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Polar Ecosystems Program

Harbor Seal Census in Southeast 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. For logistical purposes, Alaska is divided into five regions and one region is surveyed each year. The five regions are the Gulf of Alaska, northern Southeast Alaska (surveyed this year), southern Southeast Alaska, the Aleutian Islands, and the north side of the Alaska Peninsula and Bristol Bay.

The 2007 surveys were conducted 7-17 August 2007 using nine aircraft: three NOAA twin-engine planes (two AC-Shrikes and a DHC-Twin Otter) and six chartered single-engine floatplanes (Cessnas). Six planes and crew were based in Juneau, two in Sitka, and one in Yakutat. Each plane carried a single observer; eight observers were from NMML and one was from the National Park Service.

Starting in 2006, surveys were conducted under a new protocol designed to improve the spatial and temporal resolution of our surveys, data management, and workflow. The entire range of harbor seals in Alaska was divided into fixed areas (polygons) to better document seal counts and survey effort; each polygon covers several miles of coastline and can be surveyed in about 15 minutes, a period during which survey conditions (e.g., tide height, weather conditions) are relatively constant. In the past, seal haul outs and counts were associated with specific geographic coordinates. Such sites, however, were difficult to define as they were often scattered across many rocky islets or sand bars. This created problems in standardizing how different observers (using a portable GPS) were mapping site locations across years.

Observers now only have to record a track of their day’s flight and take photographs, ensuring that the area represented by each polygon is surveyed completely. The photographs are geo-referenced, and the seals counted in them are assigned to the corresponding polygon instead of a specific site. This approach allows observers to focus on sighting seals and enhances continuity in the technique across years.

In contrast to last year, surveys in 2007 were flown with few weather delays or limitations. As usual, surveys were conducted daily within 2 hours of low tide. The first few survey days entailed reconnaissance flights to search for haul-out sites within all polygons along the entire coastline. Observers then targeted specific polygons where seals were observed, during reconnaissance flights or previous years’ surveys, in order to get more replicate counts in polygons with seals.

Flight tracks were automatically recorded on a portable GPS while observers kept track of when they were on effort (i.e., when searching wasn’t hampered by visibility, turbulence, etc.). Photographs of seal haul outs were taken through side windows at elevations of 500-1,000 ft. Sites where seals were hauled out on ice were photographed using a down-looking camera linked to a GPS (see following article).

At least four replicate surveys were conducted at most, if not all, of the larger seal haul outs. Larger haul outs (>20 seals) account for the vast majority of variability in counts across days. Image processing and counting will occur at NMML over the next several months.

By Dave Withrow, John Jansen, and Josh London

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