Too young to die: Mapping nursery areas for early juveniles of the endangered sandbar shark (Carcharhinus plumbeus) to inform conservation in the Mediterranean Sea

Abstract

Globally, elasmobranch species have been declining in abundance due to fishery activities. This conservation issue calls for effective management strategies and increasing efforts to protect these species. The declining status of elasmobranchs in the Mediterranean Sea is alarming as well. Reversing such dramatic trends requires tackling fishing pressure using multiple methods, including a fine-tuned spatial resolution in conservation strategies incorporating robust evidence on species spatial use at different life stages and its overlap with fishing pressure. In particular, a scientifically sound identification of nursery grounds is crucial to define key spatial management targets promoting the recruitment of such depleted species. Here we focused on the sandbar shark (Carcharhinus plumbeus), listed as endangered by the IUCN, and on the nursery of its early juveniles such as newborns (i.e. a few months old) and young-of-the-year individuals (i.e., < 1 year old), both characterized by a total length below 71 cm. First, by monitoring a small-scale fishery fleet in the North-Western Adriatic Sea, we unambiguously identified a local multiyear nursery site for early juveniles of this species. Then, we combined such novel information with a bibliographic review on the presence and absence of early juveniles across Mediterranean regions to construct a species distribution model predicting favourable nursery areas throughout the entire Mediterranean Sea. To do so, a Bayesian approach was applied to construct a generalized linear model with spatial effect estimated by a Gaussian random field (INLA-SPDE). Model-based inference indicates that, in summer, which is the main pupping season, important nursery areas for this species are found along the North-Western coast of the Adriatic Sea, in the coastal area of the Gulf of Gabes, and in the Gulf of Iskenderun (North-Eastern Levantine Sea). Synthesis and applications: These key areas should be prioritized to apply conservation measures to foster the recruitment of this species since intense fishing activities were also documented. Model predictions also allowed to determine where unknown nursery grounds may potentially occur, a valuable information to direct future monitoring efforts and clarify any further occurrence of nursery areas.