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Biologist Carin Ashjian is not a walrus researcher, but she knew something was wrong as she stood on the deck of the Coast Guard icebreaker Healy off Alaska's northern coast.
Conducting studies in the Beaufort Sea, Ashjian and other scientists moved from station to station sampling seawater and plankton. At seven locations, they spotted walrus calves, without their mothers, swimming near the ship in deep water.
Walrus calves are dependent on their mothers for at least 18 months. Under normal circumstances, calves are fiercely protected by walrus cows. The cows leave calves on sea ice to dive to the ocean floor for clams and other invertebrates.
But Arctic temperatures in late summer 2004 were so warm, the edge of the ice sheet moved north beyond the relatively shallow continental shelf to water too deep for walrus cows to dive.
Ashjian and other scientists concluded that the nursing mothers were forced to abandon their calves to feed themselves.
"We were on a station for 24 hours, and the calves would be swimming around us crying. We couldn't rescue them," said Ashjian, a biologist at Woods Hole Oceanographic Institution in Massachusetts.
While changes in the ice sheet have been widely reported, and alarm bells have sounded for the predator at the top of the sea ice food chain, the polar bear, warming's affects are potentially just as catastrophic for pinnipeds -- seals, walruses and sea lions.
Pinnipeds are carnivorous aquatic mammals that use flippers for movement on land and in the water. All pinnipeds must come ashore to breed, give birth and nurse their young.
The marine mammals have adapted in different ways and have different relationships with sea ice, said Brendan Kelly, a seal and walrus researcher for more than 30 years with the University of Alaska and now the National Science Foundation's Arctic National Sciences program manager.
"Walruses use sea ice sort of like a conveyor belt," said Tim Ragen, executive director of the U.S. Marine Mammal Commission. "As it moves along, they go with it and it takes them over feeding areas. What happens if you don't have resting platforms, i.e., ice, to get access to these different places?"
If forced to haul out only on land, walruses will not reach feeding areas they formerly used, he said.
Federal agencies already are addressing falling numbers of other pinnipeds in the North Pacific.
The National Marine Fisheries Service reported Feb. 2 that America's northern fur seal pup population continued a marked decline that started in 1998. The western stock of Steller sea lions, covering Southcentral and Western Alaska, dropped 80 percent from 1975 to 2002.
But no one knows whether walrus numbers are down. The U.S. Fish and Wildlife Service attempted its first major count in years last summer with Russian biologists using satellite and infrared information. They expect to come up with a number by the end of the year but current population numbers are "highly speculative," said Joel Garlich-Miller, an agency walrus biologist.
Walruses and seals evolved from terrestrial carnivores but still reproduce and rear their young out of the water, making a trade-off, in evolutionary terms, between aquatic and terrestrial mobility, Kelly said.
"The cost is, they're very slow and clumsy out of the water," he said. "Hence they're vulnerable to predation, hence they need to find places to come out of the water where there's some refuge from predation."
Sites where they can do that in the low latitudes are limited, he said.
"They have to find offshore islands or rocks where there's no bears, no wolves, no humans," Kelly said. "Sea ice, on the other hand, offers this huge expanse of area which is largely predator-free."
That platform has been shrinking.
The average sea ice extent for the entire month of September was 2.3 million square miles, the second-lowest on record,
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