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Seasonal migrations of pregnant blue sharks Prionace glauca in the northwestern Pacific

Information on the movements of highly migratory species is important to understand their ecology, including habitat use, population connectivity, and stock structure, to implement appropriate management and conservation measures. The blue shark Prionace glauca (Carcharhinidae) is highly migratory, has a global distribution, and is ecologically and economically important as one of the most abundant apex marine predators; however, the migration patterns of pregnant females are unknown. Here, we used pop-up satellite archival tags to elucidate seasonal migration of pregnant blue sharks in the northwestern Pacific. Of 24 tagged adult females, archival data were subsequently obtained for 21 individuals (141.1-243.3 cm precaudal length). Based on ultrasonography or analysis of sex steroid hormones, 17 of these females were confirmed to be pregnant at the time of release. Females with small embryos moved in a northeasterly direction, from subtropical (10-30°N) into temperate (30-40°N) waters during autumn and the following spring; 2 of these females returned to subtropical waters in spring and summer. In contrast, females with large embryos moved in a southwesterly direction, from temperate into subtropical waters during spring and autumn. Tagged sharks also showed regional differences in diving behaviors, reflecting thermal habitats in the ocean environment. Our findings indicate that pregnant blue sharks undergo a seasonal northeast-southwest migration within a year to give birth to pups in productive temperate waters. This is the first report on seasonal reproductive migration of pregnant blue sharks in the global ocean.