Fig 1.
Map showing two major spawning grounds of Pacific bluefin tuna.
Colors represent the spawning ground in the southwest of the Sea of Japan (blue shade) and the Nansei Islands area (orange shade). Juvenile nursery areas are shown in the black-shaded areas. Red circles indicate catch locations of the fish samples used in this study.
Table 1.
Summary on sampling data, biological information, and δ18Ootolith analyses of Pacific bluefin tuna otoliths.
Fig 2.
Otolith sample preparation method using indium mount for SIMS δ18O analysis.
(a) Sectioned otoliths are embedded in epoxy resin using a 1 cm silicon mold. The back of the sample surface (non-sample side) is first ground with a grinding machine to a thickness of approximately 1.5 mm. The sample surface is then polished with 9 and 3 μm lapping film, followed by a final polishing with 0.3 alumina suspension. Finally, the excess epoxy resin around the otolith is trimmed off. (b) Two pieces of pre-polished UWC-3 standards are pressed into the center of an indium mount with a clean plastic. Samples are placed within a 1 cm diameter circle of the mount and then gently pressed into the indium using a hydraulic hand press. Pressing process should be done in a repeated manner with a force of 0.2 to 0.3 kN until the otoliths are fully embedded, and then the final press is done with 10 kN.
Fig 3.
SIMS analysis of the otoliths of Pacific bluefin tuna.
(a) Gold-coated indium mount containing 2 pieces of calcite UWC-3 standard and 4 otolith thin sections. (b) Cross-sectional image of an otolith taken with an electron probe micro analyzer under scanning electron microscope mode. SIMS beam spots are indicated by numbers. (c) An ion microprobe spot sputtered with a 133Cs+ primary ion beam focused to a diameter of 10 μm.
Fig 4.
The δ18Ootolith chronologies recording the early life and immature stages of adult Pacific bluefin tuna.
Error bars indicate the spot-to-spot precision of SIMS analyses (± 2SD). Dashed vertical lines indicate the location of the first opaque zone (a dark line-like structure formed in winter) in otoliths corresponding to the lowest temperature experienced by an individual fish in winter.
Fig 5.
Clustering results on core δ18Ootolith and the distance from otolith core to first opaque zone.
The ellipses show 95% confidence intervals. (a) Clustering results when k = 3. Cluster 1 (blue circles) are fish presumed to be born in the Sea of Japan and Clusters 2 (orange circles) and 3 (green circles) are fish presumed to be born in the Nansei Islands area. (b) Clustering results when k = 2. Clusters A (blue circles) and B (orange circles) are fish presumed to be born in the Sea of Japan and the Nansei Islands area, respectively.
Fig 6.
(a) Salinity, δ18Oseawater, and (b) sea surface temperature time series (1993–2017) in the two spawning grounds of Pacific bluefin tuna. Colors represent the Sea of Japan (July: blue plot) and the Nansei Islands area (May–June: orange plot). The δ18Oseawater values were estimated using salinity data derived from JCOPE2. Grey shaded areas represent the 95% confidence intervals.