POLAR ECOYSTEMS PROGRAM:
Census of Harbor Seals in Southern Southeast Alaska
Each year NMMLs 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either native digital or converted to digital from transparency, have been
archived and are being analyzed.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 Foundations 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.
quarterly July-Sept 2003 sidebar
Auke Bay Lab