Trolling the frigid waters of the North Pacific Ocean and Bering Sea, northern fur seals (Callorhinus ursinus) are truly one of nature’s sentinels of the sea. These pinnipeds spend the majority of their lives out at sea in perhaps the most extreme marine environment on earth—the Bering Sea ecosystem—and have been doing so for over 5 millions years. Northern fur seals are the oldest living genera of sea lions and fur seals in the world belonging to the Otariidae family. Their younger relative, the Steller sea lion (Eumetopias jubatus), shares similar habitat and geographic distribution as the northern fur seal.
Northern fur seals range across the North Pacific Ocean Rim extending from southern California, along the western coastline of North America, the Alaskan coastline, Russian Far East, and as far west as northern Japan. They are the most numerous and widespread otariid in the Northern Hemisphere. Unlike Steller sea lions, however, northern fur seals are not known to have established resting locations or haulouts scattered throughout their range. Instead, they spend an estimated 90% of their lives out at sea. The remainder of their time is spent on land at their natal rookery where females return to give birth and nurse their pups, and males mate with the females.
For management purposes, the six known northern fur seal rookeries have been divided into 5 distinct stocks or populations based on geographic separation. The eastern Pacific stock, which the Alaska SeaLife Center studies, includes the rookeries found at the Pribilof Islands and Bogoslof Island. The other U.S. stock is found off the southern coast of California at the San Miguel Island rookery. Currently, the eastern Pacific stock is the largest northern fur seal stock in the world. It has been estimated that 74% of the world’s population of northern fur seals are found at the Pribilof Islands during breeding with another 3% found at Bogoslof and San Miguel Islands. These percentages may have changed as recent population estimates show that the global population has declined in 2004-2005 to 1.1 million—down from an estimated 1.3 million in 1992. During 1992, approximately 984,000 northern fur seals were found at the Pribilof Islands and Bogoslof Islands combined. Preliminary assessments suggest that in 2005, between 676,000-722,000 seals represented the eastern Pacific stock—only 55% of the current estimated global population.
There have been three northern fur seal declines in reported history since 1742—all related to harvesting or management actions. By 1911 and the second reported decline, the U.S. and three other nations established a Fur Seal Treaty which officially lasted until 1985. During that time, fur seals reached population highs in the 1940s and 1950s—numbering over 2 million individuals. Harvesting from 1956-1968 killed an estimated 315,000 females leading to the most recently reported global population decline. Commercial harvesting was terminated in the U.S. by 1985, but a subsistence harvest still continues at the Pribiolof Islands where approximately 2,000 are taken each year.
During the past 50 years, trends have varied between rookeries with localized population increases or decreases. Only two new rookeries have been established since 1786—San Miguel Island in southern California in 1965 and Bogoslof Island in 1980. Geologically, Bogoslof Island only recently emerged as an island in the 1880s, and is part of the Aleutian Island Chain. Numbers have shown healthy populations at these two sites, however at the same time, the Pribilof population suffered dramatic declines. The declines in this area, especially from 1974-mid 1980s, prompted the National Marine Fisheries Service (NMFS) to list this species as depleted under the Marine Mammal Protection Act (MMPA) in 1988, and have continued to be listed as such. Although much of the decline is attributable to commercial harvesting, there are still underlying factors that have yet to be identified.
There is strong sexual dimorphism between male and female with males weighing almost 3-5 times more than females (males weigh 450-550 lbs, 200-250 kg; females weigh 100 lbs, 45 kg). Northern fur seals are a highly social and gregarious species, often forming large colonies when they return to land to breed. Both males and females return annually, when mature, to the same rookery with timely precision—often on the exact day from previous years. Juveniles will tend to remain foraging in the open ocean until about 2 years of age. Females reach reproductive maturity from 5-7 years of age and males from 8-10 years. Adult males will, on average, have only 1.5 seasons to breed.
When returning to the rookery, they exhibit strong site fidelity (returning to the same site) and philopatry (mating at the site of their own birth)—females will give birth 8-10 m from the same location in previous years. Males begin arriving in mid-May and establish territories. Females arrive in mid-June, giving birth to a single pup within a few days of their arrival. During July 4th-10th, peak pupping occurs. Males will remain on the rookery and fast, while defending their territories. Males will typically mate with females 5-14 days after parturition (giving birth). Females will alternate nursing their pup and foraging cycles for about 4-months, upon which fur seals will depart from the rookery by November. By January-February, fur seals will congregate along the continental margin where intermixing of stocks can occur, and in March, there journey back to the rookery begins.