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Polar Ecosystems Program

Harbor Seal Census Along Coastal Alaska

NMML's Polar Ecosystems Program (PEP) is responsible for monitoring and estimating the abundance of harbor seals in Alaska. The PEP conducts aerial surveys of harbor seals every August during the seals' annual molt when they spend much of their time out of the water while shedding and growing new hair. The 2009 surveys were conducted during the last week in July through the end of August.

We utilized seven aircraft, including four NOAA twin-engine planes (two AC-Shrikes and two DHC-Twin Otters) and two chartered single-engine floatplanes. The scientific crew was made up entirely of NMML employees and contractors with a significant amount of aviation safety and survival training.

Prior to 2008, for logistical purposes, Alaska was divided into five regions and one region was surveyed each year. The five regions were the Gulf of Alaska, northern Southeast Alaska, southern Southeast Alaska, the Aleutian Islands, and the north side of the Alaska Peninsula and Bristol Bay.

Beginning in 2008, the PEP developed and implemented a new system that allows annual surveys across the entire range of harbor seals in Alaska. Harbor seals range from Southeast Alaska through the extent of the Aleutians and north into Bristol Bay. We focus on surveying sites with historically large numbers of seals every year, and those sites with fewer seals are flown every 3-5 years. This should provide us with the data necessary to estimate harbor seal population and trends on an annual basis.

The 2009 surveys were the first surveys flown under this new method to cover the entire harbor seal range. In 2008, we were unable to fly much of the central and western Aleutian Islands. This region was added in 2009 and surveyed with a second NOAA Twin Otter. The Aleutian Islands survey also served as a test site for using an infrared (IR) camera to assist in the detection of harbor seals in the highly complex habitat. A FLIR A325 camera was mounted in one of the belly ports on the NOAA Twin Otter, and a live image was displayed on a laptop during the survey flights. In the end, we determined that the angle of the camera and the resolution of the image were too limited for our needs. However, we do feel this was an excellent first step and hope to work with the NOAA Aircraft Operations Center to find a better solution for future surveys.

The 2009 surveys also included our continued effort to monitor the population and trends of harbor seals that rely on tidewater glacier habitats for pupping and molting. During the month of August, biologists photo-sampled ice-hauling seals in tidewater glacial fjords. Icy and Disenchantment Bays in the Gulf of Alaska; Tracy Arm, Endicott Arm, and LeConte Bay in Southeast Alaska; and College Fjord and Columbia Glacier in Prince William Sound. Seals in these fjords (up to 5,500) are scattered across enormous fields of floating ice that shift with ocean and wind currents, which makes them particularly difficult to count.

To address these issues, an aerial sampling method was used for population estimates of ice-hauling seals in tidewater glacial fjords. The line transects were flown by NOAA Corps pilots aboard the NOAA AC-500 Shrike (N47RF). Flights were timed daily to overlap with the peak abundance of seals, which occurs between 1300 and 1600 hours.

This method allowed scientists to capture higher-quality imagery of seals at lower altitudes (1,000 ft), using a down-looking and high-resolution digital single-lens reflex (SLR) camera. The images were georeferenced and analyzed using conventional GIS software to map seal locations in non-overlapping images that represent about 40% of the ice field.
The estimated total abundance is calculated using spatial statistical models. Besides abundance estimation, the high-quality imagery also allows for detailed analyses of individual seals (e.g., discriminating mother-pup pairs and population structure) and ice characteristics.

By Josh London, Luciana Santos, and Dave Withrow

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