A team of scientists from the United States and Russia has documented the longest migration of a mammal ever recorded - a round-trip trek of nearly 14,000 miles by a whale identified as a critically endangered species that raises questions about its status.

The researchers used satellite-monitored tags to track three western North Pacific gray whales from their primary feeding ground off Russia's Sakhalin Island across the Pacific Ocean and down the West Coast of the United States to Baja, Mexico. One of the tagged whales, dubbed Varvara (which is Russian for Barbara), visited the three major breeding areas for eastern gray whales, which are found off North America and are not endangered.

Results of their study are being published this week by the Royal Society in the journal Biology Letters.

"The fact that endangered western gray whales have such a long range and interact with eastern gray whales was a surprise and leaves a lot of questions up in the air," said Bruce Mate, director of the Marine Mammal Institute at Oregon State University and lead author on the study. "Past studies have indicated genetic differentiation between the species, but this suggests we may need to take a closer look."

Western gray whales were thought to have gone extinct by the 1970s before a small aggregation was discovered in Russia off Sakhalin Island - with a present estimated population of 150 individuals that has been monitored by scientists from Russia and the U.S. since the 1990s.

Like their western cousins, eastern gray whales were decimated by whaling and listed as endangered, but conservation efforts led to their recovery. They were delisted in 1996 and today have a population estimated at more than 18,000 animals.

Not all scientists believe that western gray whales are a separate, distinct species. Valentin Ilyashenko of the A.N Severtsov Institute for Ecology and Evolution, who is the Russian representative to the International Whaling Commission, has proposed since 2009 that recent western and eastern gray whale populations are not isolated and that the gray whales found in Russian waters are a part of an eastern population that is restoring its former historical range. He is a co-author on the study.

"The ability of the whales to navigate across open water over tremendously long distances is impressive and suggests that some western gray whales might actually be eastern grays," Mate said. "But that doesn't mean that there may not be some true western gray whales remaining.

"If so, then the number of true western gray whales is even smaller than we previously thought."

Since the discovery that western and eastern gray whales interact, other researchers have compared photo catalogues of both groups and identified dozens of western gray whales from Russia matching whale photographs taken in British Columbia and San Ignacio Lagoon in Baja California, Mexico.

Protecting the endangered western gray whales has been difficult - five whales have died in Japanese fishing nets within the last decade. Their feeding areas off Japan and Russia include fishing areas, shipping lanes, and oil and gas production - as well as future sites oil sites. Their largely unknown migration routes may include additional hazards.