Trampling likely killed 131 mostly young walruses forced onto the northwest coast of Alaska by a loss of sea ice, according to a preliminary report released Thursday.
An examination of the dead animals by federal and local biologists found extensive bruising and other indications the animals had been trampled.
Young animals can be hurt in stampedes when a herd is startled by a polar bear, human hunters or even a low-flying airplane. With no human witnesses, biologists said the deaths may have been from "disturbances" that led to trampling.
Walruses in large numbers on the northern shore of Alaska is a phenomenon seen for the first time in 2007.
Walruses cannot swim indefinitely and females with young traditionally use sea ice as a platform most of the summer to dive for clams, riding the edge north as temperatures warm, then south in the fall as temperatures cool.
In recent years, however, sea ice has receded far beyond the relatively shallow outer continental shelf over deep Arctic Ocean waters where it's too deep for walruses to dive to the ocean bottom.
An estimated 3,500 walruses were spotted Sept. 12 at Icy Cape, about 140 miles southwest of Barrow. Two days later, U.S. Geological Survey researchers on their way to a walrus tagging project saw a large number of carcasses along the high tide line of the beach and no live animals.
The U.S. Fish and Wildlife Service, which manages walruses and polar bears, organized an investigation.
Most of the dead animals were found at Icy Cape, though a few were also found near the village of Wainwright to the north and locations up to 26 miles to the south.
Veterinarians and biologists from the Fish and Wildlife Service, USGS, the Alaska SeaLife Center and the North Slope Borough examined 71 carcasses and performed nine detailed necropsies.
Of the 71 carcasses examined, three were female yearlings, 25 were female calves and 43 were male calves, according to their preliminary report.
"All of the necropsied animals showed similar abnormalities, primarily extensive bruising in muscles in the neck and chest," the report said. "One animal had a fractured skull, and one animal had separation of some ribs from the backbone. Most of the animals had blood coming from the nostrils. The blood coming from the nostrils indicated damage to the neck, head, nose, or internal organs."
Walrus calves born in late April or early May weigh just 100 to 160 pounds. They can weigh 750 pounds by age 2, but before then are susceptible to trampling by females weighing 2,000 pounds, especially when a mixed-age herd is lined up shoulder to shoulder on a rocky shoreline.
Herds in 2007 were in the tens of thousands at some locations on the Russian side of the Chukchi Sea. Russian biologists reported 3,000 to 4,000 walruses died out of a population of perhaps 200,000, mostly young animals crushed in stampedes.
Investigators found no evidence of hunting or other recent human activities near the carcasses.
Several carcasses had been scavenged by polar or grizzly bears but investigators could not say whether bears were responsible for frightening the walruses and creating "disturbances."
Biologists who examined the Alaska carcasses were protected by armed guards standing watch for polar bears.
"It was definitely crisscrossed with bear tracks all over the place," said biologist Jill Prewitt of the Alaska SeaLife Center.
USGS researcher Chad Jay, who heads the research into foraging habits of walrus that have come to shore, said 16 walruses were tagged with radio satellite transmitters. Trampling may not have been the only contributor to the deaths of the calves and yearlings.
Other factors -- such as calves being separated by their mothers, or exhaustion from coming to shore and being out to sea a long time -- could be at play, Jay said.
After the carcasses were spotted, the Fish and Wildlife Service and the North Slope Borough surveyed the entire Chukchi Sea coast from Barrow to Cape Sabine. About 1,000 walruses were spotted on a barrier island northwest of Point Lay but no other dead animals were seen.
Environmental groups said the event is a sign of things to come with global warming.
"Obviously it's a real tragedy, and it's one we're going to see repeated more and more as the climate warms and the sea ice melts," said Rebecca Noblin of the Center for Biological Diversity, which has petitioned to list walrus as threatened or endangered because of sea ice loss.
The World Wildlife Fund flew over Icy Cape and recorded footage of the dead walruses.
"Were it not for the dramatic decline in the sea ice, the young walruses at Icy Cape most likely would be alive on the ice and not dead on a beach," said WWF biologist Geoff York.