Sea-ice changes threaten walrus habits

Global-warming alarms sounding for all pinnipeds

Posted: Monday, February 26, 2007

ANCHORAGE - 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.

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Conducting studies in the Beaufort Sea, Ashjian and other scientists moved from station to station sampling sea water 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 fortify 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 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 went 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 Stellar sea lions, covering south-central 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, missing the 2005 record low by 131,000 square miles but continuing the pattern of sharply decreasing sea ice, according to the National Snow and Ice Data Center at the University of Colorado.

A change in area is one thing, Kelly said. He says there also are more subtle nuances to the ecological relationship the animals have with the ice habitat

Walruses breed in the Bering Sea in the winter time. Walrus calves are born in late April or early May.

In spring, when ice retreats, males generally remain in the Bering Sea. But females migrate north with their pups through the Bering Strait and continue to be associated with the ice edge in the Chukchi Sea, Kelly said.

"Basically, this ice edge is typically lying over relatively shallow continental shelf waters," he said. "That's really important if you're a benthic feeder, if you feed on clams and snails and things that live down on the bottom, which walruses do. So by using this ice edge in the summer and fall, the females have access to food directly below them, but they can nurse their calves up on the ice. So they split their time between nursing their young on the ice and diving down to the bottom so to provision themselves.

"What we've seen in recent years with these extreme ice retreats is that the ice is going north of the continental shelf and takes the nursing habitat over water that's really too deep for the females to feed.

A maximum dive for a walrus to scour the sea bottom with their broad, flat muzzles is about 630 feet. Ashjian and the other researchers saw the abandoned calves, nine in all at seven locations, in water that was about 3,000 feet.

Walruses will not thrive in climate warming simply by moving farther north, Kelly said.

"It's not just the amount of area," he said. "It's the relationship of the ice to another feature, in this case water depth, and hence food."

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