Sealers took 150,000 northern fur seals (Callorhinus ursinus) from the Farallons between 1810 and 1813, followed by Russian fur hunters who lived on the Farallons and extirpated the pinnipeds from the islands. In 1996 West End Island became the fourth American northern fur seal rookery when a pup was born. The recolonizers bore tags from San Miguel Island in the Channel Islands, which had been itself recolonized in 1968. By 2006, nearly 100 pups were born. The fur seals are aggressive and have displaced larger sea lions from their territory. The high count for 2011 was 476 individuals, a 69 percent increase from the year before. By 2016, the pup count alone was 1,126, reflecting a 21% average (but highly variable) annual increase in new pups over the 21 years since recolonization. If the South Farallon Islands population reaches its estimated historical size of 100,000 individuals, it could account for approximately one-fifth of the world's northern fur seal population.
