Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
Sea otters may pup at any time of the year, but most births are in spring
and summer (Kenyon 1981). Mating reaches a peak in September, October and
November.
Polar Bear (ursus maritimus). The polar bear can be considered a marine
mammal because it spends a great portion of its life associated with the sea
and sea ice, and subsists almost entireiy on marine food chains. In Alaska,
polar bears winter in the flaw zones of the Chukchi and Beaufort Seas, and in
the northern Bering Sea in years when heavy pack ice has been driven southward
through the Bering Strait. Polar bears are good swimmers but pack ice is
important to bears as a solid substrate on which they can move about and hunt,
and is an important determinant of their distribution (Fay 1974) (Fig. 2.23).
Some pregnant females go onshore in November and early December to make
maternity dens in deep snow drifts (Burns et al. 1981 ) . Off Alaska, however,
most denning occurs on heavy drifting ice (Lentfer 1978; Amstrup 1987). Cubs
are born in late December and early January and remain in the lair with the
mother until late March or early April. Upon emerging from terrestrial dens,
the mother and cubs move out onto the pack ice. Terrestrial Alaskan denning
areas are found to be along the coasts of the Chukchi and Beaufort Seas from
Point Hope to the Canadian border. There are few records of maternity dens in
the Bering Sea, but some have been reported in the St. Lawrence Island and
Cape Prince of Wales areas. Areas suitable for terrestrial maternal dens are
determined by snowfall, ambient temperature, topography and wind, since
successful denning requires snowdrifts that do not thaw during the denning
period. Other important requirements are the presence of nearby seals, and
ice conditions that enable bears to successfully hunt ringed seals during preand
post-denning periods. Alaskan polar bears feed primarily on ringed seals,
although some bearded seals are also taken. Both these prey species are
associated with the sea ice throughout the year.
A recent study has documented the importance of pack ice as denning
habitat for Beaufort Sea polar bears (Amstrup 1987). Seventy-one free ranging
females were radio-tagged and tracked to their maternity dens. Only 13 of
these dens were on land, four were on shore-fast ice and the remaining 54
(76%) were on pack ice. These marine maternity dens were found throughout the
Beaufort Sea from sites just a few km from shore to as far as 550 km north of
the coast. This study found that all terrestrial dens of radio-tagged polar
bears were within or adjacent to the Alaska National Wildlife Refuge.
In general, the polar bear prefers areas where the sea ice is kept in
motion by winds and current, and where open water and newly frozen ice
facilitate seal hunting. These areas are found around the rim of the polar
basin within 200 miles of land masses. In summer polar bears move north
within this zone as ice recedes from coastal areas. The breeding season is
from April through June, when both males and females are active on the sea
ice, and gestation lasts about eight months. The one or two cubs remain with
the mother for about 28 months.