Ontogenetic and Sex-Specific Isotopic Niches of Blue Sharks (Prionace glauca) in the Northwestern Pacific

The blue shark (Prionace glauca) is a pelagic species widely distributed in the northwestern Pacific Ocean. The trophic roles of blue sharks across different developmental stages and between sexes remain poorly understood. Fifty-four sharks were sampled (October 2022–March 2024) for precaudal length (PCL) and stable isotope levels (δ13C, δ15N) in the muscle tissue (n = 52). Mean PCL varied based on the month of sampling (p = 0.034), with the smallest individuals occurring in July (143.0 ± 4.3 cm) and the largest in October (178.0 ± 2.6 cm). Stable isotope analysis (δ13C and δ15N) indicated consistent offshore habitat use (δ13C: from −20.70 to −18.82‰) and significant nitrogen isotopic differences among life history (δ15N: from 10.23 to 15.72‰; Kruskal–Wallis test, p = 0.037). The elevated δ15N values observed in the subadult group (relative to juvenile individuals) are likely due to trophic enrichment associated with morphological development. Females exhibited markedly larger isotopic niches (SEAc = 2.42‰2) than did males (0.57‰2), and niche overlap was greater within each sex (40–52%) than between sexes (<21%). These results revealed sex-specific ecological roles and trophic strategies throughout the life history of P. glauca. Understanding these foraging differences can help with catch reduction and habitat-protection measures in the transboundary pelagic fisheries of the northwestern Pacific.