New insights into spatial segregation by sex and life-history stage in blue sharks Prionace glauca in the northwestern Pacific

Spatial segregation is a key component to understand the ecology of highly migratory species; however, this aspect is poorly known for many pelagic shark species. We investigated spatial segregation by sex and life-history stage in blue sharks Prionace glauca in the northwestern Pacific, using satellite tracking data gathered from 74 electronic tags as well as fisheries-dependent size-measurement datasets, which allowed us to update a schematic diagram of the migration patterns of this population. Blue sharks were tracked for 30 to 271 d (mean 125 d) during which they moved extensively between temperate and subtropical waters. Juveniles were distributed mainly in the North Pacific Transition Zone (30–45° N), but expanded their range southward as they grew, while adults in the entire northwestern Pacific showed clear spatial segregation by sex. Adult females migrated seasonally between temperate and subtropical areas for reproduction, while adult males occupied broad distribution area yet mainly in temperate waters (30–40° N), and their habitats partially overlapped with those of juveniles of both sexes. These findings provide new insights for updating the schematic migration diagram of this population, especially because adult males were previously thought to distribute mainly in tropical and subtropical waters. Furthermore, a majority of adult females found north of 30° N exhibited fresh mating marks; this observation suggests that the mating ground of blue sharks in the northwestern Pacific is broader (20–40° N) than previously thought (20–30° N), and partially overlaps with the species' parturition and nursery grounds (30–50° N).