Federal wildlife biologists have erected a 250-foot-long fence to stop walruses from accidentally plummeting off cliffs to their death on a Bristol Bay beach.

About 30 bulls took the fatal plunge last year, said Rob MacDonald, a biologist with the Togiak National Wildlife Refuge. More than 150 went over the edge between 1994 and 1996, he said.

The mysterious walk-offs seem to occur only at Cape Peirce, where thousands of walruses sometimes gather to rest between meals, MacDonald said.

If too many squeeze onto Maggy Beach, a quarter-mile-long strip of dark-brown sand, dozens may traipse up a chute and onto a grassy plateau, he said. When it’s time to feed, the animals seem to beeline for the water, which leads them across the plateau and over a cliff that’s up to 150 feet above shore, he said.

Sand dunes used to block the chute, he said. But over time, the 3,000-pound walruses wore away the dunes, with help from the wind, and made their way up the chute.

Last week the biologists planted a $2,500, 3-foot-tall, wooden-slatted fence across the chute, MacDonald said. But it won’t be the barricade. It should work like a snow fence, creating sand dunes that pile up to 16 feet high, MacDonald said. That should keep the walruses from straying up to the plateau.

More than 12,000 walrus gathered at Cape Peirce annually in the 1980s, said federal wildlife biologist Joel Garlich-Miller, making it the largest concentration of Pacific walruses in North America at the time.

But the visiting herds dropped sharply for several years, with sometimes only dozens of animals showing up, he said. They may have gone elsewhere, even to Russia.

About 5,500 returned last fall. Yup’ik Natives from the village of Togiak, about 60 miles east, counted 29 carcasses at the cliff’s bottom in October and even saw a walrus fall, MacDonald said.

Refuge managers flew to the scene and confirmed the deaths. They inspected the bloated carcasses and herded about 50 walruses off the bluff and back to the beach, said refuge manager Paul Liedberg.

To temporarily deter the animals from waddling up the chute, refuge managers hung blue tarps from metal fence posts and parachute cord, MacDonald said. The wind-snapping tarps kept the walruses away but didn’t work when the weather was calm.

Biologists aren’t worried the deaths will make a significant dent in the walrus population, MacDonald said. No one knows the number of Pacific walruses, but the last estimate in 1990 exceeded 200,000, he said. A new count is under way.

The fence was erected because Natives in Bristol Bay want walruses protected _ they hunt them for meat, he said. They also use ivory tusks for artwork.

"The local people don’t like to see walrus falling over," said Pete Abraham, an occasional walrus hunter and refuge information technician in Togiak. "It’s such a waste, but it’s nature’s way of doing things."