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Abandoned Walrus Calves Reported in the ArcticMelting sea ice may be forcing mothers to strand their pups in deep water |
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| Enlarge ImageScientists spotted nine walrus calves like this one in the Arctic Ocean in 2004, swimming unaccompanied by their mothers far from shore in deep water. The scientists speculate that melting sea ice may be forcing mothers to strand their pups in deep water. (Photo by Carin Ashjian, Woods Hole Oceanographic Institution) |
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| Enlarge ImageAbandoned by their mothers too far offshore to swim back to land, walrus calves would likely succumb to starvation and drowning. (Photo by Phil Alatalo, Woods Hole Oceanographic Institution) |
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| Enlarge ImageSea surface temperatures from satellite imagery during Aug. 12-18, 2004, shows a plume of warm Bering Sea water extending into the area where scientists saw unaccompanied walrus calves. Black tones denote either land or sea surface areas where cloud cover or sea ice interfered with remote measurement. White squares show locations of walrus calf sightings and red squares indicate other stations occupied during the research cruise. The open red box northeast of Wainwright indicates locations where groups of adult walruses, some with calves, were observed during the cruise. Bathymetric contours are shown as white lines. Numbers of lone calves sighted at each location are listed next to each white box. Stations in open black box east of Barrow are a transect line that is shown as a vertical cross section in the figure below. (Cooper et al, Aquatic Mammals) |
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| Enlarge ImageCross section plots comparing water temperatures on a transect line east of Point Barrow during (a) July 29 - August 4, 2002 and (b) July 28 to Aug. 8, 2004. Data points are depths and locations of water samples collected. Insets show the locations of transect lines relative to the northern coast of Alaska. (Cooper et al, Aquatic Mammals) |
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Stepping aboard the U. S. Coast Guard Cutter Healy to explore the
Arctic Ocean, Carin Ashjian expected to ride the sea’s winds and
swells, but not an emotional roller coaster.
In spring and summer of 2004, Ashjian and colleagues were investigating
the potential impacts of a warming climate on the delicately balanced
Arctic Ocean ecosystem, when they discovered an unexpected phenomenon:
nine sightings of baby walruses swimming alone far from shore—apparently abandoned by
their nursing mothers.
“The young can’t forage for themselves and are dependent on their
mothers’ milk for up to two years,” said Ashjian, a biologist at Woods
Hole Oceanographic Institution. The lone calves, about two months old
and too far offshore to swim back to land, would likely succumb to
starvation and drowning, the researchers concluded.
“We would sail up to a particular location and stay there for 24 hours
at a time, and one or two of these pups would swim up to us, and the
poor little guys would just bark at us for hours on end,” Ashjian said.
“It was really awful. I wouldn’t go outside.”
Nearshore ice disappeared
Adult walruses forage for clams, snails, crabs, worms, and other invertebrate
animals on the shallow seafloor of the continental shelf, diving to
depths of no more than 656 feet (200 meters). Walrus mothers leave their
calves on sea ice while they dive, returning to nurse them.
In short, walruses depend on sea ice that usually persists above
shallow nearshore waters in summers, and in 2004, the ice disappeared.
The researchers observed a mass of water as warm as 44°F (7°C) over
the continental shelves of the Chukchi and Beaufort Seas—more than six
degrees higher than temperatures measured in the same region in 2002.
Sampling with plankton nets, they found zooplankton species
characteristic of warmer, more southern waters.
The scientists reported their findings in the April 2006 issue of
Aquatic Mammals. Lee W. Cooper, a biogeochemist at the University of
Tennessee, was lead author of the report, which concluded that warmer
water came north into the Chukchi and Beaufort Seas, melting nearshore
sea ice or moving it further north. That left the only remaining ice far offshore and over the deep
water.
The researchers think that the mothers had to swim farther and farther
from shore to find ice for the calves to rest on and eventually had to
abandon them in waters too deep for the mothers to reach food. Ice was
"virtually absent" throughout the area where the scientists saw the
lone calves. The area was 53 to 134 miles (85 to 215 kilometers) from
shore in water 9,842 feet (3,000 meters) deep. The researchers
saw mothers and calves together only in water less than 328 feet (100
meters) deep and 20.5 miles (33 kilometers) from shore.
Rescues impossible
“If walruses and other ice-associated marine mammals cannot adapt
to caring for their young in shallow waters without sea ice available
as a resting platform between dives to the seafloor, a significant
population decline of this species could occur,” the scientists wrote.
When the Healy appeared in the ice-free waters, the calves were drawn to
it.
“Imagine the pups barking at us for 24 hours,” Ashjian said. “We
couldn’t rescue them.”
The Marine Mammal Protection Act of 1972
prohibits the capture or removal of any marine mammals in U.S. waters
and by U.S. citizens on the high seas—and there was no place they could
have taken the still-suckling calves.
— Kate Madin
The National Science Foundation and the Office of Naval Research funded
the research. Also participating in the study were Sharon L. Smith from
the Rosenstiel School of Marine and Atmospheric Sciences at the
University of Miami, Louis A. Codispoti of the University of Maryland
Center for Environmental Science, Jaqueline M. Grebmeier of the
University of Tennessee; Robert G. Campbell of the University of Rhode
Island Graduate School or Oceanography, and Evelyn B. Sherr of the
College of Oceanographic and Atmospheric Sciences at Oregon State
University.
Posted: June 5, 2006 [top] |
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