Discrimination of eastward trans-Pacific migration of the Pacific bluefin tuna Thunnus orientalis through otolith δ13C and δ18O analyses

The Pacific bluefin tuna Thunnus orientalis is a highly migratory species, some tuna exhibiting a trans-Pacific migration between the Western Pacific Ocean (WPO) to the Eastern Pacific Ocean (EPO) at the juvenile stage. To detect a signature for the eastward trans-Pacific migration, we analyzed otolith stable isotope ratios of carbon (δ 13 C oto ) and oxygen (δ 18 O oto ) along the growth trajectory for 20 migrants (age 3–5 years) from the EPO and 10 residents (age 2–3 years) from the WPO. The δ 18 O oto values for migrants recorded during the eastward trans-Pacific migration were higher than those for residents during the same period. The higher δ 18 O oto values for migrants reflect the low water temperatures encountered during their migration. The δ 13 C oto values were higher in the migrants than in the residents, suggesting that the higher values of carbon isotope ratios of both dissolved inorganic carbon and the diet in the EPO are recorded in the δ 13 C oto profiles. Our study demonstrates that otolith isotope analyses can successfully detect trans-Pacific migration of the Pacific bluefin tuna.