Distribution and Population Structure of Discarded Parastichopus regalis (Holothuroidea) from the Shrimp Trawling Fishery in the Sea of Marmara

Royal sea cucumber Parastichopus regalis is frequently caught as a bycatch in shrimp fishery with beam trawl in the Sea of Marmara (SoM). This study presents the first assessment of the distribution patterns and population structure of the royal sea cucumber in the SoM. The dataset was obtained based on the area scanning method with underwater visual counting techniques (<50 m) and beam trawling (≥50 m). A total of 1,377 royal sea cucumbers were sampled off the Kapıdağ Peninsula and Marmara Islands between May 2022 and April 2023 at monthly intervals. The gutted lengths of the samples ranged from 6.65 to 30.70 cm. No gonads were found from October to March according to macroscopic examinations; thus, the gonadal cycle starts in April, peaks in July where there gonadosomatic index gain its maxima for both sexes, and completes in September. The mean annual biomass was calculated as 30.5 kg·km−2, and the highest value obtained in a fishery operation reached 100.8 kg·km−2. The distribution patterns of royal sea cucumber seasonally varied however further studies on the relationships between food availability, social interaction, and biotic factors are needed to understand relevant spatiotemporal patterns.