perch (29.6% and 28.7%, respectively) and roach (18.5% and 29.0%); pikeperch longer than 50 cm preyed on roach (57.1% and 36.9%) and perch (42.9% and 17.3%). The food spectrum of pike of up to 50 cm in length was dominated by roach (29.7% by the frequency of occurrence and 22.9% by weight of the stomach content), Prussian carp (respectively 24.3% and 20.5) and perch (16.2% and 12, 5%); that of the pike with a length of 50 cm and more - Prussian carp (45.5% and 22.6%) and bream (36.4% and 31.6%). The main prey objects of perch of up to 20 cm in length were monkey goby (42.9% by the frequency of occurrence and 47.6% by weight of the stomach content) and perch (21.4% and 25.8%, respectively); perch with a length of 20 cm and more - monkey goby (58.3% and 25.8%) and their own juveniles (33.3% and 20.2%). Juveniles (0+-1+) of the silver, bighead carps and their hybrids were recorded in the stomach contents of piscivorous fishes only in pikeperch with a length of more than 50 cm and pike, their averaged weight share was 8.1 and 0.7%, respectively. Based on the abundance of piscivorous fishes in 2019, the calculated consumption of 20 g Chinese carps seeds is 17600 ind. that is 51.6% from planned amounts of stocking of the Velykoburluts’ke reservoir. Originality. Original data on feeding of piscivorous fishes in the conditions of special table fish farm on the middle reservoir have been obtained. The effect of piscivorous fishes on Chinese carps seeds with weights lower than the traditional ones has been assessed quantitatively. Practical value. The obtained data can be used for the determination of targets of stocking efficiency of the young-of-the-year and yearlings in the case of fish ranching.