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October 27, 2003
Seattle, Washington, USA

Airborne Thermal Remote Sensing Surveys of Pacific Walrus (Odobenus rosmarus divergens) in the Bering Sea

Douglas M. Burn¹, Marc A. Webber^2
1Marine Mammals Management Office, U.S. Fish and Wildlife Service, 1011 East Tudor Road, Anchorage, AK, 99503, USA, Phone 907-786-3807, Fax 907-786-3816, Douglas_Burn@fws.gov
²Marine Mammals Managment Office, U.S. Fish and Wildlife Service, 1011 East Tudor Road, Anchorage, AK, 99503, USA, Phone 907-786-3479, Fax 907-786-3816, Marc_Webber@fws.gov

The life history of Pacific walrus is tied to the seasonal advance and retreat of sea ice in the Bering and Chukchi seas. Pack ice floes serve as a resting substrate for walrus groups, and provide greater access to shallow water feeding areas. Previous aerial surveys of Pacific walrus conducted from 1975-1990 used observers to both detect and count walrus groups. The results of these surveys suffered from low precision, and experts have determined that additional visual surveys of this kind would be of little value in monitoring the walrus population.

In April 2002 and April 2003 we conducted field tests using an airborne thermal scanner to detect walrus groups in the pack ice of the Bering sea. Walrus have considerable thermal contrast from their background environment, and thermal imagery represents one of the best means to locate groups hauled out on ice floes. After visually locating walrus, we first overflew each group at 457-792m altitude to collect digital photography, and then at altitudes ranging from 792-3,200m to collect thermal imagery at 1-4m spatial resolutions. Survey swath widths at these altitudes ranged from 1.5-6km. Walrus groups were visible in thermal imagery at all resolutions, and there is a significant relationship between the number of walrus in a group and the total amount of heat they produce.

In April 2003 we conducted a pilot survey during which we sampled approximately 30,000km² of sea ice habitat in the Bering Sea. Results of this survey will be used to plan for a range-wide survey of the entire walrus population. Pacific walrus are an important subsistence resource for Native people in both the U.S. and Russia; it will be important to accurately monitor future population trends in relation to changes in sea ice conditions.

Abstract Categories: Changes in the Sea

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