Post-weaning migration of northern fur seal Callorhinus ursinus pups from the Pribilof Islands, Alaska

The post-weaning migration of northern fur seal Callorhinus ursinus pups from the Pribilof Islands in the eastern Bering Sea was investigated using satellite-linked dive recorders deployed in November 1996 and 1997, with tracking duration ranging up to more than 6 mo. Prior to abruptly departing on the migration by early December, pups did not stray far from their natal islands. After a median travel time of 17 d, most pups transited various Aleutian Island passes, though 3 animals remained in the Bering Sea until at least late January. Pups that left the Bering Sea subsequently dispersed into a vast area of the North Pacific some 2500 km wide from the central Aleutians to the Gulf of Alaska and 1000 km from 60 to 45 degrees N. Most pups remained in offshore pelagic areas; however, 3 individuals also spent time over continental shelf waters. While the spatial dispersion and habitats used by migrating pups varied considerably, their diving behavior was remarkably consistent. All migrating pups dove largely during the night, and during evening and morning hours, but practically ceased diving during the day, suggesting they foraged on vertically migrating prey that is unattainable or too costly to reach during daytime. Dives were typically shallow and brief; 77 reached depths less than 10 m, and 81 % lasted less than 1 min. Wide dispersal presents a potential selective mechanism for differential survival, in that benefits (food availability) and risks (storms, predation, and, potentially, fishery interactions) likely vary considerably across the oceanic habitats occupied by these naive foragers on their first migration.