2021 Northern Fur Seal Pup Production And Adult Male Counts On The Pribilof Islands, Alaska
Northern fur seal pup and adult male counts.
Numbers of northern fur seal, Callorhinus ursinus, pups were estimated using a mark-recapture method, shear-sampling, on St. Paul Island during August 2021. Due to restrictions during the Covid 19 pandemic, 5 sample rookeries were sheared and dead pups counted (Table 1). Adult male counts were counted on 14 sites, but pup production was only estimated on 13 sites since no pups were observed on Little Polovina in August, 2021. An estimate of the number of pups alive at the time of marking for St. Paul Island (excluding Sea Lion Rock) was obtained using a jackknife ratio estimate of pups to harem males on sampled rookeries. The jackknife ratio estimate was then multiplied by the total number of harem males counted (see below, excluding Little Polovina) to get the estimate of pups alive (69,696, SE = 10,902) for rookeries on St. Paul Island. Pup mortality was estimated for the whole island using the estimated mortality rate from sample rookeries. The observed pup mortality rate on sample rookeries was 4.27% (SE = 0.06%) on St. Paul Island. We estimated 72,806 (SE = 11,389) pups were born on St. Paul Island. The larger variance on the estimates was expected since a sample of rookeries, versus all rookeries, was used in the estimation process. The 2021 pup production estimate for St. Paul Island is 3.84% less than the estimate in 2018 (Figure 1). Since 1998 pup production on St. Paul Island declined 59.4%, or at an annual rate of 4.02% (SE = 0.33). No pup census work was undertaken in 2021 on St. George Island. Pup production on St. George shows no significant trend in production from 1998 to 2018.
Adult male northern fur seals on St. Paul and St. George Islands were counted over the period July 10 to 19, 2021 (Table 2). Due to the pandemic and other logistical concerns, bull counts on both islands have not been obtained consistently for the past 5 years. To provide similar assessments, 2021 counts have been compared to 2018, the last year of complete bull counts at both islands. Counts of territorial males with females (class 3; "harem" males) on St. George Island increased by 18.9% compared to 2018. Idle males (classes 2 and 5) on St. George Island decreased in comparison to 2018 by 21.1%. On St. Paul Island the "harem" males increased by 5.2% and the idle males decreased by 8.7% compared to 2018 (Figure 2). Overall, the total number of all classes of adult males counted on the Pribilof Islands in 2021 decreased by 1.1% from 2018 to 7,524.