Morphological peculiarities, age structure and growth rate of rudd (Scardinius erythrophthalmus L., 1758) of the Dnieper-Bug estuary system

Purpose. To analyze basic biological characteristics of the commercial stock of rudd. Scardinius erythrophthalmus (Linnaeus, 1758) of the Dnieper-Bug estuary system in the conditions of the regulated flow of the Dnieper River. Methodology. Morphological peculiarities were determined based on the analysis of meristic and plastic features. When assessing the biological state of the commercial stock rudd, the main attention was given to the age, sex structure, fecundity, characteristics of length-weight growth and condition factor. The ichthyological material was collected during the work of monitoring stations of the Institute of Fisheries NAAS of Ukraine, the operation of which was regulated by protocol decisions of the Scientific and Commercial Council of the State Agency of Fisheries. Field and laboratory processing of collected data was carried out using conventional ichthyological methods and guidelines. Findings. The analysis of meristic features of rudd of the Dnieper-Bug estuary system showed that fin formulas were as follow: D ? III 7-9 (10) (М=8.19±0.14); A - III 10-12 (М=10.81±0.17); P - I (12) 13-15 (16) (М=14.26±0.18); V - II 7-8 (М=7.78±0.08). The lateral line had from 38 to 47 scales (М=42.63±0.65). 7-8 (М=7.67±0.09) transverse rows of scales were located above lateral line, while 5-6 (М=5.6±0.9) under it. A peculiarity of the current biological state of rudd stock is stable age structure with the domination of young age groups. An increase in the ratio of the maximum age groups against the background of an insignificant trend for the decrease of the ratio of age-5 fish indicates on an increased pressure (both commercial and recreational) on rudd recruitment. A confirmation of it is an increase of the mean weighed age of the commercial stock from 4.16-4.17 to 4.26-4.31 years. The sex structure is typical for cyprinids. A trend of the domination of males in younger age groups and females in older age groups is observed. In total, sex ratio was practically equal (1:0.96) but with insignificant predominance of females. The length-weight growth rate is intensive enough. The most variable growth features were observed in age-4 fish that was related to sexual maturation. The absolute individual fecundity varied from 11.02±0.26 thousand eggs in age-3 fish to 132.56±1.48 thousand eggs in age-7 fish. At the same time, the variability (Cv) gradually decreased from 5.27% to 2.50%, respectively, in the indicated age groups. The condition factor of rudd is satisfactory. Depending on the age, it varied from 2.27±0.02 to 2.58±0.03 with Cv=1.46-2.49%. Originality. The data of the morphological structure of the rudd of the Dnieper basin have been significantly improved, especially in the lower reach in the conditions of the transformed flow. The existing information on the age, sexual structure, fecundity and length-weight growth has been supplemented. Practical value. Current data obtained during the study have to become a scientific justification of the optimization of commercial pressure on native fish fauna of the Dnieper-Bug estuary system that becomes important in the aspect of nature protection measures aimed at preserving biodiversity of the transformed water bodies of Ukraine.