Summer migration to temperate foraging habitats and active winter diving of juvenile loggerhead turtles Caretta caretta in the western North Pacific

Endangered loggerhead turtles (Caretta caretta) are highly migratory marine reptiles that spend extended maturation periods at sea. Although an understanding of the biology and ecology of juveniles is essential for effective conservation of this species, such information is still limited, especially for the North Pacific population. The first quantitative bycatch survey in northern Japan revealed that the Sanriku coastal area served as an important summer foraging habitat for large juvenile loggerhead turtles in the western North Pacific. To examine their year-round distribution and diving behaviour, we deployed satellite-relayed data loggers on six turtles (mean SCLn–t ± SD = 69.2 ± 9.2 cm, range = 58.3–82.5 cm). While turtles predominantly utilize the Sanriku coastal area and surrounding oceanic waters during summer, all turtles spent most of their time travelling in oceanic waters (>200 m) several kilometres east of Japan and sometimes in association with the Kuroshio Extension Current during winter. Although water temperature experienced by turtles significantly decreased during winter, they predominantly stayed in water >15 °C. There was no strong seasonal pattern in diving behaviour, and daily average dive duration was 20.9 ± 11.6 min regardless of summer or winter. In addition, turtles sporadically performed deep dives, including the deepest dive (>340 m) ever recorded for this species, during winter. These results suggest that juveniles remained active and possibly foraging in oceanic habitats throughout the winter rather than adopting the quiescent overwintering strategy (i.e. undertaking prolonged dives lasting >300 min) previously reported in adult females.