Nursery-specific contribution estimates of Pacific Bluefin Tuna to the eastern North Pacific Ocean

Objective The Pacific Bluefin Tuna Thunnus orientalis is a highly migratory species currently managed as a single stock with two discrete spawning areas in the western Pacific Ocean: one extending from the Nansei Islands through Taiwan and the Ryukyu Archipelago (East China Sea) and the other in the Sea of Japan. While age-0 fish remain near Japan, an unknown portion of age-1 and age-2 Pacific Bluefin Tuna migrate to the eastern Pacific Ocean, returning to the western Pacific Ocean after several years. The dynamics and timing of these migrations are poorly understood, and questions remain about how many fish migrate to the eastern Pacific Ocean and from which spawning (i.e., nursery) area they originate. This study used natural tracers (i.e., otolith chemistry) to estimate the nursery origin of subadult and adult Pacific Bluefin Tuna (ages 4–7) from the eastern Pacific Ocean to provide a better understanding of Pacific Bluefin Tuna movement and connectivity. Methods We analyzed otolith cores from age-0 Pacific Bluefin Tuna from the two western Pacific Ocean nursery areas for trace elements (Li, Mg, Mn, Sr, Zn, and Ba) and stable isotopes (δ13C and δ18O) to establish baseline signatures spanning a 4-year period (2011–2014). These baseline signatures were then compared against otolith cores from subadult and adult Pacific Bluefin Tuna collected in the eastern Pacific Ocean to estimate the relative contributions of each nursery area and assess whether these contributions vary with age-class and year. Results Distinct chemical signatures existed in the otolith cores of age-0 Pacific Bluefin Tuna collected from the two nursery areas. Subadult and adult Pacific Bluefin Tuna collected in the eastern Pacific Ocean over a 4-year period (2016–2019) were then age-class matched to baselines using two mixed-stock analysis approaches. Nursery-specific contribution estimates to the eastern Pacific Ocean were generally higher from the East China Sea averaging 61% (SD = 26%) versus 39% (26%) from the Sea of Japan. However, contribution estimates varied largely by year and age-class ranging from 18% to 91% from the East China Sea and 9–82% from the Sea of Japan. Conclusions This study expands upon the application of these natural tracers for population connectivity of this economically important species and highlights the importance of both nursery areas supporting Pacific Bluefin Tuna in the eastern Pacific Ocean. Results suggest that nursery-specific recruitment dynamics can shift substantially across cohorts. The high interannual variability in relative contribution estimates underscores that both the East China Sea and Sea of Japan support migratory Pacific Bluefin Tuna biomass in the eastern Pacific Ocean.