ANCHORAGE, Alaska – Yukon River smokehouses should be filled this summer with oil-rich strips of king salmon — long used by Alaska Natives as a high-energy food to get through the long Alaska winters. But they're mostly empty.

The kings failed to show up, and not just in the Yukon.

One Alaska river after another has been closed to king fishing this summer because significant numbers of fish failed to return to spawn. The dismally weak return follows weak runs last summer and poor runs in 2007, which also resulted in emergency fishing closures.

"It is going to be a tough winter, no two ways about it," said Leslie Hunter, a 67-year-old store owner and commercial fisherman from the Yup'ik Eskimo village of Marshall in western Alaska.

Federal and state fisheries biologists are looking into the mystery.

King salmon spend years in the Bering Sea before returning as adults to rivers where they were born to spawn and die. Biologists speculate that the mostly likely cause was a shift in Pacific Ocean currents, but food availability, changing river conditions and predator-prey relationships could be affecting the fish.

People living along the Yukon River think they know what is to blame — pollock fishery. The fishery — the nation's largest — removes about 1 million metric tons of pollock each year from the eastern Bering Sea. Its wholesale value is nearly $1 billion.