Post-spawning migration and habitat use of adult Pacific bluefin tuna (Thunnus orientalis) in the western North Pacific

Pacific bluefin tuna (PBF, Thunnus orientalis), migrate widely throughout the north Pacific Ocean during all life stages. However, studies on the migration of adult fish in the northwestern Pacific Ocean, where spawning occurs, are limited. This study investigated the post-spawning migration and habitat use of PBF from the major spawning grounds of the Nansei Islands (approximately 23–31° N), Japan, using pop-up satellite archival transmission (PAT) tags. The PAT tags were deployed on 15 PBF individuals (estimated fork lengths: 170–260 cm) in the late spawning season (May–June) of 2021, and tracking data were collected for 3–103 days. Four tagged PBF individuals moved > 2,500 km north to the Kuroshio-Oyashio transition region at high latitudes (40° N) in July–September, immediately after the spawning season. During this northward movement, these PBF showed a spatiotemporal change in their vertical behavior, adapting to regional thermal habitat differences. At low latitudes near the spawning grounds (23.6–29.6° N), the PBF dived to deep depths (mean: 162.8 m) to avoid the warm surface layer (mean: 28.6 °C); however, once they reached higher latitudes (36.6–44.9° N), they spent most of their time in the cool surface layer (mean: 18.5 °C; 22.3 m). Our findings suggest that adult PBF move from spawning grounds to physiologically suitable low-temperature, food-rich waters through long-distance movements, and then exhibit a habitat preference behavior characterized by repeated short-distance movements in response to the feeding environments at high latitudes.