State of the Commercial Stock of Rapa Whelk Rapana venosa in the Coastal Zone of the Northwestern Section of the Black Sea

Abstract

The paper deals with the age and size-mass composition of the most important commercial object - veined rapa whelk <i>Rapana venosa </i>(Valenciennes, 1846) and spatial distribution of its population in the northwestern section of the Black Sea based on analysis of samples from 857 beam trawl catches and 7 diving sampling over 2018-2021. Maximal biomass of the mollusk is concentrated at the depths up to 25 m. Rapa whelk abundance decreased sharply below 25 m owing to insufficient food and low water temperature. The beam trawl surveys over 2020-2021 showed that in summer hypoxic zones were common in the bottom horizons in significant areas of the shelf water. The black sludge with a sharp smell of hydrogen sulfide often occurred in the catches. The mass death of the filtering mollusks and rapa whelk was registered as a consequence of these phenomena in the shelf zone. At the depths of 5-25 m, rapa whelk population comprised twelve age groups from 2+ to 13+ and the main portion consisted of the groups 6+-8+. At the same time, at the depth below 25 m, most specimens were 7-9 years old. Rapa whelk enters the deep-water zone due to migration from shallower areas where it reproduces. The stock of rapa whelk in the Ukrainian waters estimated by the direct trawl accounting varied within the range of 28.5-72.7 th. tons. At the same time stock assessment using the BSM model showed about 35.7 thousand tons. Intensification of the mollusk harvesting in recent years has not led to overexploitation of this stock. The size and age structure of the population did not significantly change. The areas accessible to fishermen are less than 15-20% of total shelf inhabited by rapa whelk. Its resource demonstrated high ability for sustainable reproduction due to good feeding conditions and constant migration of mollusks from areas where it is not harvested.