In 1786 the Pribilof Islands probably supported a sea lion population considerably in excess of 15,000 animals. These bred on two rookeries on St. George Island, one on St. Paul Island, and one on Walrus Island. A few may have bred also on Sea Lion Rock. The Walrus Island colony disappeared in 1827; between 1867 and 1914 both St. George rookeries were exterminated, and the St. Paul rookery was reduced to less than 150 animals. A measure of protection was given the sea lion in 1914. By 1960, the population had increased to 5,700–6,700 adults. The only breeding ground today on the Pribilofs is on Walrus Island, where about 3,000 young were born in 1960. Newborn pups were last seen on St. Paul Island in 1957. All extinct rookery sites are now regularly used as hauling grounds. Otter Island, never a breeding ground, is a regular winter hauling ground. Unregulated exploitation and harassment by man probably played an important role in the reduction of the Pribilof sea lion herd and the shifts in rookery locations. The failure of the Pribilof population to approach its aboriginal size and to reoccupy old breeding grounds during a 40-year period of moderate exploitation is unexplained. Unknown ecological factors are suggested as a contributing cause.