A young gray whale has been found dead after swimming 20 miles (32.2km) inland up a river in Washington state, a local scientific research group said, pointing to hunger as a possible cause.

It added the group was evaluating the safety of the locaction to carry out an examination.

The juvenile whale, affectionately dubbed "Willapa Willy" by locals, was first spotted last Wednesday in the north fork of the river, about 145 miles southwest of Seattle, the marine mammal research group said.

The whale, "although thin, was behaving normally," the group said at the time, with no sign of injuries.

Its teams had hoped the whale would find its own way out of the river but it failed to do so.

Gray whales undertake long spring time migrations north up the Pacific Coast to feed in the Arctic using up their nutritional reserves along the way, John Calambokidis, a research biologist with the Cascadia Research Collective, told the Associated Press. "When that happens, you often see gray whales in a more desperate search for new areas to feed," he added. "That's the most likely context for this whale."

Gray whales in the eastern part of the Pacific Ocean have faced reduced food availability in the northern Bering and Chukchi seas off Alaska's coast in recent years, Calambokidis said.

"Gray whales are facing a major crisis and the heart of it does seem to be feeding on their prey in the Arctic," he added.

They face threats from vessel strikes to entanglement in fishing gear on their long migration routes. They also tend to travel alone, "long-term bonds between individuals are thought to be rare," it said.

Two adult gray whales washed up dead in the nearby Ocean Shores area earlier in April, Cascadia Research Collective said. "These 42 ft adults, one male and one female, were both malnourished. The male also had significant trauma to the head, consistent with ship strike," it added.