Half a century after Pacific walruses began recovering from industrial-scale hunting, marine biologists are growing worried that they face a mounting threat from global warming.

Masses of the lumbering Arctic denizens have been crowding on beaches and rocks along the Russian and American sides of the Bering Strait in the absence of the coastal sea ice that normally serves as a late-summer haven and nursery.

While the retreats in sea ice around the Arctic this summer were not as extensive as in 2008 or 2007, the Chukchi Sea, at the heart of the subspecies’ range, was largely open water.

On Thursday, biologists from the U.S. Geological Survey issued a report concluding that 131 walruses found dead near Icy Cape, Alaska, on September 14 died from being crushed or stampeded. Several thousand walruses had been congregating in the area, a situation that scientists from the agency said was highly unusual.

In September, Russian scientists and the World Wildlife Fund had reported several thousand crushing deaths among tens of thousands of walruses crowding along the western shores of the Chukchi Sea.

The tusked marine mammals have endured more than 15 million years of climatic ups and downs, so walrus specialists do not foresee the species going extinct, particularly if hunting remains controlled. (Thousands are legally killed each year by indigenous communities in both countries.)

But there has been growing confirmation that the walrus is suffering substantial losses as the sheath of sea ice in coastal waters erodes in the summer.

The floes normally provide a floating nursery for pups while adults dive to root for clams and other food in the seabed in shallow coastal waters along the continental shelf. In September, the federal Fish and Wildlife Service, responding to a lawsuit by the Centre for Biological Diversity, an environmental group, concluded that there was sufficient scientific evidence of rising stress on the animals from climate change to consider granting the Pacific walrus protection under the Endangered Species Act.

That review is under way, and the service is taking public comment until November 9.

The polar bear, similarly dependent on sea ice, was listed as threatened under the species act last year.

“I think there is reason to be concerned,” said Dr. Brendan P. Kelly, a marine biologist at the University of Alaska — Fairbanks, who has been studying walruses for several decades and is also a researcher at the National Marine Mammal Laboratory in Juneau.

Fatal stampedes among walruses have occurred in past years, he said, citing research he conducted on a similar event in 1978 and reports by hunters on islands in the Bering Sea more than a century ago. But the expansion of open waters along the coasts raises the odds and adds to other pressures on the animals, he said.

For the moment, the Pacific walrus remains abundant, numbering at least 200,000 by some accounts, double the number in the 1950s.

The Atlantic walrus, a subspecies in Canada, Norway, Russia and Greenland numbering about 22,000, never recovered from sustained slaughter.

Kelly said the long-term forecast of warming and less summer ice for the Arctic did not bode well for the Pacific walrus.

“The Pacific population did recover,” he said. “But it is hard to imagine that it will not decline in the coming century.” — © 2009 The New York Times News Service

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