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NOAA Technical Memorandum NMFS-AFSC-375

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Fur seal investigations, 2015-2016

Abstract

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 2015-2016.

Population parameters monitored in 2015 and 2016 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 2016. On St. Paul Island, annual counts of harem males were nearly constant from 2014 to 2016. On St. George those counts increased 13% between 2014 and 2015 and decreased 9% from 2015 to 2016. The total estimated number of pups born on St. Paul Island in 2016 was 80,641 (SE = 717), a 12% decline from 2014 (P < 0.01) after a loss of over half the annual production from 2000 to 2010. On St. George, the estimate was 20,490 (SE = 460), an 8% increase from 2014 and continuation of the moderate upward trend that began around 2006. Pup mortality at one month of age was approximately 3% on St. Paul and 1% on St. George. The subadult male harvest on the Pribilof Islands was 373 and 345 in 2015 and 2016, respectively. Harvests of 57 and 46 male pups were made in 2015 and 2016 on St. George Island.

From 2007 to 2016, 883 adult and subadult female fur seals were flipper-tagged in the fall at Polovina Cliffs rookery, St. Paul Island. From 2009 to 2016, 536 were tagged at South Rookery on St. George Island. One thousand, two-hundred and fifty-five female pups were tagged at Polovina Cliffs from 2008 to 2016 and 4,011were tagged from 2010 to 2016 at Zapadni Reef rookery on St. Paul Island; 8,691pups of both sexes were tagged from 2009 to 2016 at South Rookery, St. George. Re‑sightings were made in July-August every year after the initial tag deployments, and in Sept.-Oct. 2012-2016 for juveniles at South and Zapadni Reef rookeries. Tag loss is a significant problem in survival estimates, but using tags with the lowest estimated loss rate (~1% annually) the average estimated survival at Polovina Cliffs was 0.80 (95% CI = 0.76-0.83); at South Rookery it was 0.81 (95% CI = 0.78-0.84). Pupping rates among adult females 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. Age at first pupping was approximately 1 year younger than estimated in the 1950s and 1960s, based on age-specific rates among the tagged pup cohorts. Estimated survival of three pup cohorts to a recruitment age of 4 years ranged from 0.18 to 0.21, with 95% confidence intervals in the range of 0.15-0.21. Models incorporating these estimates produce an annual rate of decline much greater than observed during the study period, suggesting significant biases in survival estimates that may be due to emigration of tagged seals from the study sites.

At San Miguel Island the index count of territorial bulls at Adams Cove was 111 and 110 in 2015 and 2016, respectively. These were 50% less than the highest historical count of 265 obtained in 2014. In 2015 and 2016, the total numbers of pups born at Adams Cove were estimated at 2,035 and 2,325. At nearby Castle Rock those estimates were 998 and 1,709. Pup mortality from birth to 3 months was 17% and 29% in 2015 and 2016 at Adams Cove. At Castle Rock pup mortality in those years was 7% and 6%. Pup weights standardized to 1 October at Adams Cove were ~35% below the long-term average in 2015, but near that average in 2016. Counts from aerial photographs of the Farallon Islands, California, continued to increase from the first counts in 2013, reaching 1,126 pups and 2,238 in all age-sex categories in 2016.




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