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NOAA Technical Memorandum NMFS-AFSC-257

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Fur seal investigations, 2012


Researchers from the Alaska Fisheries Science Center's National Marine Mammal Laboratory conduct field investigations on the population status of northern fur seals (Callorhinus ursinus) on the Pribilof Islands, Bogoslof Island in the eastern Bering Sea, and on San Miguel Island off the coast of California. This report summarizes these monitoring efforts in 2012, and presents an introduction to ongoing demographics research based on tagged animals on the Pribilof Islands that began in 2007.

Population parameters monitored on the Pribilof Islands included the size of the subsistence harvest and the number of adult male fur seals. On St. Paul Island, 3,336 territorial male seals with females were counted in 2012, which represented a 12.9% annual decline from 2011. On St. George Island the total was 852, which represented a 1.2% increase. The subadult male harvest was 383 and 63 on St. Paul and St. George, respectively. The estimate for the total number of pups born in 2012 was 96,828 (SE = 1,260) on St. Paul Island (not including Sea Lion Rock) and 16,184 (SE = 155) on St. George Island. Pup mortality from birth to late August was 3.7% on St. Paul Island and 3.1% on St. George Island. The number of pups born on St. Paul Island in 2012 was not significantly changed from the previous estimate in 2010 (P = 0.9), while at St. George the number of pups born was 9.9% lower (P < 0.01).

From 2007 to 2012, 632 adult and sub-adult female fur seals were flipper-tagged in fall at Polovina Cliffs rookery, St. Paul Island. From 2009 to 2012, 462 were tagged at South Rookery on St. George Island. Seven hundred and thirteen female pups were tagged at Polovina Cliffs from 2008 to 2012, 1,921 were tagged from 2010 to 2012 at Zapadni Reef rookery on St. Paul; 6,605 pups of both sexes were tagged from 2009 to 2012 at South Rookery, St. George Island.

Resightings were made in July-August each year after the initial tag deployments at Polovina Cliffs and South rookeries, and in Sept.-Oct. 2012 for juveniles at South. Pupping rates at both rookeries were high (0.86-0.91 at Polovina Cliffs and 0.79-0.88 at South Rookery), consistent with recent and historic estimates of pregnancy rates in northern fur seals. Tag loss varied by tag manufacturer (Dalton Superflexitag loss > Monel metal tags > Allflex sheep tags), tag age (1st year > later years) age class (pups > adults), and rookery (South > Polovina Cliffs). Estimated rates of losing both tags were low for adults (0-6%), but were ~15% for Dalton tags after 2-3 years in the first cohort of pups from South rookery. Preliminary estimates of adult survival were lower than historic estimates (0.71-0.86 at Polovina Cliffs and 0.76-0.84 at South vs. ~ 0.88 historically), without consideration of permanent emigration and tag loss, which cause negative bias. At San Miguel Island in 2012 the index count of territorial bulls was 178. On the two rookeries there, Adams Cove and Castle Rock, the estimated numbers of live pups were 1,692 (SE = 22.5) and 1,163 (SE = 2). Total pup production in 2012 was 4% higher than the peak recorded in 1997, just prior to a strong El Niño event, though the recovery since that time has been sporadic. Pup mortality in recent years has been high, and remained so in 2012 (31%). Pup weights were near the long-term average.

The estimate of the total stock for the Pribilof Islands population in 2012 was about 547,000 fur seals. The total stock size for the United States, which includes Bogoslof, the Pribilof, and San Miguel Islands, was approximately 664,000 fur seals.

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