Isotopic insights into migration patterns of Pacific bluefin tuna in the eastern Pacific Ocean

Understanding regional migration, residency dynamics, and associated trophic ecology can inform recovery strategies for pelagic species such as Pacific bluefin tuna (Thunnus orientalis) (PBFT). PBFT residency duration in the eastern Pacific is uncertain, particularly for larger individuals (here, >100 cm or ~3+ years of age). We applied a previously tested “chemical tracer toolbox” (Fukushima-derived radiocesium and 13C and 15N stable isotope signatures) to examine migratory and residency patterns and dietary inputs of 428 age 1-6+ PBFT collected from 2012 to 2015 in the eastern Pacific Ocean. Age 1-3 individuals were a mix of residents and recent (≤500 day) migrants, while 98% of age 3-4 and 100% of age 4-6.3 year old PBFT were resident for >500 days in the eastern Pacific. Zooplanktivorous forage (e.g., sardine, anchovy, pelagic red crab, and trophically similar species) of the California Current Ecosystem constituted 57%-82% of diet across PBFT sizes. Migration timing estimates show that PBFT may spend 2-5 years in the eastern Pacific Ocean before returning to the western Pacific.