The cause of this decline remains a mystery.

"While the population trends were up in specific areas and certain sectors of the population, the Pribilof Islands pup count is a major marker, and it was down by 4.9 percent since the 2006 count," said center director Doug DeMaster after a 2008 population count.

Analysis of 2008 data produced an estimate of 121,000 northern fur seal pups born in 2008. The total number of adult males counted on the Pribilof Islands increased by 4.6 percent to 10,600 from 2007 to 2008.

Lowell Fritz, a biologist in the center's Alaska ecosystem program, said no recommendations were expected at this time with regard to changes in areas for commercial fisheries.

"We are not seeing more dead pups; we are just seeing fewer pups born," Fritz said in an interview Jan. 15.

Biologists hope to learn more from studies of northern fur seal pregnancies, he said. For the last couple of years, the center has begun doing ultrasounds on some of the pregnant females to determine what, if anything, goes wrong during the pregnancy.

Northern fur seals mate during the summer, but the fertilized egg lies dormant for a while before implanting itself in the uterus. Pups are born the following summer, Fritz said. A male may mate with a number of females, he said.

The northern fur seals feed mainly on a diet of pollock, but also other species that include Atka mackerel, flatfish and herring. Fritz said there is no indication that food is declining.

The complexity of ecosystem interactions and limitations of data and models make it difficult to determine how fishery removals may have influenced this population. Other factors that may have contributed to declines include entanglement in marine debris, parasites and disease, pollutants, general nutrition and predation.

"We have a very long, scientific record of the population of northern fur seals on the Pribilof Islands and not since 1916 have the islands produced this few seal pups," DeMaster said. "Adult male counts began in 1909 and pup counts were initiated in 1912. At that time, the northern fur seal population was rebounding at a healthy 8 percent per year, following the end of extensive at-sea seal hunting."

The northern fur seal population rose steadily from the end of unregulated sealing into the 1950s, when scientists estimated the population at about 2 million seals. A harvest of adult females from 1956 to 1968 reduced the population through the 1970s.

The total Pribilof population size stabilized briefly from about 1980 through the mid-1990s, but since 1998, the population has declined, DeMaster said.

Margaret Bauman can be reached at margie.bauman@alaskajournal.com.