Ecological function of the Ariake Bay as a scalloped hammerhead shark (Sphyrna lewini) nursery in the northwestern Pacific

The scalloped hammerhead shark ( Sphyrna lewini ), a cosmopolitan shark inhabiting temperate and tropical waters, is highly vulnerable to fisheries owing to late maturity and low reproductive output. Although nurseries have been identified in the Americas and Australia, ecological knowledge of individuals in the northwestern Pacific remains limited. We analyzed 1,167 individuals collected as bycatch across 4 regions of southwestern Japan, with a focus on the Ariake Bay. Size- and season-specific occurrence patterns, umbilical scar condition, and vertebral age estimation revealed that the shallow inner Ariake Bay functions as a nursery—the first one to be documented in the northwestern Pacific. Age-0 individuals, born May–June, comprised 88% of the inner bay population and stayed 3 months to 2 years before dispersing. Larger mature individuals >10 years were concentrated at the bay mouth, with intermediate sizes and ages largely absent. Age-0 individuals occurred in environments with water temperatures of 18.6–27.3 °C and salinities 25.9–32.8. Condition factor and hepatosomatic index analyses suggested that this nursery, characterized by turbid and moderately saline waters, provides favorable nutritional conditions in addition to predator protection. Substantial annual bycatch resulting from the eagle ray control program highlights strong fishing pressure on juveniles. Overall, we identified the Ariake Bay as a highly productive nursery of global significance, thereby advancing current understanding of S. lewini habitat use in the northwestern Pacific. Protecting this habitat is essential for sustaining regional populations and informing conservation strategies for this critically endangered species.