Osamu Abe
Head, Pacific Bluefin Tuna Biology Group, National Research Institute of Far Seas Fisheries

Important information on the stock management
Earlier information of annual juvenile abundance is essential for planning appropriate stock management of the Pacific bluefin tuna (PBT). It would contribute to earlier estimation of annual recruitment level which facilitates more accurate, rapid stock assessment and the rational stock management.
However, as it is now, it is difficult to evaluate the annual juvenile abundance at early stage. Because its “migration route” from its spawning area has not yet been clarified until they are targeted by the pole and line fishery near Tosa Bay and Goto Islands for aquaculture seedling, we still do not have the adequate methods to monitor the juvenile abundance before its recruitment.

Difficulties in catching juveniles
The difficulties to monitor the annual juvenile stock abundance arises for several reasons: its large fluctuation in relation with the oceanographic conditions; separation of the juvenile schools from Nansei Islands into two migrating areas in the Pacific side and in the Sea of Japan; mixture of the juvenile schools in the Sea of Japan having different origins from Nansei Islands and from the Sea of Japan; and so on. Another reason was we did not have adequate methods to catch smaller juveniles (10 – 14 cm) before they are targeted by pole and line fishery.
High speed trawling technique is required to catch PBT juvenile which has high swimming ability; however, it was difficult to tow trawl nets in the high speed Kuroshio Current. Therefore, we have devised the trawl nets to be operated stably even in the strong Kuroshio Current, resulting in the first successful catch of 10 – 14 cm PBT juveniles around Amami-Oshima Islands and Yakushima Island from June to July 2011, and showed the juveniles distributed densely especially from the center to the north edge of Kuroshio off the west coast of Yakushima. The similar surveys conducted in 2012 confirmed the results obtained in the previous year.

Migration route becoming more obvious
Previous studies showed that small PBT larvae, which were less than 20 days old after being hatched, distributed widely around Nansei Islands including of f Yaeyama Islands and Okinawa Main Island, while PBT juveniles of 1 – 2 months old (2 – 10 cm) were found mainly near Kuroshio Current region and the larger fish appeared coming up north. Together with the current study, the “migration route” of PBT juveniles was suggested; their schools migrate northward with Kuroshio, then some part of which separate away from Kuroshio off-west of Yakushima Island to move up northwardly off the west coast of Kyushu, and the remaining schools migrate to the Pacific side of Honshu and Shikoku.
Our study indicates that the PBT juveniles before their recruitment probably pass through off the west coast of Yakushima Island. For the future, focusing around this area, we would like to investigate developing the appropriate methods to monitor the annual juvenile abundance reasonably. Also, we would like to proceed with analyzing the marine environments to find out the key oceanographic factors to evaluate the separate ratio of juveniles to the Pacific side and to the Sea of Japan. Through these studies, we would like to contribute to improving accuracy in PBT stock assessment, and then consequently sustainability of the stock.

Contact: Tel: +81- 54-336-6033, E-Mail: turtlea@affrc.go.jp