National Marine Mammal Laboratory (NMML)

Polar Ecosystems Program

Harbor Seals Census in Bristol Bay and Along the Alaska Peninsula

Figure 4.  Aerial survey image of harbor seals on a sandbar in Bristol Bay, Alaska, (left) and that image after count processing in the laboratory.

The Polar Ecosystems Program (PEP) conducts aerial surveys for abundance of harbor seals in Alaska every August during the seals’ molt, when they spend much of their time out of the water. For logistical purposes, Alaska is divided into five regions, and one region is surveyed each year. The five regions are the north side of the Alaska Peninsula and Bristol Bay (surveyed this year); Gulf of Alaska (2006), northern Southeast Alaska (2007), southern Southeast Alaska (2008), and the Aleutian Islands (2009).

This year’s surveys were conducted 10-25 August utilizing five aircraft and seven researchers from NMML, Alaska Department of Fish and Game (ADF&G), and U.S. Fish and Wildlife Service (USFWS). Routes were based out of Cold Bay, King Salmon, and Dillingham. All surveys were conducted within 2 hours on either side of low tide. Observers initially spent the first 3-4 days in reconnaissance covering the entire coastline in their zone. The location of all seals was recorded and marked as to whether the site was known from previous surveys or recently discovered. The remaining surveys were flown from site-to-site. Four to five replicate surveys were conducted to ensure adequate precision of the abundance estimates. High resolution digital cameras were used to photograph the seals. Image processing and counting will occur back at the laboratory during the next several months (Fig. 4 above).

By Dave Withrow
 

Photogrammetry Sampling of Molting Harbor Seals in Icy and Disenchantment Bays

Harbor seals observed on floating ice during the 2005 surveys.

PEP completed its field studies for 2005 on the disturbance of harbor seals by cruise ships in Disenchantment Bay, Alaska, near Yakutat. Nearby Icy Bay is not visited by cruise ships and is studied as a control. Both sites are tidewater glacial fjords where seals haul out on floating ice throughout the year. From May to September 2005, spanning the pupping and molting periods for harbor seals, Disenchantment Bay received about 170 visits from cruise ships. The Yakutat Tlingit Tribe relies on harbor seals for subsistence and has raised concerns about a perceived decline in the abundance of harbor seals since cruise ship visits began increasing from the 15 that occurred in 1989.

Nineteen aerial surveys were flown during the pupping season in late spring, producing about 41,000 digital images. Eighteen additional surveys were flown in July-September, producing an additional 24,000 images. These late summer surveys will provide a detailed time-series of information on abundance and distribution of harbor seals during their annual molt at glacial haulouts. Transects and images were spaced to provide a sample of the ice field (with seals) that represents about 15% of the total area. Harbor seal and ice distributions will be mapped by analyzing the images in a GIS (geographic information system) and then modeled with other environmental covariates including measures of ship traffic.

A plane used in the harbor seal aerial surveys.

Throughout both survey periods, observers from the PEP and the Yakutat Tlingit Tribe continued to board cruise ships in Disenchantment Bay to collect data on ship movements and seal density in relation to ship approach distance. Meteorological data were collected via automatic weather stations installed at both sites. Because the molting season can be energetically demanding for seals, requiring them to haul out for longer periods without feeding when compared to other times of the year, the seals may be more sensitive during molting to disturbance by cruise ships.

In addition, large-format photogrammetry (AFSC Quarterly Report October - December 2004) of the entire area was conducted daily from 9-11 August 2005 to provide an estimate of the total number of seals present during molting. Although these surveys are part of our ongoing efforts to estimate the statewide abundance of harbor seals, we were able to conduct our sampling photogrammetry at the same time on each of the 3 days to allow for direct comparisons between the two methods. These comparisons will ultimately provide the means to cross-validate the two methods and examine potential biases.

By John Jansen and Shawn Dahle

AFSC Quarterly
Research Reports July-Sept 2005