Alaska Science Center
Polar Bear-Sea Ice Relationships
Polar bears are tied to the sea ice for nearly all of their life cycle functions. Most important of these is foraging, or access to food. Polar bears almost exclusively eat seals, and they are equally as dependent upon the sea for their nutrition as are seals, whales, and other aquatic mammals. Polar bears are not aquatic, however, and their only access to the seals is from the surface of the sea ice. Over the past 25 years, the summer sea ice melt period has lengthened, and summer sea ice cover has declined by over half a million square miles. In winter, although sea ice extent has not changed as greatly, there have been dramatic reductions in the amount of old ice, predominantly in the western Arctic. This loss of stable old ice has set up additional losses of sea ice cover each summer because the thinner younger ice is more easily melted during the recent warmer summers.
Because of their dependence upon the sea ice for food, these changes can directly affect the carrying capacity of the Arctic for polar bears. Our studies are documenting numerical responses to these changes in sea ice. In Western Hudson Bay near the southern extent of the range of polar bears, the Canadian Wildlife Service has long term data on population characteristics and changes in sea ice. Our analysis of those data has shown that longer ice-free seasons have resulted in reduced survival of young and old polar bears and a population decline over the past 20 years. Recent observations of cannibalism and unexpected mortalities of prime age polar bears in Alaska are consistent with a population undergoing change. Ongoing studies are designed to gain an understanding of movement patterns of polar bears in response to changing ice conditions and of associated population responses. If we know how polar bears respond, functionally and numerically, to ice quantity and quality, we will be able to predict how forecasted changes in the ice may affect future polar bear populations. This will give managers the best chance of adapting strategies to assure long term polar bear survival in a changing ice environment.
We have been developing Resource Selection Functions (RSF) of habitat use by female polar bears in the Beaufort Sea since 2001. Earlier results were published in a report to Minerals Management Service (2003) and in the proceedings of a conference on resource selection by animals in Laramie, WY (2004). This new report includes a longer record of satellite location data and corresponding National Ice Center and Canadian data, as well as ocean depth data. The data selection procedures were similar to those used in the 2003 and 2004 reports. The model building procedures in this new report, however, are different than those used in the earlier reports. Rather than creating seasonal models, we built monthly RSFs. We used Receiver Operator Characteristic (ROC) curves to build models by a forward stepwise procedure using cross validation at each step. Parameter estimates and their standard errors were derived from bootstrapping. In general, models for winter months were largely driven by proximity to the edge of major ice types. During other times of the year, models were more complex and included covariates of ocean depth, sea ice concentration, ice thickness, and floe size. Late summer melt in the Beaufort Sea resulted in polar bears selecting the deep water regions of the polar basin where ice persisted. These new RSF models allow a closer examination of annual changes in habitat use by polar bears in the Beaufort Sea.
Durner, G. M., S. C. Amstrup, R. M. Nielson, and T. L. McDonald. 2003. Use of sea ice habitat by female polar bears in the Beaufort Sea. Pages 60-61 in Proceedings of the Ninth MMS Information Transfer Meeting. U. S. Minerals Management Service, Anchorage, Alaska. Report for OCS Study 042. 118 pp.
Polar bears (Ursus maritimus) in the northern Alaska region den in coastal areas and on offshore drifting ice. We evaluated changes in the distribution of polar bear maternal dens between 1985 and 2005, using satellite telemetry. We determined the distribution of maternal dens occupied by 89 satellite collared female polar bears between 137°W and 167°W longitude. The proportion of dens on pack ice declined from 62% in 1985–1994 to 37% in 1998–2004 (P = 0.044) and among pack ice dens fewer occurred in the western Beaufort Sea after 1998. We evaluated whether hunting, attraction to bowhead whale remains, or changes in sea ice could explain changes in den distribution. We concluded that denning distribution changed in response to reductions in stable old ice, increases in unconsolidated ice, and lengthening of the melt season. In consort, these changes have likely reduced the availability and quality of pack ice denning habitat. Further declines in sea ice availability are predicted. Therefore, we expect the proportion of polar bears denning in coastal areas will continue to increase, until such time as the autumn ice retreats far enough from shore that it precludes offshore pregnant females from reaching the Alaska coast in advance of denning.
Through this study we made predictions of polar bear autumn distribution in the Chukchi Sea with a Resource Selection Function (RSF) developed from 1198 satellite radio-collar locations on 124 adult female polar bears, 1987 – 1994. The RSF was created to assist in an aerial survey design for polar bears proposed by the U.S. Fish and Wildlife Service. The RSF was based on bathymetry and daily sea ice covariates extracted from passive microwave satellite imagery within the region > 25 km from shore. The RSF indicated that polar bears in the Chukchi Sea during autumn selected habitats with intermediate amounts (~50%) of ice cover in close proximity to higher ice concentrations, and over relatively shallow waters. The RSF showed good predictive abilities for the years of its construct, worked best in October, and was robust to inter-annual variability. When evaluated with recent (1997 – 2005) data, the RSF performed well for October and November but poorly for September. This loss of predictive abilities appeared to be related to recent changes in habitat due to longer melt seasons and younger sea ice, and to testing the retrospective model with a small sample of recent polar bears locations from a limited region of the Chukchi Sea. Contemporary applications of this RSF must consider three factors that could limit its utility: 1) different sea ice phenology; 2) distributions of males and sub-adults; and 3) occupancy in near shore habitats.
Durner, G. M., D. C. Douglas, R. M. Nielson, and S. C. Amstrup. 2006. A model for autumn pelagic distribution of adult female polar bears in the Chukchi Sea, 1987-1994. U.S. Geological Survey, Alaska Science Center, Anchorage, Alaska. Contract Completion Report: 70181-5-N240. 67 pp. [PDF file 1.62 MB]