In this Aug. 17, 2011 photo provided by National Marine Mammal Laboratory, herds of Walrus line the shore off Point Lay Alaska. In winter, Pacific walruses are found at the edge of sea ice in the Bering Sea. Older males remain in the Bering Sea for the summer, but mothers and their young ride the edge of sea ice as it melts north through the Bering Strait and into the Chukchi Sea.(AP photo/National Marine Mammal Laboratory)

(AP) -- Large herds of Pacific walrus have begun gathering on the northwest coast of Alaska, again forsaking sea ice for sand in what has become a symbol of climate warming in the region.

A contractor taking part in federal marine mammal aerial surveys over the Chukchi Sea spotted about 5,000 walruses Wednesday on a beach north of Point Lay. A second herd of 3,000 walruses rested a short distance away.

In winter, Pacific walruses are found at the edge of sea ice in the Bering Sea. Older males remain in the Bering Sea for the summer, but mothers and their young ride the edge of sea ice as it melts north through the Bering Strait and into the Chukchi Sea.

Female walruses and their young have come ashore during late summer and fall in four of the last five years on Alaska's northwest coast. The phenomenon began after sea ice melted far beyond the relatively shallow continental shelf to waters of the deep Arctic Basin.

Walrus can dive to 600 feet to find clams, snails and other food on the ocean bottom. Water beyond the continental shelf can be 10,000 feet or more.

In 2007, several thousand walruses gathered on Alaska's northwest shore for the first time. They returned in 2009, and last year gathered in unprecedented numbers. Upward of 20,000 animals were counted near Point Lay, an Eskimo village 300 miles southwest of Barrow and 700 miles northwest of Anchorage.

Walrus face danger from stampedes when they gather on shore. More than 130 mostly young walruses were crushed in September 2009 at Alaska's Icy Cape in a disturbance that could have been caused by a polar bear, human hunters or an airplane.

"Walrus are sensitive to human activity and to machine activity," said U.S. Fish and Wildlife Service spokesman Bruce Woods. The agency has already issued alerts to pilots and coastal communities asking that airplanes stay at least 1,500 feet above herds and a half-mile away if it's safe. Likewise, hunters in boats have been asked to stay a half-mile away to avoid spooking walrus herds.

Woods said the herds could continue to grow. Sea ice in Arctic waters generally melts through mid- to late September.

"We could see bigger concentrations," he said.

The walruses were spotted by a COMIDA flight, which stands for Chukchi Offshore Monitoring in Drilling Area. The aerial surveys are funded by the Bureau of Ocean Energy Management, Regulation and Enforcement, which oversees offshore petroleum drilling, and coordinated by the National Marine Mammals Laboratory. A 2008 sale sold 2.76 million acres in leases of Chukchi Sea ocean bottom and Shell Oil hopes to drill during the open water season next summer.

U.S. Geological Survey scientists plan to be in Point Lay next week to tag 35 walruses with satellite radio tags that will monitor foraging activity from shore. The tracking project is part of a study on how walruses respond to reduced sea ice.

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