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Denning


Catalogue of polar bear (Ursus maritimus) maternal den locations in the Beaufort Sea and neighboring regions, Alaska, 1910-2010

This report presents data on the approximate locations and methods of discovery of 392 polar bear (Ursus maritimus) maternal dens found in the Beaufort Sea and neighboring regions between 1910 and 2010 that are archived by the U.S. Geological Survey, Alaska Science Center, Anchorage, Alaska. A description of data collection methods, biases associated with collection method, primary time periods, and spatial resolution are provided. Polar bears in the Beaufort Sea and nearby regions den on both the sea ice and on land. Standardized VHF surveys and satellite radio telemetry data provide a general understanding of where polar bears have denned in this region over the past 3 decades. Den observations made during other research activities and anecdotal reports from other government agencies, coastal residents, and industry personnel also are reported. Data on past polar bear maternal den locations are provided to inform the public and to provide information for natural resource agencies in planning activities to avoid or minimize interference with polar bear maternity dens.

Detecting denning polar bears with Forward Looking Infra-Red Imagery (FLIR)

Polar bears give birth in snow dens in mid winter, and remain in dens until early spring. Survival and development of neonates is dependent on the stable environment within the maternal den. In Alaska, petroleum related activities currently span approximately 200 km of the Alaskan Beaufort Sea coastal area. New and proposed developments are expected to dramatically expand the area influenced by petroleum activities. These activities are a potential threat to denning polar bears, especially as they might disturb denning females.

To provide information that help manage and mitigate potential disruptions of to polar bear denning, we tested the ability of Forward Looking Infrared (FLIR) viewing devices to detect the heat signature of maternal polar bear dens. We tested this concept by flying transects over habitat containing known dens with FLIR equipped aircraft. We recorded flight and weather conditions at each observation and tallied whether or not the den was detected.

We conducted FLIR surveys, between 1999 – 2001, on 23 known polar bear dens on 67 occasions. Four dens were never detected (17%), but 3 of those only were visited under marginal conditions. Nine dens were detected on all visits and 10 dens visited more than once were detected on some flights and not on others. Detection was dependent on weather conditions and solar radiation. For every one-degree (C) increase in temperature dew-point spread, the odds of detecting a den increased 3X. We were 4.8X more likely to detect a den when airborne moisture (snow, blowing snow, fog etc.) was absent than when it was present, and we were approximately 28X more likely to detect a den at night than we were after sunrise. Our data suggest some dens never will be detectable with FLIR. Conversely, we feel FLIR surveys conducted during conditions that maximize odds of detection will locate most dens most of the time and can be an important management/mitigation tool.

Amstrup, S. C., G. York, T. L. McDonald, R. Nielson, and K. Simac. 2004. Detecting denning polar bears with Forward Looking Infra-Red Imagery (FLIR). BioScience 54:337-344.

FLIR image of a polar bear in a den
Figure 1. FLIR image of a polar bear in a den in a coastal bank in northern Alaska.

Mapping polar bear maternal den habitat in the Arctic National Wildlife Refuge, Alaska

Polar bears give birth during mid-winter in dens of ice and snow. Denning polar bears subjected to human disturbances may abandon dens before their altricial young can survive the rigors of the Arctic winter. Because the Arctic coastal plain of Alaska is an area of high petroleum potential and contains existing and planned oil field developments, the distribution of polar bear dens on the plain is of interest to land managers. Therefore, as part of a study of denning habitats along the entire Arctic coast of Alaska, we examined high-resolution aerial photographs (n = 1655) of the 7994 km2 coastal plain included in the Arctic National Wildlife Refuge (ANWR) and mapped 3621 km of bank habitat suitable for denning by polar bears. Such habitats were distributed uniformly and comprised 0.29% (23.2 km2) of the coastal plain between the Canning River and the Canadian border. Ground-truth sampling suggested that we had correctly identified 91.5% of bank denning habitats on the ANWR coastal plain. Knowledge of the distribution of these habitats will help facilitate informed management of human activities and minimize disruption of polar bears in maternal dens.

Durner, G. M., S. C. Amstrup and K. Ambrosius. 2006. Polar bear maternal den habitat on the Arctic National Wildlife Refuge, Alaska. Arctic 59(1):31-36.

Study area and the distribution of polar bear maternal den habitat, on the ANWR, Alaska, August 2004
Study area and the distribution of polar bear maternal den habitat, on the ANWR, Alaska, August 2004.

Mapping polar bear maternal den habitat in northern Alaska with Interfermetric Synthetic Aperture Radar Data

Much of the northern Alaska coastal plain has been mapped for potential polar bear maternal den habitat, including the central coastal plain and the Arctic National Wildlife Refuge (ANWR; Durner et al., 2001, 2005). In the National Petroleum Reserve – Alaska (NPRA) limited exploratory activity has begun, particularly in the northeastern section of NPRA. Over the past 24 years, we have identified many polar bear maternal dens in NPRA, indicating that NPRA is an important denning region. Den habitat, however, has not been mapped in NPRA and hence can not be included in resource management plans. The primary reason for this is because high resolution aerial photography, such as that used for mapping den habitat east of the Colville River (Durner et al. 2001, 2005), is not available for NPRA. Recently, however, the USGS Alaska Science Center (ASC), has obtained very high resolution Interferometric Synthetic Aperture Radar (IFSAR; Intermap Technologies Corp., Ontario, Canada) data in standard geographic information system (GIS) format for the NPRA. IFSAR technology provides a superior alternative to traditional photogrammetric methods in many applications (Tighe 2003). We are currently examining the applicability of IFSAR digital terrain model (DTM) data for delineating polar bear maternal den habitat in the NPRA. This will provide useful information on the distribution of polar bear denning habitat on the Alaska Beaufort Sea coastal plain.

Ground-level features at den 732
Typical landscape feature underlying previous polar bear den site in the NPRA, Alaska.

Post-emergence observations of polar bears at den sites in northern Alaska

Polar bear sow and yearling runningThe activity budgets of undisturbed animals provide a basic understanding of their behavior patterns as well as a benchmark against which human impacts can be evaluated. Denning in polar bears is an integral part of the reproductive process rather than a response to resource scarcity, and involves only pregnant females. The den’s primary role is to provide a secure environment for the gestation and bearing of young. However, following den emergence, continued den residence by cubs is beneficial in that it provides opportunities for acclimatization to the harsh arctic environment, development of locomotor skills, and an increase in body weight and overall size. It seems unlikely mothers and cubs would emerge and remain for weeks at the den site if the only function of dens was to provide a suitable environment for gestation and parturition. However, polar bears at den sites forage minimally, are susceptible to predation, and are thought to be sensitive to anthropogenic disturbance. Depending on the cub’s maturation level, disruption that results in premature abandonment of the den could reduce cub survival. During the winters of 2002-03, several radio-tagged polar bears denned in close proximity to the Prudhoe Bay oil field. This provided an opportunity for us to document the post-emergence behavior of family groups at den sites in Alaska.

We used focal scan sampling procedures to document the behavior of radio tagged polar bears from observation blinds positioned approximately 400 m from dens. Using computers we continuously logged behaviors of adult polar bears. We visited den sites daily to determine when bears first emerged. Upon emergence, our observations continued daily, weather permitting, until family groups abandoned the den site. We logged 459 hours of direct observation at 8 den sites in March of 2002 and 2003. Details of this effort can be found in the following publication:

Smith, T. S., Partridge, S. T., Amstrup, S. C., and Schliebe, S. 2007. Post-den emergence behavior of polar bears (Ursus maritimus) in northern Alaska. Arctic. 60(2)187-194.

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