Commercial-biological characteristics of tulka (Clupeonella cultriventris Nordmann, 1840) of the Dnieper-Bug estuary system

Purpose. To give biological assessment of the commercial stock of tulka Clupeonella cultriventris (Nordmann, 1840) of the Dnieper-Bug estuary system in the conditions of the reduced Dnieper flow. Methodology. Analysis of the biological state of the tulka stock of the Dnieper-Bug estuary system was performed based on the data on the dynamics of its age structure, linear-weight growth and condition factor. Primary materials were obtained from scientific and commercial catches done during the current century. Field and laboratory processing of the ichthyological materials were performed according to generally accepted methods. Findings. Current commercial fishery situation in the Dnieper-Bug estuary system is characterized by reduced intensity of tulka harvest. Currently, major fish harvest is concentrated in the Mykolaiv fishing ground. As a result of continuous underfishing, a gradual growth of the mean weighed age of the tulka stock is observed with the accumulation of maximum groups of the right wing of the age series. At the beginning of the study (2001-2004), when age group was represented by three groups, the nucleus of the population was formed by age-1 and age-2 fish – 95.7%. However, last years, due to an increase in the number of age-4 fish, a reduction in the percentage of modal groups (age-1 and ahe-2 fish) to 76.7% was observed. At the same time, the share of age-3 and age-4 fish gradually increased from 15.8% in 2013 to 23.2% in 2017. Against the background of a relatively instable age structure, the linear growth and body weight growth rates did not change significantly. The highest variability of linear parameters was observed in age-1 fish. With a mean body length of 40.55±0.53 mm (lim=36.2-45.2 mm), deviations from the modal value (Cv) were 5.10%. With an increase in the age, the linear growth rate decreased and no significant variability was observed. The highest variability of body weight was typical for age-1 and age-2 fish (Cv=17.42 and Cv=15.92%, respectively). The variability of body weight in fish of the right wing of the age series depended by a stable prevalence of females in the stock structure, which were characterized by more significant fluctuations of mean weight due to differences during sexual gland development. The condition factor of tulka was high, Fulton's condition factor values varied from 0.85 to 1.04. At the same time, a gradual growth of the condition factor with an increase in age was observed. The highest variability of the condition factor was typical for age-2 and age-3 fish – Cv=12.8-15.2%. Originality. The paper contains new as well as updated already existing data on major biological characteristics of the commercial stock of tulka in transformed conditions of fish fauna existence in the Dnieper-Bug estuary system. Practical value. The obtained results are a scientific basis for rational fishery and optimization of fishing pressure on this clupeid species of the Dnieper-Bug estuary system. Under the current conditions, the provided information is topical from the point of view of the improvement of the trophological status of a valuable fishery water body in the context of the reduction of the level of trophic competition between juveniles of semi-migratory fish species and typical zooplantonivorous species such as tulka, which currently increased its abundance.