Evidence of Genetic Continuity in the Shortfin Mako Shark ( Isurus oxyrinchus ) Between the Eastern Atlantic and Mediterranean Sea

ABSTRACT Highly migratory sharks such as the shortfin mako ( Isurus oxyrinchus ) undertake extensive oceanic movements that cross ecological and political boundaries. This wide‐ranging behavior increases their exposure to human impacts, primarily from fishing activities, as the species is frequently caught as bycatch in longline fisheries. Consequently, the shortfin mako is listed as Endangered globally and Critically Endangered in the Mediterranean Sea. Despite growing conservation concerns, knowledge of its population structure within the Mediterranean, and its genetic and ecological connectivity with the Atlantic Ocean, remains limited, hindering the development of effective management measures. We investigated the population structure and demographic history of shortfin mako across the Eastern Atlantic Ocean and Mediterranean Sea using double‐digest restriction‐site associated DNA (ddRADseq) sequencing on 66 individuals collected from six regions in the two areas. After sequence filtering, we retained 4349 neutral SNPs to assess population structure and demography. Our results indicated a single, genetically mixed population across the Eastern Atlantic‐Mediterranean range, suggesting high connectivity and gene flow. Demographic analyses revealed a historically stable population size, although recent declines may not be detectable given the resolution of our genetic markers. While such connectivity may enhance resilience to localized pressures, it also implies that intense Mediterranean fishing could have population‐wide consequences. These findings challenge the assumption of strong genetic isolation between basins but do not preclude demographic differentiation. Given that ICCAT currently treats the Mediterranean as a separate, data‐poor stock, our results highlight the need for coordinated, basin‐wide monitoring and management to safeguard this endangered species.