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Alaska Ecosystems Program

Alaska Northern Fur Seal Demographic Research, 2009

Figure 1, adult female northern fur seal
Figure 1.  Adult female northern fur seal and dependent pup on St. Paul Island, Alaska.  Photo by Louise Taylor.
 
 

At their peak in the 1950s, northern fur seal (Callorhinus ursinus) rookeries on St. Paul and St. George Islands in the Bering Sea produced over 400,000 pups annually.

An experimental harvest of females in the 1960s, intended to reduce resource competition and increase reproductive rates, was followed by a post-harvest population decline, a period of relative stability, and a more recent decline that has occurred at an annual rate of 5.2% since 1998. Understanding the demographic mechanism underlying the decline is an important part of unraveling the ecological causes for this most recent downturn.

The Alaska Ecosystems Program (AEP) initiated a tagging program on St. Paul Island in 2007, began resighting studies in 2008, and continued tagging and resighting efforts in 2009. Resightings of tagged fur seals are focused on the north end of Polovina Cliffs rookery on St. Paul Island, one of the few locations offering favorable vantage points for detecting and reading flipper tags without recapturing the seals.

Past captures at this location have been motivated by various objectives, so a combination of tag types exists. Not all tags are designed to maximize tag retention or readability needed for demographic research.

Captures at Polovina Cliffs rookery in November 2007 and 2008 included 133 juvenile and adult females that were captured for pregnancy diagnosis by ultrasonography. These animals were tagged with a conspicuous large Allflex tag and a VHF radio tag in either flipper. While the large Allflex tags are the most visible and easily read of the tags, they also are lost at a higher rate than the smaller tags deployed primarily to estimate demographic parameters.

The VHF tags do not have permanent lettering for resighting purposes, and their transmitters functioned for only 1 year. As such, we expect animals with these tags to provide more limited demographic information than the additional 143 juvenile and adult females tagged specifically in those years to address demographic questions. In addition, a small number of females were tagged in studies prior to 2007.

In July and August 2009, the resighting effort was led by our collaborators at Oregon State University: master's student Erin Kunisch and her assistants Stephen Meck and Brett Miller. They identified 218 tagged females on the beach at Polovina Cliffs. Nearly 90% of these females were observed interacting with a pup, indicating a true mother-pup bond (Fig. 1) and a likely maximum bound on annual pupping rate (assuming that some of the tagged females that were not seen were alive and their pupping status, although unknown, was more likely to be negative).

Of the tagged females known to be alive in 2008, 75% were seen in summer 2009, which is a minimum estimate of annual survival (assuming that some were not seen and that some had lost their tags). Our estimation of both pupping and survival rates relies on capture-mark-recapture statistical methodology and requires multiple years of observations to make inferences about the animals not seen in a given year. Our resighting effort in future years will provide corrections to the biases in these preliminary estimates.

Demographic research on northern fur seals at the Pribilof Islands is expanding. Tagging efforts were significantly increased during fall 2009 with a large deployment of tags on fur seal pups and initiation of a tagging program at South Rookery on St. George Island.

On St. Paul Island, a field team of 9-12 scientists tagged 478 female pups and 156 juvenile and adult females at the intensive study site at Polovina Cliffs, resulting in a total of 470 tags deployed on adult females on St. Paul Island during 2007-09.

For ageing studies, teeth were collected from 164 anesthetized animals in 2008-09, including 107 of the juvenile and adult females tagged in 2009. Ten of these adult females also received satellite transmitters to monitor overwinter movements.

On St. George Island, a crew of 9-12 scientists tagged 1,964 pups of both sexes, as well as 91 juvenile and adult females, 83 of which had a tooth removed for ageing studies. Males generally start reappearing on shore at an earlier age than females do (ages 2-3 instead of ages 5-7), so the tagging of male pups will allow an initial assessment of pup survival in 2012.

By Ward Testa, Rolf Ream, and Tom Gelatt
 

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