Evidence of westward transoceanic migration of Pacific bluefin tuna in the Sea of Japan based on stable isotope analysis
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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.
KeywordsAge SIA Movements Marine predators Stock structure
We are grateful to Ishihara Yukio, Tottori Fishery Research Center and Minoru Kato, JAPAN NUS CO., LTD. for their assistance with sample collection. We thank Mikio Watai, Satoru Nisimoto and Tomoko Ota, National Research Institute of Far Seas Fishery for their assistance with sample measurement. Finally, the paper was improved by the detailed comments of two anonymous reviewers.
Compliance with ethical standards
This study was partially funded by Fisheries Agency, Japan through the research project on Japanese bluefin tuna.
Conflict of interest
The authors declare that they have no conflict of interest.
All bluefin tuna specimens were caught by commercial purse seine or set net operations and not killed for our study. The stock size of this species has been assessed by the Western & Central Pacific Fisheries Commission (WCPFC; http://www.wcpfc.int/), and the catch of this species has been controlled by Fisheries Agency, Japan since 2015.
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