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Alaska Ecosystems Program

Changes in Northern Fur Seal Foraging Behavior with Increasing Population Density on Bogoslof Island, Alaska

figure 1, see caption
Figure 1.  Northern fur seals were studied at Bogoslof Island, Alaska (53.92°N, 168.03°W), which is located just north of the Aleutian Island chain in the Bering Sea.  The largest breeding population of northern fur seals occurs on the Pribilof Islands, Alaska.  Map inset: Aerial image of Bogoslof Island.

The northern fur seal (Callorhinus ursinus) population in the United States has been declining at a rate of more than 3% per year since 1998. This is primarily the result of continued declines in pup production at the largest breeding colonies on the Pribilof Islands, Alaska (Fig. 1). In contrast, a recently established northern fur seal colony on Bogoslof Island, Alaska (Fig. 1), is showing a very different trend.

The first report of breeding northern fur seals on Bogoslof Island came in 1980, after previous surveys reported only young males hauling out on the island. Since 1980, the population has grown rapidly with 17,574 843 pups born on Bogoslof Island in 2007 (Fig. 2).

figure 2, see caption
Figure 2.  Northern fur seal pup production on Bogoslof Island, 1979-2007.  Pups were directly counted from 1979 to 1997, and the shear-sampling method for estimating production was employed from 2005 to 2011.  Error bars represent variance estimates based on shear-sampling count replicates.



figure 3, see caption
Figure 3.  An adult female northern fur seal instrumented with a satellite-tracking transmitter (PTT) and a time-depth recorder (TDR), posterior to the PTT.



figure 4, see caption
Figure 4.  A comparison of trip durations (circles) and maximum travel distances (squares) by year.  Foraging trip duration and maximum distance from the rookery increased from 1997 to 2006 but did not change between 2006 and 2007.
 
 

This recent colonization event and the continued increase in abundance has provided a unique opportunity for researchers from the Alaska Ecosystems Program (AEP) at the National Marine Mammal Laboratory (NMML) to examine the impact of increasing population density on northern fur seal foraging behavior. During the breeding season, female fur seals are reliant upon local prey resources as they alternate time on land nursing a growing pup with time at sea acquiring resources.

In seabird studies, researchers found that as colony size increased, prey resources near the colony became depleted due to the higher number of predators. As a result, the birds had to expend greater foraging effort and travel further from the colony to feed. To determine if there is evidence of resource depletion occurring around Bogoslof Island, we examined changes in northern fur seal foraging behavior intermittently over an 11-year period (1997, 2005, 2006, and 2007). Specifically, we examined differences in dive behavior, movement patterns, and space use among years focusing on adult female fur seals with a dependent pup.

The foraging behavior of 57 fur seals was measured by equipping seals with a time-depth recorder (TDR) and satellite-tracking transmitter (PTT) (Fig. 3). The TDRs recorded detailed information about dive depths and durations, whereas the PTTs provided at-sea locations making it possible to reconstruct foraging trips.

For all years, average dives by fur seals on Bogoslof Island were short (37.7 0.5 seconds) and shallow (9.2 0.2 m), similar to the dive behavior reported for northern fur seals on other islands. For most dive characteristics (e.g., mean depth, duration, post-dive surface interval), we found no differences among years or obvious patterns of change.

The one significant difference among years was an increase in the maximum dive depth recorded during a foraging trip. Maximum depths increased from 28.3 1.9 m in 1997 to 70.2 4.0 m in 2006, and some dives in 2006 reached depths well over 130 m. This suggests that fur seals may have been exploring deeper areas of the water column to find prey in the later years of the study.

In contrast to dive behavior, we found significant differences among years in all of the movement patterns examined. Trip durations increased from just under 1 day in 1997 to nearly 3.5 days in 2006 (Fig. 4). During these longer duration foraging trips, the maximum distance a female fur seal travelled from the rookery also increased (Fig. 4). This led to an expansion in the overall foraging range from 1997 to 2006 (Fig. 5). Interestingly, trip durations, distances traveled, and overall foraging habitat did not change between 2006 and 2007.

The changes we found in fur seal foraging behavior, specifically increased trip durations and greater foraging range, were similar to what has been reported for expanding seabird colonies. For these seabird studies, the authors concluded that this trend was a result of an increased number of predators depleting local resources. We believe that fur seals on Bogoslof Island may also be experiencing localized resource depletion associated with the rapidly growing population.

figure 5, see caption
Figure 5.  Tracks of northern fur seal foraging trips for each study year, showing the increase in foraging range from 1997 to 2006.  Circle denotes Bogoslof Island.  Click image to enlarge.
 

In August 2011, researchers from the AEP and staff from the Alaska Regional Office returned to Bogoslof Island. During this trip, pup production for 2011 was estimated and 10 adult female fur seals were equipped with GPS tracking instruments to examine at-sea behavior. Although the analysis is ongoing, preliminary results show that the population is continuing to grow, as approximately 23,000 fur seals were born on Bogoslof Island in 2011.

This newly collected data will be combined with the analysis to date for a forthcoming publication entitled "Changes in northern fur seal (Callorhinus ursinus) foraging behavior with dramatically increasing population density."

By Carey Kuhn
 

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