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Census of Harbor Seals in Southern Southeast Alaska

Each year NMML’s Polar Ecosystems Program (PEP) surveys a portion of the harbor seal population in Alaska. Abundance estimates of Alaskan harbor seals are mandated by the Marine Mammal Protection Act to ensure that population levels are monitored regularly. Because harbor seals haul out daily – usually at low tide during daylight – they are visible and can be counted accurately. Alaskan harbor seals, however, haul out in a wide range of habitats on intricate coastlines, generally scatter in small groups, and occupy an extensive range from Southeast to Bristol Bay and the Aleutians – some 62,000 km of coastline. Moreover, abundance surveys need to be completed during the short peak of seal molting when the largest proportion haul out. Thus, harbor seals in Alaska are logistically difficult and costly to survey. A state-wide abundance estimate takes 5 years to derive.

This year surveys were conducted from 8 to 17 August in Southeast Alaska from the Canadian border to the north end of Kupreanof Island. Staff from NMML and the Alaska Department of Fish and Game flew the surveys while based in either Ketchikan or Petersburg. Eight single-engine, high-winged aircraft (e.g., Cessna 185 and DeHavilland Beavers with floats) were used to photograph (for later counts) and visually estimate numbers of seals hauled out. Surveys were conducted within a 4-hour period centered on low tide. The first few survey days involved extensive reconnaissance searching the entire coastline to locate new and previously mapped seal haulouts. The remaining surveys were flown from site to site in order to acquire up to six replicate counts at each site, thereby reducing the confidence intervals of the final abundance estimates. The resulting images, either native digital or converted to digital from transparency, have been archived and are being analyzed.

With all survey imagery in digital format, PEP staff developed new techniques to increase the accuracy and repeatability of seal counts. Image analysis software was tested and customized to increase counting accuracy. Additional methods were employed to streamline image enhancement, integrate count data within images, and better access archived images. These efforts resulted in an improved image-centric database that will allow for efficient data retrieval and more detailed analyses.

By John Jansen, Dave Withrow, and Shawn Dahle.

Marine Mammal Surveys in the Chukchi and Beaufort Seas

The shelf, slope, and basin zones of the western arctic provide productive habitats for polar marine mammals. Determining the seasonal patterns of marine mammal abundance and distribution is key to understanding the ecological interactions involving these apex predators and the ecosystem “hotspots” where they are often found. From 5 July to 18 August 2003, we participated in a research cruise to the Chukchi and Beaufort Seas as part of the National Science Foundation’s Shelf Basin Interactions (SBI) Program. The research vessel Nathaniel B. Palmer served as an excellent platform for conducting this research within the sea ice zone. During the cruise, although abundance and distribution data on all marine mammal species observed were recorded, our main focus was on two species of seals in sea ice habitats: bearded seals (benthic foragers), and ringed seals (fish and crustacean predators). Our principal research objectives were to determine marine mammal distribution, relative abundance and habitat associations via visual surveys, and to relate these patterns with measures of mesoscale oceanographic structure and potential prey availability.

Pinniped surveys were conducted both from helicopters and the ship. Despite foggy weather during most of the cruise, approximately 3,655 km of linear transects of sea ice habitat were surveyed during 18 helicopter flights. Six species of marine mammals were seen: 4,100 walrus, 48 ringed seals, 16 bearded seals, 3 gray whales, 24 beluga whales, and 6 polar bears. Shipboard surveys yielded sightings of 310 walrus, 33 bearded seals, 5 ringed seals, and 6 polar bears. Density estimates based on these tallies and their relationships to environmental features await further data processing and analysis. Across the study area, lower densities of ringed and bearded seals were observed than expected, presumably due to declining haul-out rates following the seals’ annual molting period. Consistently high densities of walrus were observed hauled out on bands of ice just inside the outer fringe of the marginal ice zone, where several thousands of walrus were seen in localized areas. Relatively high densities of bearded seals were encountered on the continental shelf in the western portion of the study area, presumably in a zone where the benthic productivity is high.

By John Bengtson and Michael Cameron.

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