The 13-year-old, male western Pacific gray whale dubbed Flex is being tracked by U.S. and Russian researchers.
Its last location was logged Thursday about 400 miles off the coast of British Columbia, said Bruce Mate, director of Oregon State University's Marine Mammal Institute.
Mate said it was possible the whale's satellite tag had fallen off, or that bad weather interfered with transmissions.
"We have not heard from the animal for the last three days," Mate said. "There is a pretty good lump out there in terms of swell, but it's nothing like what was out in the southeast Bering Sea. We may be coming to an end of this, but it's a little early to say."
Western Pacific gray whales are the second-most- threatened species of large whales after North Pacific right whales. Only 130 of the gray whales remain.
In contrast, there are about 18,000 eastern Pacific gray whales. Those whales 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.
Western Pacific gray whales spend summers near Sakhalin Island at the south end of the Sea of Okhotsk near Russia. Little is known of their winter habits. North America waters were not high on lists of suspected winter sites.
Last October, researchers from Oregon State and the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences tagged Flex. The whale spent more than two months feeding near Sakhalin Island and swam to the west coast of the Kamchatka Peninsula.
Within a few weeks, the whale was headed across the Bering Sea.
On Jan. 13, Flex was about 80 miles north of Alaska's Pribilof islands. He turned south and was tracked on the south side of the Alaska Peninsula.
A week ago, he was in the Gulf of Alaska about 400 miles south of Cordova. Researchers don't know where he wants to go.
His trajectory now puts him on a course to be close to shore along the central Oregon coast by Thursday.
Researchers are prepared to shadow the whale if he gets close to shore.
Mate hopes swells are blocking transmissions from the whale. Satellite monitored radio tags have lasted as long as 385 days on a gray whale but average four months. The tag transmits four hours a day to conserve battery power.
The chances of finding the whale if the tag is off are minimal.
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