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Pacific bluefin tuna Thunnus orientalis has 2 spawning grounds with different spawning seasons. One is in the northwest region of the Philippine Sea during May to June and the other is in the Japan Sea from late July to August. The oxygen isotope level in otoliths is affected by water oxygen isotope composition and negatively correlated to the ambient water temperature. We developed a technique using the temporal profiles of otolith δ18O to discriminate the natal origin of Pacific bluefin tuna. For Age-0 tuna caught in the Japan Sea (n = 35), the δ18O values varied from –2.5 to –2.2‰ between the otolith core and a distance of ca. 600 μm from the core, and then the values increased (enriched) to about –0.8 to –1‰ at 700 to 1000 μm. For Age-0 tuna caught in the Pacific Ocean (n = 41), the otolith δ18O values varied between –2.8 and –2.2‰ at 0 to 1000 μm from the otolith core and then increased to ca. –1.5 to –1‰ at around 1200 to 1600 μm from the core. The different profiles of otolith δ18O values between tuna caught in the Pacific Ocean and the Japan Sea evidently reflect different water temperatures along the migratory route from their natal origin. Fish hatched in the Japan Sea experience water temperatures of <15°C in winter, about 4 mo after hatching, while fish hatched in the Philippine Sea experience winter water temperatures of 17 to 18°C about 6 mo after hatching. The temporal difference in experienced water temperatures produces an earlier increase of otolith δ18O values for tuna caught in the Japan Sea than in the Pacific Ocean. The location of the first winter signal recorded as enriched δ18O in the otolith is a valuable proxy for tracking the natal origin of Pacific bluefin tuna.

Natal origin of Pacific bluefin tuna Thunnus orientalis inferred from otolith oxygen isotope composition