Biological and ecological indicators of European bitterling Rhodeus amarus (Bloch, 1782) of the Zaporizhzhia (Dnipro) reservoir

The results of studies on the biological and ecological characteristics of the European bitterling (Rhodeus amarus (Bloch, 1782)) in the Zaporizhzhia (Dnipro) Reservoir during the growing season are presented. The aim of the work was to study the population structure, age groups, sex structure, length-weight indicators, and reproductive characteristics of the species in various sections of the water body. The object of the study was the European bitterling, collected from the littoral zone of the Zaporizhzhia Reservoir. Fish from three age groups (0+, 1+, 2+) were caught using a fry seine net with a mesh size of 4 mm. The age structure, body length, weight, sex ratio, and reproductive parameters were analyzed using classical ichthyological research methods. The results of the study indicate the significant distribution of the European bitterling in the water bodies of Dnipropetrovsk Oblast, particularly in the Zaporizhzhia Reservoir, where it dominates in numbers among short-cycle littoral fish. The proportion of fry (0+) in the population was 47.88%, yearlings (1+) – 38.56%, and two-year-old fish (2+) – 13.56%. The main reasons for the species' high abundance include a rich food base in shallow waters and a reduced population of predatory fish (zander, perch, pike). It was established that the average length-weight indicators of mature individuals in the 2+ age group were as follows: for males – 6.03 ± 0.06 cm and 3.34 ± 0.14 g; for females – 5.95 ± 0.03 cm and 2.94 ± 0.04 g. The sex structure of the population showed a predominance of males in the 1+ age group (68%), whereas in the 2+ group, the sex ratio stabilized to 1:1. The reproductive characteristics of the European bitterling demonstrate a fractional spawning pattern, which lasts from June to August. The absolute individual fecundity was 176.01 ± 12.73 eggs, of which mature eggs (IV–V stages) accounted for only 23.5%. The remaining 76.5% of the eggs were at stages II–III, forming a reserve for future spawning. The conclusions of the study confirm the significant abundance of the European bitterling in the examined areas, its ecological plasticity, and its adaptability to reservoir conditions. The obtained data form a basis for assessing population status and further studies of the species' reproductive potential, which is essential for monitoring aquatic ecosystems and the rational use of bioresources.