Spatial and temporal variability in the trans-Pacific migration of Pacific bluefin tuna (Thunnus orientalis) revealed by archival tags

Archival electronic tags were internally implanted in 713 age-0 Pacific bluefin tuna (PBF) caught in their nursery waters off the southern coast of Japan and in the East China Sea over an extended study period (1995-2015) to clarify the spatial and temporal variability of their trans-Pacific migration. Two hundred twenty-five of these tagged tuna were recaptured by fisheries (31.6%), and we successfully retrieved tag data from 14 of 21 individuals recovered in the Eastern pacific. Furthermore, one archival tag recovered in the Western Pacific revealed that the individual had performed a trans-Pacific migration, so in total 21 tagged PBF were shown to have migrated to the Eastern Pacific (2.9% of the total tags released). We successfully downloaded data from 15 of these 21 archival tags, which revealed that some age-1 PBF migrate rapidly (123.9 +/- 82.8 km day-1) and directly from waters offshore of Japan to the eastern Pacific (160.0 E to 130.0 W), a journey that takes an average of 2.5 months (ranging from 1.2 to 5.5 months) through relatively cool waters (14.7 2.0 degrees C). All juvenile PBF began their trans-Pacific migration shortly after exposure to cooler water temperatures ( < 14 degrees C), suggesting that sustained residence in lower water temperatures presents a physiological challenge for this age class. Three patterns were identified in the timing of the departure of juvenile PBF from the western Pacific: departing 12-14 months post-hatch (N = 7) in early summer (May-July), departing 17-19 months post-hatch (N = 7) in late autumn (October-December), and departing 21 months post-hatch (N = 1) in late winter (February). The PBF tagged along the southern coast of Japan (SCJ) arrived in the eastern Pacific earlier than those tagged in the East China Sea (ECS), most likely due to the shorter travel distance. Additionally, the PBF that began their trans-Pacific migration in the earlier period remained in an offshore foraging zone (the Kuroshio-Oyashio transition region) for shorter periods (2.8 months on average) and at lower latitudes (35.0 N) during the spring, while the PBF that delayed their migration spent more time (6.7 months on average) in the productive waters between 35.0 and 45.0 degrees N during the spring-autumn months. The variability in the departure timing of the trans-Pacific migration of age-1 PBF may be related to geographic differences between nursery areas in addition to oceanographic conditions and foraging opportunities encountered by the tuna in the offshore waters of Japan during their first year.