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Pacific bluefin tuna ( Thunnus orientalis ) has two spawning grounds, and each is used at different times: May–June in the Nansei Islands of western North Pacific Ocean and July–August in the Sea of Japan. Although previous studies developed body-length based methods to differentiate between individuals originating in each spawning ground, these methods are only applicable to the young-of-the-year (YOY). In this study, we evaluate a simple technique for discerning the natal grounds of Pacific bluefin tuna greater than one year in age by observing the first annulus on the vertebrae of fish caught around Japan. We found a significant linear relationship between fork length and vertebral radius, and determined that the first annulus forms in boreal winter. Both the back-calculated fork length at the time of first annulus formation and the measured fork length of YOY in February and March had a bimodal frequency distribution, and the peaks of the two frequency distributions had similar means. The larger fork length individuals appear to have originated near the Nansei Islands, whereas the smaller fork length individuals appear to have originated in the Sea of Japan. This is because PBF hatched around the Nansei Islands will have longer growth time for the YOY to develop the first annulus at longer distance from the vertebrae center. Similarly, we observed bimodal distributions for the vertebral first annulus radius of mature Pacific bluefin tuna (>120 cm) in both spawning grounds. Our technique offers improved sample treatment efficiency and lower costs. In addition, this method may help reveal the contribution rate of each spawning ground. [ABSTRACT FROM AUTHOR]

The spawning fraction (SF) is an important reproductive trait used to assess the reproductive potential of indeterminate spawning fishes. To estimate the SF more accurately, the durations of spawning markers in ovaries need to be determined. The SF of the Pacific bluefin tuna Thunnus orientalis (PBF) in the Sea of Japan (SOJ) is higher than the SFs in its other main spawning grounds. The present study aimed to revisit the SF of the PBF in the SOJ by focusing on diurnal changes in spawning markers and estimating the duration of spawning markers in PBF ovaries. According to diurnal changes in the postovulatory follicle (POF) degeneration process and final maturation process of oocytes, POFs were observed in ovaries up to 38.5 h after ovulation. The revisited total SF throughout the spawning season was 0.34–0.43 in Area 1 and 0.50–0.64 in Area 2; these values are smaller than the SF previously estimated in the SOJ. The estimated SF of the PBF in the present study was equal to or slightly higher than that in the water between the Nansei Islands, Japan, and the northeast of the Philippines and higher than that in the Kuroshio-Oyashio transition area. • Postovulatory follicles were retained up to 38.5 h after ovulation. • The spawning fraction, estimated by Day 0 spawners, may have been overestimated. • The revisited total spawning fraction throughout the Sea of Japan was 0.43. • The estimated spawning fraction depended on the fishing area and fishing date. [ABSTRACT FROM AUTHOR]

Abstract: Age determination of wild captured Pacific bluefin tuna, Thunnus orientalis, was conducted using sagittal otoliths of 806 specimens (47–260cm in fork length) caught in the waters off Japan and Taiwan. Otoliths were transversely sectioned and the opaque and translucent zones were analyzed. Opaque zones mainly appeared on the otolith edge from April to July, indicating that the opaque zone is formed annually. The opaque zones formed during later life (age 10+) were more distinct than the earlier zones. The estimated ages of specimens ranged from 1 to 26 years. Parameters of the von Bertalanffy growth function were estimated to be 249.6cm, 0.173, and −0.254 years for L ∞ , k, and t 0, respectively. Growth of younger fish was rapid up to 5 years old attaining about 150cm, and then growth rate decreased. After that, fish attained about 200cm at 9 years old and about 225cm (90% of L ∞ ) at 13 years old (50% of maximum age). This paper updates the biological information on length at age with a large size range to support stock assessment model analyses for this commercially valuable species. [Copyright &y& Elsevier]

Abstract: We followed three patches of larval Pacific bluefin tuna (Thunnus orientalis) using Lagrangian drifters in 2004 and 2005 from May to June and determined the age-specific larval instantaneous mortality rates, which were estimated to be between 0.06 and 2.75day−1. The mortality rates differed significantly between patches and between ages within the same patch, which were higher than most of the estimations for other marine fish larvae. The results indicate that for the purpose of understanding the process of the larval survival, the use of larval patches is appropriate as a unit of observation. Evidence of the occurrence for net avoidance is presented and also it is concluded that net extrusion was observed for the larvae. Length-specific catch probability of the larvae was estimated to correct for length distribution biases derived from the net avoidance and net extrusion. [Copyright &y& Elsevier]