A female grey whale from Russia crossed the Pacific, swam to Mexico and back in the longest migration ever recorded for a mammal.

The journey reveals some disconcerting news about a species thought to be critically endangered.

Prior to the new study by U.S. and Russian scientists, it was estimated there were just 130 Western North Pacific grey whales left in the world. The whales were so rare that scientists thought they had gone extinct until a small population was discovered off the coast of Sakhalin Island in Russia in the 1970s.

U.S. and Russian researchers tagged whales off Sakhalin Island with satellite tags and tracked them over the course of several months. (Craig Hayslip/Oregon State University Marine Mammal Institute)

But satellite tracking shows that some of those whales might not be Western North Pacific grey whales after all.

Bruce Mate, director of the Marine Mammal Institute at Oregon State University, led the study that tagged three whales near Sakhalin Island. The whales were tracked as they travelled down the west coast of North America to Baja California, Mexico.

One female whale tracked from Russia made a total round trip of 22,511 kilometres — more than four times the distance from Vancouver, B.C., to St. John's, N.L., and the longest migration ever recorded in a mammal.

Mistaken identity?

The area near Mexico where the whales migrated is where the similar eastern grey whale breeds. The eastern grey whale isn't endangered — there are estimated to be about 18,000 of them.

One female whale tracked from Russia made a total round trip of 22,511 kilometres – more than four times the distance from Vancouver, B.C., to St. John's, N.L., and the longest migration ever recorded in a mammal. (Craig Hayslip/Oregon State University Marine Mammal Institute)

"The ability of the whales to navigate across open water over tremendously long distances is impressive and suggests that some western grey whales might actually be eastern greys," Mate said in a statement.

B.C. pictures

The researchers also compared photographs of the whales off Sakhalin Island to those of grey whales spotted off the coast of British Columbia and in Baja California and turned up 10 that were spotted on both sides of the Pacific Ocean.

The results were published in the journal Biology Letters this week.

Scientists have previously found genetic differences between the two types of whales, suggesting that at least some "true" Western North Pacific grey whales do exist.

The researchers also compared photographs of the whales off Sakhalin Island to those of gray whales spotted off the coast of British Columbia and in Baja California and turned up 10 that were spotted on both sides of the Pacific Ocean. (Craig Hayslip/Oregon State University Marine Mammal Institute)

In some sense, that would be bad news.

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

In fact, the researchers wrote in the paper, it is possible that Western North Pacific Grey whales are extinct.

"Overall," the paper concluded, "the tagging and photo ID data indicate that the population identity of whales off SI [Sakhalin Island] needs further evaluation."