An estimated ten thousand Pacific walruses have huddled together on a remote island in the Chukchi Sea (map), an unusual phenomenon that's due to a lack of sea ice, experts say.

The giant marine mammal is known to "haul out"—literally haul its body onto ice or land to rest or warm up—on various places along the Arctic coast.

But with the Arctic warming up and melting much of its floating ice, there are limited areas for the walruses to gather. This forces them to cluster on land in huge aggregations rarely before seen. (See more walrus pictures.)

In 2011, 30,000 walruses hauled out along a stretch of beach less than a mile long, according to the National Oceanic and Atmospheric Administration, which took aerial pictures of the most recent walrus gathering.

Scientists first noted that such large terrestrial haulouts along Alaska's coast in 2007 and reports have increased in the past five years, said Pam Tuomi, senior veterinarian at the Alaska SeaLife Center in Seward.

That mirrors the effect of warming temperatures in the Arctic, which is in the throes of a "long-term, downward trend" in sea ice cover, according to the National Snow and Ice Data Center.

In 2013, the Arctic experienced its sixth lowest minimum extent, or the period when sea ice cover is at its smallest.

The walrus haulouts are "another one of the symptoms of the changes that are occurring in the Arctic Ocean," Tuomi said, "and they are causing cascading effects."

On Thin Ice

The Pacific walrus as a species is suffering due to its shrinking habitat—the animal's numbers are declining, and it is currently listed as "threatened" and may soon be upgraded to "endangered" under the U.S. Endangered Species Act, Tuomi said.

Meanwhile, the large haulouts are putting individual animals at risk. For one, if something like an airplane flying overhead spooks one of the mammals, it may spark a stampede into the water. During their panic, the heavy animals—which can weigh up to 1.5 tons (1.4 metric tons)—may trample other walruses to death, especially young ones, she said.

"It's like yelling fire at a movie theater," she said.

In addition, so many animals in such close quarters could increase the likelihood of a disease outbreak. In 2011 a mysterious, fatal disease swept through a population of ringed seals in Alaska and there was concern that some walruses might also have been affected, she noted. (Also see "New Diseases, Toxins Harming Marine Life.")