Spawning migration and habitat use of adult Pacific bluefin tuna (Thunnus orientalis) in the Northwestern Pacific Ocean

Pacific bluefin tuna (PBT, Thunnus orientalis) is a highly migratory species that mainly inhabits temperate regions of the North Pacific Ocean. To examine the population dynamics of this commercially and ecologically important species, it is essential to understand their spawning migration, since Northwestern Pacific Ocean is their important spawning ground. A total of three PBT were tagged with pop-up satellite archival tags (PSATs) in the spawning season (May) of 2021, and the tags remained affixed for 13, 54 and 61 days, respectively. The linear displacement ranged from 797 to 2743 km from deployment locations to pop-up locations. The deepest descent recorded was 1458 m, and the coldest temperature visited was 2.6 °C. The time spent at depth was significantly different between the daytime and nighttime, where the fish displayed regular crepuscular patterns of ascending into the surface layer at dusk and remaining there until the following dawn, when they descended past the mixed-layer depth. At the spawning grounds, PBT exhibited shallow oscillatory diving behavior and frequently visited the surface during daytime and nighttime with longitudinal movements correlated with sea surface height anomalies and mesoscale eddies. This study tracked adult PBT on their spawning grounds, and insights were gained into spawning migration, seasonal movements, and habitat use of this species in the Northwestern Pacific Ocean.