Polar bear encounters on the North Slope oil fields have risen to record levels the last two years, a sign that increasing numbers of the white giants may be prowling on land because the sea ice they prefer is shrinking, scientists said.

    Oil field sightings along the southern Beaufort Sea coast jumped to 321 in 2007 and 313 in 2008, said Craig Perham, a U.S. Fish and Wildlife Service biologist in Anchorage.

Polar bear sightings along the southern Beaufort Sea coast in Alaska have jumped, signaling a possible migration from shrinking ice. (Photo: Paul Nicklen / National Geographic)

    That's more than double the 15-year average of 138. It's also a sharp rise from 232, the previous high in 2005.

    Oil companies are legally required to report a polar bear encounter to Fish and Wildlife that involves any change in the animal's behavior, even if the bear simply lifts its head to sniff the air, Perham said.

    The measure is designed to protect polar bears and humans by providing information on the bears' whereabouts and behavior. Increased sightings might also be up because new exploration work has taken place on the near-coast areas where polar bears are usually found, he said.

    For example, ships doing seismic exploration in the Beaufort Sea have waited out storms on barrier islands those years, giving workers the chance to spot polar bears numerous times, sometimes the same animal.

    Perham hasn't "teased out" the data to find out exactly what it means, he said.

    "Truth be told, we don't know," he said.

    But the rising encounters are in line with what scientists expect to find - as sea ice shrinks, polar bears spend more time on land, he said.

    Polar bears prefer the ice because it provides access to ringed and bearded seals, he said. But the seals are harder to find when the ice recedes so far - more than 70 miles off the coast - that it no longer sits over the shallower and biologically productive continental shelf.

    The sea ice has shrunk to record levels in recent summers. One day last August, there were 400 miles of open water between the Alaska coast and pack ice.

    Interestingly, the majority of oil field sightings have come in August and September, Perham said. In September, the sea ice is farthest from shore.

    In those two months, oil field workers have reported spotting polar bears more than 240 times in 2007 and 2008.

    During the winters of 2007 and 2008, oil field workers reported seeing polar bears no more than 14 times.

    In the past, most sightings took place in the winter.

    Despite the increased sightings, the oil industry hasn't reported killing a polar bear since 1993, Perham said.

    If bears come too close, workers might shoo them away by shouting or slamming a car door. If that doesn't work, they might scare it away by firing off loud cracker shells.

    Scientists believe the shrinking sea ice has lead to declining numbers of polar bears. If sea-ice loss continues as forecasted, the bears could someday disappear from large areas, including Alaska's coastal waters, said Eric Regehr, a federal wildlife biologist. "It's pretty grim," he said.

    He said the Fish and Wildlife Service tentatively estimates the southern Beaufort Sea polar bear population at 1,526 in 2006, a drop from the previous estimate of 1,800.

    The world population of polar bears is estimated between 20,000 and 25,000, he said.

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