Nine lone walrus calves were reported swimming in deep waters far from shore
by researchers aboard the U.S. Coast Guard icebreaker Healy during a cruise in
the Canada Basin in the summer of 2004. Unable to forage for themselves, the
calves were likely to drown or starve, the scientists said.
Lone walrus calves far from shore have not been described before, the researchers
report in the April issue of Aquatic Mammals. The sightings suggest that increased
polar warming may lead to decreases in the walrus population.
"We were on a station for 24 hours, and the calves would be swimming around
us crying. We couldn't rescue them," said Carin Ashjian, a biologist
at Woods Hole Oceanographic Institution and a member of the research team.
The researchers found evidence of warmer ocean temperatures that may have rapidly
melted seasonal sea ice over the shallow continental shelf where walruses dive
to feed on bottom-dwelling animals such as clams and crabs. Walrus need the ice
to rest themselves and to leave the pups to rest while the mothers feed. Ice
remained over very deep water.
Cooper, Ashjian and other researchers made the unexpected walrus calf sightings
during a cruise to investigate the impact of
global climate change on the oceanic ecosystem over the continental shelf of
Alaska. Their work focused on the shallower waters of the continental shelf in
the Chukchi Sea to deeper waters in the Beaufort Sea of the Western Arctic Ocean.
The project was funded by the National Science Foundation and the Office of Naval
Research.
Adult Pacific walrus, Odobenus rosmarus divergens, forage for food by diving
as far as 200 meters about (630 feet) down to the seafloor and using sensitive
facial bristles to locate prey. Sea ice normally forms over the continental shelf
north of Alaska and persists even in summer. Adult walrus use the sea ice as
a resting platform; mothers leave the calves there and dive to the bottom for
food.
The researchers measured a mass of water as warm as 44°F (7°C) moving
onto parts of the shelf from the Bering Sea to the south in 2004. This warm-water
intrusion was more than six degrees higher than temperatures at the same time
and location in 2002. The warmer water apparently caused seasonal sea ice to
melt rapidly over the shallow continental shelf and retreat to deep water over
the Arctic Ocean basins, where the water remained colder.
In the areas where ice remained, the bottom is up to 3,000 meters (about 9,300
feet) deep, too deep for even adult walrus to dive to feed. When sea ice retreats
to such deep water, as it did in 2004, there are no platforms in shallow waters
for mothers to rest and to leave their calves while they feed, and the pairs
become separated.
Scientists on the Healy used geographic positioning, digital
photography, ship bridge logs, and other observations to record the calves' positions
and bathymetric charts and depth sounder data to
identify water depth. They documented the very warm water using both conductivity-temperature-depth
(CTD) profile sampling and plankton-net sampling, which revealed zooplankton
species that prefer warmer waters.
