ANCHORAGE, Alaska (AP) — A study of the migration of rare western Pacific gray whales has led U.S. and Russian scientists to question whether they're a separate population or simply California gray whales that have expanded their feeding grounds.

Researchers for decades have considered the two subpopulations to be distinct, with different territories and genetics.

California gray whales occupy mostly waters closer to North America, while the much smaller western Pacific gray whale population was thought to roam the eastern Asia coast.

Recently, scientists over two migratory seasons tagged western Pacific gray whales off Russia's Sakhalin Island, hoping to discover exactly where they spend winters.

The scientists were expecting western whales to migrate to breeding and calving grounds somewhere in the south China Sea, said Bruce Mate, director of the Marine Mammal Institute at Oregon State University and lead author of a research paper published this week in the journal Biology Letters.

"It didn't turn out that way," Mate said.

Instead, three whales with satellite tags were tracked swimming east and across the north Pacific to Alaska waters and into the migratory route of California gray whales, also known as eastern Pacific gray whales.

"It made us realize that many of these animals were born in Mexico," Mate said.

Western Pacific gray whales, once thought to be extinct, are the most endangered of the large whales, Mate said. Only 150 remain as the result of overhunting.

To protect them, environmental groups have campaigned against industrial activities such as oil and gas development off Sakhalin.

Federal and international whale experts have considered the two subpopulations of gray whales to be distinct because their territories were thought not to overlap, based on historic whaling data, and because of genetics analyses, Mate said. Those conclusions are being reevaluated, he said.

The tagging project began in September 2010 and was done with scientists from the Institute of Ecology and Evolution of the Russian Academy of Sciences, the Kronotsky State Nature Biosphere Reserve and the Kamchatka Branch of the Pacific Institute of Geography.

The scientists were startled when a 13-year-old male whale they named Flex swam east to U.S. and Canada waters. He was tracked to the Oregon coast until his tag was lost.

A year later, a 6-year-old female named Agent was tracked halfway across the Gulf of Alaska before her tag was lost.

A third whale, a 9-year-old female dubbed Varvara, was tracked all the way to Baja Mexico, where most California gray whales breed and give birth.

Varvara spent 42 days off Baja California and visited the three main breeding areas of the California gray whale. She then swam north and west and returned to Russia waters, crossing the Bering Sea near retreating sea ice in May 2012. Her trip took 172 days.

The nearly 14,000-mile swim is the longest recorded migratory journey by a mammal by more than 1,200 miles, Mate said. The bigger question, however, is whether the western whales are remnants of a distinct population of endangered animals or the westernmost feeding group of fully recovered California gray whales.

"I don't have an answer to that," Mate said. "I know everybody would like me to. The sample size is just too small with three animals."

Biopsies and photo identifications indicate 30 of the western whales have made the crossing to North America, Mate said.

However, there also is evidence that western Pacific gray whales are not extinct, according to the paper. Four western gray whales died in fishing nets off Japan between 2005 and 2007; a gray whale was stranded off southern China in November 2011; another was sighted in Mikawa Bay, Japan, in March 2012.

More study is needed, Mate said, but a conclusion that they're part of the eastern population could change how whales are treated and studied.

Margaret Williams, Arctic program director for the World Wildlife Fund, had not seen the paper but said Thursday protecting critical gray whale habitat off Sakhalin from chronic pollution, noise and industrial disturbances remains important.

"It doesn't negate the big conservation effort to protect the habitat for this migratory species in a place that would otherwise be made more vulnerable by offshore development," she said.