A lone female Western North Pacific grey whale has set a record for long-distance migration, according to a study Tuesday.

Nicknamed Varvara, the nine-year-old whale was tagged off Russia's Sakhalin Island in 2011.

She crossed from the northwestern to the northeastern side of the Pacific and followed the western Canadian and US coast down to breeding grounds off Baja California, Mexico.

She swam 10,880 kilometres (6,800 miles) in 69 days -- "the longest recorded distance travelled during a mammal migration," the scientists said.

She then returned home by a slightly more southerly route, completing a 172-day, 22,511-km (14,069-mile) trek, also a record for round-trip migration.

Until now, the acknowledged record-holder for mammal migration has been the humpback whale, one of which made a one-way trip of 9,800 kms from waters off Brazil to Madagascar, according to a 2010 study.

The new research, published in the British journal Biology Letters, raises intriguing questions about grey whales -- Latin name Eschrichtius robustus.

Grey whales exist in both the eastern and western North Pacific and conservationists consider the two populations to be separate.

Commercial whaling ravaged numbers on both sides of the ocean.

However, those in the east have largely recovered while those in the west are teetering on the edge of extinction -- and indeed at one point were thought to have been wiped out.

The International Union for Conservation of Nature (IUCN) lists the western grey whale population at around 130, making it "critically endangered".

But if Varvara's trek is any guide, there could be a genetic mix between the eastern and western populations, the investigators said.

In other words, some presumed western grey whales could actually be eastern grey whales.

"The population identity of whales off Sakhalin Island needs further evaluation," said the scientists, led by Ladd Irvine at Oregon State University.

Six other whales were also given satellite-monitored tags.

Along with Varvara, two others also crossed into regions inhabited by non-endangered eastern grey whales.

They were "Flex", a 13-year-old male that crossed to waters off Oregon, a journey of 7,611 km, and "Agent", a six-year-old female, which headed to the Gulf of Alaska, 5,464 km away.

The migration patterns break with the idea that grey whales follow exclusively north-south migratory routes along coasts and demonstrate that these mammals have "navigational skills across open water," the paper said.