| 總結: | The blue shark (<i>Prionace glauca</i>) 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 (δ<sup>13</sup>C, δ<sup>15</sup>N) in the muscle tissue (<i>n</i> = 52). Mean PCL varied based on the month of sampling (<i>p</i> = 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 (δ<sup>13</sup>C and δ<sup>15</sup>N) indicated consistent offshore habitat use (δ<sup>13</sup>C: from −20.70 to −18.82‰) and significant nitrogen isotopic differences among life history (δ<sup>15</sup>N: from 10.23 to 15.72‰; Kruskal–Wallis test, <i>p</i> = 0.037). The elevated δ<sup>15</sup>N 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‰<sup>2</sup>) than did males (0.57‰<sup>2</sup>), 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 <i>P. glauca</i>. Understanding these foraging differences can help with catch reduction and habitat-protection measures in the transboundary pelagic fisheries of the northwestern Pacific.
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