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Alaska Ecosystems Program

Northern Fur Seal Research in Alaska

Figure 1, see caption
Figure 1.  Argos satellite (blue triangle) and GPS
(red circle) locations acquired from an adult female northern fur seal during a summer foraging trip from St. Paul Island in 2008.
 
 

The National Marine Mammal Laboratory's (NMML) Alaska Ecosystems Program (AEP) conducted a number of studies on northern fur seals (Callorhinus ursinus) during 2008, with field components that began in June and continued until late November.

The primary research objectives of monitoring population trends and investigating the ecology and health of the fur seal population on the Pribilof Islands (St. Paul and St. George) were accomplished through a variety of projects, including studies of pup condition, causes of mortality, and habitat use during the winter migration.

However, the majority of the AEP's research efforts involved three projects that collected data on fur seal abundance, fine-scale foraging behavior of adult females in the Bering Sea, and age-specific vital rates of females on St. Paul Island.

Northern fur seal abundance and population trends were assessed on the Pribilof Islands during 2008 by obtaining counts of adult males and estimates of the numbers of pups born. Recently completed analyses of data collected during the summer indicated that, although the numbers of adult males increased, overall pup production has continued to decline on the Pribilof Islands.

Numbers of adult males increased on both St. Paul and St. George Islands from 2007 to 2008. The total number of adult males (10,612 individuals) counted on the Pribilof Islands during 2008 represented an increase of 4.6%. The trend in the number of territorial males with females was even more notable, having increased by 14.2% from 2007 to 2008.

Pup production estimates were obtained using a mark-recapture (shear-sampling) method, and data were collected at both St. Paul and St. George Islands during August. We estimated that 102,674 (SE = 1,084) pups were born on St. Paul Island and 18,160 (SE = 288) pups were born on St. George Island during 2008.

Since 2006 when the previous estimates were obtained, pup production declined by 6.6% at St. Paul Island and increased by 6.4% at St. George Island. The combined pup production estimate for St. Paul and St. George Islands declined approximately 4.9% from 2006 to 2008.

Since 1998, pup production at these primary Pribilof Island breeding colonies has declined at an annual rate of 5.2% (SE = 0.40) and is now at a level of production that was last observed around 1916. Pup production was also estimated at Sea Lion Rock, a small island approximately 500 m from St. Paul Island, for the first time since 2002. The estimated number of pups born in 2008 at Sea Lion Rock was 6,380 (SE = 80), which represents a decline of 22.8% since 2002.

Fine-scale foraging behavior studies of northern fur seals were conducted during August–October 2008 at St. Paul Island, expanding the efforts of a pilot project that was initiated during 2007 by Dr. Carey Kuhn (AEP). Specific objectives of the project were to obtain higher-resolution location and diving data during adult female foraging trips in the Bering Sea and to identify important feeding areas using stomach temperature telemetry technology in combination with the fine-scale movement and diving data.

Twenty-nine females were captured and sedated with gas anesthesia for instrumentation during August 2008. Each female was equipped with a GPS tag (MK10-AF, Wildlife Computers), stomach temperature recorder (MK10-L), and VHF tag. Six of the females were followed for a single trip while the remaining animals were tracked for two to six trips each.

Current analyses are focused on quantifying differences between tracking methods using standard Argos satellite data and GPS data (Fig. 1 above). The high-quality GPS data are also being used with a new movement model developed by Devin Johnson (AEP) to test methods to improve satellite tracking data analysis.

Tracking female fur seals using the Argos satellite data yielded an average of 12.0 (± 2.9) locations per day and, even though the location data had previously been filtered to remove poor quality locations, the location error was estimated to range from 0.8 km to more than 45 km (Fig. 1 above).

By contrast, GPS tracking resulted in a greater number of highly accurate at-sea locations per day (31.6 ± 16.0), making it possible to define fine-scale high resolution foraging habitat. In addition, the GPS location data were more consistently distributed throughout the day; on average, a location was acquired every hour.

The maximum time between location acquisitions was significantly greater for Argos satellite data, averaging slightly less than 18 hours for all females, compared to an average of 8.2 hours for the GPS data. Preliminary analysis also showed that females retained stomach temperature telemeters for approximately twice as long (for up to 5 days) in 2008 than in 2007.

Research efforts by the AEP on age-specific vital rates of northern fur seals were expanded at St. Paul Island during 2008 and involved collaborations with biologists from Canada, Australia, and Scotland. Fieldwork at St. Paul Island focused on demographic objectives: tagging females for mark-recapture studies, extracting teeth for age estimation, reproductive ultrasonography to determine early-term pregnancy rates, and initiation of tag resighting and behavioral observations.

Ninety-two females were captured 15–24 November, and 51 of these females were sedated with gas anesthesia for tooth extraction. Members of the AEP received training in tooth extraction techniques from Dr. Brad Page, a fur seal specialist from the University of Adelaide in Australia. Sixty-five of the captured females were also examined with ultrasound by reproductive specialists, Dr. Gregg Adams (University of Saskatchewan) and Dr. Don Bergfelt (U.S. Environmental Protection Agency).

November is the period when embryonic diapause in fur seals ends and renewed growth of the embryo becomes detectable by either gross necropsy, in the case of animals historically collected for this purpose, or by ultrasonographic examination of the uterus of live females.

Statistical modeling of the temporal progress of embryo growth and detection is part of the analysis of these data. However, our preliminary estimate of the pregnancy rate (83%) appears within the range expected for this period of November, and it is nearly identical to at-sea pregnancy rates observed in December–January during collections in the 1960s.

The AEP will continue resighting work next summer to provide longitudinal data on survival and reproductive success of fur seals tagged in November.

By Rolf Ream
 

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