Thousands of Pacific walrus are coming to Alaska's northwest shore again in the absence of summer sea ice and

not all are surviving.

A survey Monday of a mile of coastline near the Inupiaq Eskimo village of Point Lay found 64 dead walruses, the U.S. Fish and Wildlife Service told The Associated Press.

Most of the animals were younger than a year old. The cause of death is not known, said agency spokeswoman Andrea Medeiros, but stampedes — set off when startled walruses rush to the sea, crushing smaller animals — are a likely suspect.

"Our thinking is, because of the age of the animals — they were young animals — it's likely that it was caused by a stampede, probably more likely than disease, given the age class," Medeiros said.

Bears, hunters, airplanes can be the spark

A polar bear, hunter, airplane or boat can cause a stampede. Alaska Native residents of Point Lay, who may legally hunt walrus for food, expressed concern after seeing an airplane flying near the herd and possibly circling.

"That certainly is a concern," Medeiros said. "That's not what we want people to be doing."

Fish and Wildlife Service guidelines instruct pilots of single-engine planes to stay at least a half-mile away from walruses

on land or ice, and if closer, to fly above 610 metres.

The guidelines call for helicopters and multi-engine aircraft to stay a mile away, or if closer, above 915 metres. The

agency warns that it is only guidance but creating a disturbance is a violation of federal law.

Walruses gather to rest on the shores of the Chukchi Sea near the coastal village of Point Lay, Alaska, in 2013. (Ryan Kingsbery/USGS via AP)

No ice, busy beach

Several hundred walruses came ashore near Point Lay on Aug. 3, the earliest recorded appearance of a herd in a phenomenon tied to climate warming and diminished Arctic Ocean sea ice.

A week later, the number had grown to 2,000. In the past month, 30,000 to 40,000 walruses at times have crowded the beach, Medeiros said.

Walrus dive hundreds of feet to eat clams on the ocean bottom, but unlike seals, they cannot swim indefinitely.

Historically, sea ice has provided a platform for rest and safety far from predators for mothers and calves north of the Bering Strait.

However, sea ice has receded much farther north in recent years because of global warming, beyond the shallow continental shelf, over water more than 3,050 metres deep. That's far too deep for walruses to reach the ocean bottom.

Instead of staying on sea ice over the deep water, walruses have gathered on shore to rest.

No witnesses

Calves born earlier this year are especially vulnerable when shoulder to shoulder with mature females that weigh more than a ton.

Residents of Point Lay reported three to five dead walruses in early August. A community member who works with the Fish and Wildlife Service counted 64 dead walruses Monday and tagged them so they would not be counted in a later survey.

The agency hopes to send a veterinarian to determine the cause of the deaths. No one has witnessed a stampede.

"Depending on when the last time he did his survey, it may be an accumulation over several weeks," Medeiros said.

Climate change the biggest threat

Shaye Wolf, climate science director for the Center for Biological Diversity, who wrote the 2008 petition to list walruses

as threatened or endangered species, said the Fish and Wildlife Service should review guidelines for protecting walruses.

"These animals are suffering a great deal of stress from climate change, and when they're pushed ashore, they should get very strong protections from disturbances," she said.

The ultimate threat to walruses is the rapid loss of sea ice due to climate disruption, she said, adding that rollbacks of climate change protections by the Trump administration will further endanger the animals.

Ice in the Chukchi Sea has not reached its minimum for 2017. Walruses likely will keep coming ashore until ice starts to re-form with the onset of winter, Fish and Wildlife said.