Effects of fishing capacity and environmental parameters on landings of Hexanchus griseus in the Sea of Marmara

Between July 1967 and December 2023, 136 bluntnose sixgill sharks, Hexanchus griseus, were caught in the Sea of Marmara (SoM). Although a generalised linear model (GLM) and Mann-Kendal trend analysis both showed an increasing trend in landings per year, the estimated smooth splines of the GLM regression trend line highlights a drastic decline in landings of H. griseus after 2017. An accumulation curve generated based on annual landings and registered fishing capacity per year in the SoM predicted an increase in landings per fishing capacity; however, despite fluctuations in landings between 2009 and 2016, landings declined drastically after this period. While the depths at which landed bluntnose sixgill sharks were caught ranged from 10 to 1,000 m since the early 2000s, the majority of specimens were caught in the shallow waters of the continental shelf. Deteriorating environmental conditions and increasing deoxygenation in the deep waters of the SoM coincide in time with a decrease in the depth at which bluntnose sixgill sharks are caught over the continental shelf. The available evidence suggests an ongoing process of vertical habitat compression that appears to be increasing the vulnerability of H. griseus to commercial fisheries in the SoM.