Natal Origin and Trans‐Pacific Migration of Pacific Bluefin Tuna ( Thunnus orientalis ) Inferred From Otolith Oxygen and Carbon Isotope Analysis

ABSTRACT Pacific bluefin tuna ( Thunnus orientalis , PBF) are highly migratory, with a single Pacific‐wide stock facing complex management challenges due to extensive transoceanic movements and a recent population recovery. This study addresses persistent knowledge gaps regarding the quantitative contributions of different natal origins to key feeding grounds and the proportions of trans‐Pacific migrants in spawning grounds. We used stable isotope analysis of otoliths (δ 18 O oto and δ 13 C oto ) to investigate natal origins and migratory patterns of PBF collected from three key locations: the Philippine Sea–East China Sea (PS‐ECS), the Sea of Japan (SoJ), and the California Current Large Marine Ecosystem (CCLME). Our analysis of δ 18 O oto values effectively discriminated natal origins, revealing a strong dominance of PS‐ECS‐origin fish in catches across all areas. Specifically, 91% of PBF captured in the CCLME originated from the PS‐ECS spawning grounds, a proportion that surpassed estimates from those captured in SoJ and PS‐ECS. Additionally, the δ 13 C oto analysis successfully distinguished between trans‐Pacific migrants and resident individuals, confirming that a substantial proportion (87%) of adult PBF caught in the PS‐ECS spawning grounds have previously migrated to the CCLME. These results confirm the high connectivity of PBF across the Pacific and support its management as a single stock. The high proportion of PS‐ECS‐origin fish utilizing the CCLME as a critical feeding ground underscores the necessity of managing juvenile exploitation in Eastern Pacific waters to ensure sufficient survivorship and maximize the reproductive output of the adult spawning stock in the Western Pacific. The quantitative estimates of natal origins and migratory patterns derived from this study provide crucial data for future stock assessments and the development of improved spatially explicit management models.