Biological Parameters and Biomass and Abundance Indices of Two Demersal Species, Turbot (Scophthalmus maximus) and Thornback Ray (Raja clavata), Estimated by a Trawl Survey in Western Black Sea

In this study, we determined the biological characteristics and indices of abundance and biomass of two demersal species, turbot and thornback ray, through a scientific trawl survey of Bulgarian Black Sea waters in the autumn of 2020. Turbot is among the most valuable fish species in the Black Sea, and thornback ray is a sensitive cartilaginous species with a significantly declining abundance throughout the Mediterranean region. The stock biomass of turbot was estimated at 1467.6 tons, with a relative abundance of 896,922 individuals, and those of the thornback ray were 1187.9 tons and 519,606 individuals, respectively. Mean turbot abundance, interpolated into 0.5° latitude/longitude grid cells, ranged between 52 and 120 ind·km−2, with a mean biomass of 78.26–238.31 kg·km−2, and for the thornback ray, these indices were within the limits of 0–107 ind·km−2 and 0–219 kg·km−2. The distribution of the different length classes of the two fish species by depth layer was analyzed. Length–weight relationships were estimated based on combined samples of both sexes and separately for female and male individuals, allowing a better understanding of growth patterns.