A polar bear assessment
USGS reports estimated number of bears and dens in Beaufort Sea coastal area
for Petroleum News
The U.S. Geological Survey Wildlife Program has published a report presenting a statistical analysis of the number of polar bears and their dens in the southern Beaufort Sea region, which includes the Beaufort Sea coastal region of the North Slope. The data presented are of importance to the North Slope oil and gas industry, and to the communities of the region, since they may impact the permitting of activities in the region. Polar bears have been listed as threatened under the Endangered Species Act and are protected under the terms of the Marine Mammal Protection Act.
The new report comments that the U.S. Fish and Wildlife Service, in meeting its obligations under these two acts, must determine whether planned activities will have a negligible impact on the bear population. Estimates of the bear population and the number of bear dens are important in making these determinations, the report says.
Shrinkage of sea iceMoreover, the shrinkage of the Arctic sea ice extent in recent decades, coupled with the reduced stability of the offshore ice, have caused a shift of bear denning from offshore to onshore. And the reduction of the season during which sea ice covers biologically productive offshore areas has resulted in polar bears spending more time onshore during the summer and autumn, the report says.
The growing reliance on land by SBS (southern Beaufort Sea) polar bears elevates the importance of mitigating potential disturbance to polar bears denning on the Arctic Coastal Plain of Alaska, the report says.
Counting the bearsBetween 2001 and 2016, to obtain polar bear population data, scientists tattooed bears and sampled their tissue, to enable individual bear identifications. Starting in 2013 microchips and telemetry devices were used to track bears.
The tracking data obtained were used in sophisticated statistical models to obtain population estimates and associated confidence limits for bears and bear dens in nearshore and offshore areas along the Beaufort Sea coast. The scientists also developed models for bear survival.
The marking and later re-identification of some of the bears resulted in 1,224 observations of 868 individual bears, the report says. The subsequent analysis indicated that the bear survival rate was relatively high in the years 2001 to 2003, dropped during the years 2004 to 2008, and became higher from 2009 onwards, except in 2012. These findings appear consistent with a previous bear population study, the report says. Overall, it appears that the southern Beaufort Sea population has remained mostly stable since the population decline that occurred in the mid-2000s.
Estimated den distributionThe USGS study found that the total annual number of dens in the region probably lies somewhere between 69 and 198, with a median of 123. Onshore there may have been 66 dens, with 14 of these dens occurring in the 1002 area of the Arctic National Wildlife Refuge, 26 dens between the Colville and Canning Rivers of the central North Slope, and 12 dens in the National Petroleum Reserve-Alaska. These estimates are subject to significant ranges of uncertainty. However, it appears that the coastal region of the central North Slope, including the areas of the Prudhoe Bay and Kuparuk River oil fields, sees the highest density of bear denning, followed by the 1002 area of ANWR and the NPR-A.
Previous studies have indicated that more than 60% of pregnant polar bears in the region now den on land rather than in sea ice, a 50% increase in on-land denning since the 1980s, the report says.