ANCHORAGE, Alaska - America's northern fur seal pup population continues a marked decline this decade, federal biologists reported Friday. The number of pups born between 2004 and 2006 in Alaska's Pribilof Islands, home of the world's largest rookeries, fell by 9 percent from the previous two year estimate, according to researchers from the National Oceanic and Atmospheric Administration. "We have seen a significant decline in the abundance of fur seals on the Pribilof Islands starting about 1998, and we have not been able to identify the factors responsible," said Doug DeMaster, director of NOAA's Alaska Fisheries Science Center.Less pollock, herring, squid and other seal food because of global warming or commercial fishing are possibilities, as is increased predation by killer whales. Entanglement in marine debris, parasites, disease and pollutants may have contributed to past or present declines, according to federal researchers.The Pribilofs are a group of five islands in the Bering Sea about 300 miles from the mainland. St. Paul is the largest at just more than 40 square miles and St. George, 47 miles to the south, is just under 35 square miles.The remote points of land in the Bering Sea are the breeding grounds for more than half of the world's northern fur seals. Just 15 years ago, they accounted for 74 percent of the population.The most recent federal estimate of fur seals in the Eastern Pacific stock, which includes breeders on Bogoslof Island in the Aleutians, is 721,935, according to the National Marine Fisheries Service. The new pup counts likely will mean those figures will go down when the agency updates its population estimate later this year, said NMFS zoologist Rolf Ream. Federal biologists estimate the worldwide population at 1.1 million.The latest pup estimate decreased most sharply on St. Paul Island, where the agency in 2004 estimated 122,825 pups. The new estimate on St. Paul was 109,937 pups — a 25 percent drop from 145,716 estimated in 2002.The agency in 2004 counted 16,876 on St. George, which has now increased to 17,070 pups. While that's a slight increase, researchers says it represents a 3 percent decline from the 17,593 in the 2002 estimate.The Pribilofs were discovered in 1786 by Russian fur traders. Two years later, the Russian American Co. enslaved and relocated Aleuts from Siberia, Atka and Unalaska to the Pribilofs to hunt fur seals. Their descendants live on the two islands today.The United States government also sanctioned sealing after the purchase of Alaska from Russia in 1867. By 1890, fur seals had been severely over-harvested and the 1910 Fur Seal Act ended private leasing on the islands.DeMaster said the agency has a long scientific record of the population of northern fur seals on the Pribilofs. "Adult male counts began in 1909 and pup counts were initiated in 1912," he said. "At that time, the northern fur seal population was rebounding at a healthy 8 percent per year, following the cessation of extensive pelagic (at sea) sealing."In the 1950s, scientists estimated the population at about 2 million, but a harvest of adult females from 1956 to 1968 reduced the population again. The Pribilof population size stabilized from about 1980 through the mid-1990s but has since declined.The National Marine Fisheries Service in 1988 declared the stock of northern fur seals to be "depleted," or below its optimal sustainable population under the Marine Mammal Protection Act.Pribilof Island fur seals spend only summer months foraging in the Bering Sea. The rest of the year they migrate south of the Aleutians and forage without coming to shore.Males arrive at rookeries in late spring. Breeding females arrive in mid-June, give birth a few days later and nurse on the islands through October.Besides the Pribilofs, northern fur seals also breed on Bogoslof Island in the Aleutians and on California's San Miguel and South Farallon islands. Northern fur seals also breed on the Kurile Islands northwest of Japan, the Commander Islands off Russia's Kamchatka Region, and Robben Island in the Sea of Okhotsk.