The Northern fur seal is a pinniped that is widely distributed in the North Pacific and is migratory, coming ashore en masse at specific breeding sites during the summer. There is marked sexual dimorphism in body size; males establish territories at breeding grounds and practice polygyny. Mothers raise their young by making repeated short foraging trips, and after weaning the juveniles join the migration. Diet consists mainly of fish and squid; bycatch, reductions in prey resources, and marine debris are factors contributing to population declines, and the species is classified as Vulnerable (VU) by the IUCN.