Vanishing summer sea ice imperils marine mammals

As federal marine mammal experts in Alaska scramble to study how global warming will affect walrus, polar bears and ice seals, they warn there are limit to the protections they can provide.

They can restrict hunters, ship traffic and offshore petroleum activity, but they acknowledge there are limits if the animals’ basic habitat -- sea ice -- disappears every summer.

“Ultimately it’s beyond my scope,” said Joel Garlich-Miller, a walrus expert for the U.S. Fish and Wildlife Service in Anchorage. “I can’t make ice cubes out there.”

Garlich-Miller spoke after confirming that 3,000 to 4,000 mostly young walrus died this year in stampedes on land on the Russian side of the Chukchi Sea, the body of water touching Alaska and Russia just north of the Bering Strait. Instead of spending the summer spread over sea ice, thousands of walrus were stranded on land in unprecedented numbers for up to three months.

If current ice trends continue, and walrus are based on coastlines every summer, they will put tremendous pressure on nearby foraging areas rather than on rich offshore feeding areas -- sort of like putting all the cattle from a farm into one small pasture, said Tony Fischbach of the U.S. Geological Survey.

Experts on summer sea ice say it’s not likely to suddenly reappear. Arctic sea ice this summer plummeted to the lowest levels since satellite measurements began in 1979, according to the National Snow and Ice Data Center at the University of Colorado.

“Certainly we look like we’re on a death spiral right now,” said Mark Serreze, senior research scientist. “Losing that summer sea ice-over by 2030, within some of our lifetimes, is a reasonable expectation.”

Sea ice loss could have a devastating effect. The U.S. Fish and Wildlife Service within weeks will decide whether to list polar bears as “threatened” under the federal Endangered Species Act because of the loss of sea ice from global warming. Polar bears hunt and breed on sea ice and are poor candidates for survival if they are based on land, where grizzly bears dominate.

Polar bears’ primary prey are ringed seals, as many as 43 per year. They’re the only seals that thrive under sea ice, digging breathing holes with their thick claws and creating lairs on top of the ice where they birth their young. With warming, those lairs collapse earlier in springtime, leaving hairless pups susceptible to freezing, foxes, polar bears and even ravens and gulls.

Then there’s the Pacific walrus, in line for a triple whammy. Their ocean habitat may be changing, they may be forced to shore for long periods and their weakest members are in danger when crowded on land.

Walrus dive to the ocean bottom to eat clams, snails, crabs, shrimp and worms. Research suggests that diminished sea ice and warmer water may decrease the plankton that dies and feeds creatures on the bottom. More plankton in the water column helps fish but could decrease the prey of walrus and gray whales.

Unlike seals, walrus can’t swim indefinitely. Females and their young traditionally use ice as a diving platform, riding it north like a moving sidewalk over offshore foraging areas, first in the northern Bering Sea, then into the Chukchi Sea.

Ice disappeared this summer in the Chukchi and walrus hauled out on Alaska’s northwest shore in groups of up to 2,500 animals. On the Russian side, where the trend started about a decade ago, one coastal haul-out reached 40,000 animals.

If animals are on shore for three months every summer, they can’t reach offshore foraging areas. Chad Jay, chief walrus researcher for the USGS, said there were concerns with how much energy walrus would expend swimming to near-shore foraging areas.

An adult walrus can eat 200 pounds of clams a day. If the walrus population stays within 30 miles of shore in summers, they could over-harvest available clams and other food.

“I suspect they won’t do very well as totally shore-based animals,” said Vera Alexander, one of three members of the U.S. Marine Mammal Commission.

Walrus, she said, are not as dependent on ice as polar bears, but near-shore sediment is less productive than offshore foraging areas. The effect is likely to show up in the conditions of walrus and eventually will affect their numbers.

Hauled out on land, walrus stack up shoulder to shoulder. A polar bear, a human hunter or even a low-flying airplane can trigger a stampede.

Anatoly Kochnev, who conducts walrus research for Russia’s Pacific Institute of Fisheries and Oceanography, said the loss of 3,000 to 4,000 animals this year from mostly one demographic could be disastrous.

Biologists did not detect stampede deaths on the U.S. side. But anecdotal evidence has contributed to their concerns -- single animals showing up, or mother-calf pairs appearing on shore, or animals that did not flee when hunters approached.

“It’s my hypothesis that these animals were in fact very exhausted from long swims from offshore locations,” said Garlich-Miller of the U.S. Fish and Wildlife Service. “This is something that our Russian colleagues have reported before. The animals are so tired they sort of lose their flight response.”

U.S. biologists are planning an aggressive year of research in 2008 to continue studying the effects of global warming.