LAUSR

Pacific bluefin tuna (Thunnus orientalis; PBFT) is a highly migratory species, with some individuals migrating between the western Pacific Ocean (WPO) and eastern Pacific Ocean (EPO). In this study, we used stable isotope analysis to identify PBFT that had recently undergone westward transoceanic migration on the Sea of Japan. A total of 155 PBFT individuals were examined. Their ages ranged from 2 to 17 years, with most individuals being 2–7 years of age. Individuals from each year class were classified as WPO residents or recent EPO migrants using cluster analysis of δ15N values. Individuals aged 2, 6, and over 7 years had unimodal distributions of δ15N values, while individuals aged 3, 4 and 5 years showed a bimodal distribution with high- and low-δ15N groups. Due to the overall higher baseline of δ15N values in the EPO, high δ15N individuals were considered to represent PBFT that had migrated from the EPO. Though individuals aged 6 and over 7 years showed unimodal distributions in the cluster analysis, discriminant analysis indicated that these PBFT also included some migrants from the EPO. We preliminary estimated the percentages of migrants and residents in the Sea of Japan. Such information can improve stock assessments models for PBFT and contribute to the sustainable stock management of this species.