A highly endangered whale that spends summers in Russian waters has crossed from the Bering Sea into the Gulf of Alaska.

US and Russia researchers have tracked the 13-year-old male western Pacific grey whale, dubbed Flex, from Russia across the Bering Sea, through the Aleutian Islands into the Gulf of Alaska about 400 miles south of the Alaska fishing community of Cordova.

Bruce Mate, head of Oregon State University’s Marine Mammal Institute, called the whale’s location “pretty darn amazing”.

No one has documented winter habits of western grey whales, he said. Others of the species may spend winters elsewhere, but a route over deep water in the Bering Sea and the Gulf of Alaska is “something of a paradigm shift” given that eastern grey whales are considered near-shore animals.

“Flex is writing a new chapter for western grey whales, but there may be several chapters to be written yet,” he said.

Western Pacific grey whales are the second-most threatened species of large whale after North Pacific right whales. Just 130 of the animals remain. They spend summers near Sahkalin Island at the south end of the Sea of Okhotsk.

In contrast, eastern Pacific grey whales number about 18,000 animals. They breed and give birth in warm water, mostly along Baja California, and migrate north to spend summers on feeding grounds in Alaska’s Bering, Chukchi and Beaufort seas.

Scientists have been monitoring the tagged whale since he showed up as a calf with his mother in 1997.

It is unknown if the whale has company on its journey, but baleen whales generally do not travel together.