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Fur seal investigations, 2013-2014
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2016
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Description: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 (St. Paul and St. George) and Bogoslof Island in the eastern Bering Sea, and on San Miguel Island and the Farallon Islands off the coast of California. This report summarizes these monitoring efforts in 2013-2014. Population parameters monitored in 2013 and 2014 on the Pribilof Islands included the size of the subsistence harvest and the number of adult male fur seals. Biennial estimates of the number of pups including mortality, size, and sex ratio were made in 2014. Harem males increased by 13.3% between 2012 and 2013 and decreased 10.7% between 2013 and 2014. The total estimated number of pups born on St. Paul Island in 2014 was 91,737 (SE = 769); on St. George the estimate was 18,937 (SE = 308). Pup mortality at one month of age was approximately 3% on both islands. Pup production estimates on St. Paul in 2012 and 2014 were similar to that in 2010, with no significant decline over the last three estimates (2010-2014) after ~ 6% annual decline from 1998 to 2010. On St. George, pup estimates have been stable or slightly increasing since 2002 or 2004. In 2014, pups of both sexes were significantly longer and heavier at St. George than at St. Paul. Since 2002, pup production has been below that estimated in 1919, when the population was recovering at 8% annually from a pelagic harvest that ended in the early 20th century. The subadult male harvest on the Pribilof Islands was 379 and 365 in 2013 and 2014, respectively. An inaugural harvest of 214 pups was conducted in autumn 2014 on St. George. Modeling of possible pup harvests on St. Paul projected a mostly minor impact on the population after 25 years, unless the sex ratio of the harvest (expected to be targeted on males) approached 1:1. From 2007 to 2014, 759 adult and subadult female fur seals were flipper-tagged in the fall at Polovina Cliffs rookery, St. Paul. From 2009 to 2014, 471 were tagged at South Rookery on St. George. Eight hundred and ninety-five female pups were tagged at Polovina Cliffs from 2008 to 2014; 3,010 were tagged from 2010 to 2014 at Zapadni Reef rookery on St. Paul; 8,691 pups of both sexes were tagged from 2009 to 2014 at South Rookery, St. George. Re-sightings were made in July-August every year after the initial tag deployments, and in Sept.-Oct., 2012-2014 for juveniles at South and Zapadni Reef rookeries. Pupping rates at both rookeries were high (0.80-0.90 at Polovina Cliffs and 0.80-0.88 at South), 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 cumulative loss of both tags from an individual were low for adults (0-11% after 3-5 years, depending on tag type), but were as high as 25% (95% confidence interval = 17%-29%) for Dalton tags after 5 years in the first cohort of pups from South rookery. Dalton tags were largely unreadable after 5 years. At San Miguel Island the index count of territorial bulls at Adams Cove was 166 and 224 in 2013 and 2014, respectively, the latter representing the highest value observed since 1997. In 2013, the total numbers of pups at Adams Cove and Castle Rock were estimated at 2,051 and 1,218, respectively. Only Adams Cove was estimated in 2014, where 2,288 births were estimated, 2% higher than the previous peak in 1997. Pup mortality from birth to 3 months of age in recent years has been high and remained so in 2013 (39%) and 2014 (28%). Pup weights standardized to 1 October were near the long-term average in 2013, but they were more than 2 kg lower in 2014. Counts from aerial photographs of the Farallon Islands, California were presented for the first time: 401 and 656 pups and 265 and 363 non-pups were counted in 2013 and 2014, respectively. The total stock size of northern fur seals in the United States, which includes Bogoslof, the Pribilof, and San Miguel Islands was approximately 634,000 fur seals, 83% of which are from the Pribilof Islands. [doi:10.7289/V5/TM-AFSC-316 (http://dx.doi.org/10.7289/V5/TM-AFSC-316)]
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Content Notes:by J.W. Testa (editor).
"April 2016."
doi:10.7289/V5/TM-AFSC-316 (http://dx.doi.org/10.7289/V5/TM-AFSC-316)
System requirements: Adobe Acrobat Reader.
Includes bibliographical references (pages 103-111).
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Rights Information:Public Domain
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