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.

King salmon get caught in the huge pollock trawl nets, and the dead kings are counted and most are thrown back into the ocean. Some are donated to the needy.

“We do know for a fact that the pollock fishery is slaughtering wholesale and wiping out the king salmon stocks out there that are coming into all the major tributaries,” said Nick Andrew Jr., executive director of the Ohagamuit Traditional Council.

Since 2000, the incidental number of king salmon caught has skyrocketed, reaching over 120,000 kings in 2007. A substantial portion of those fish were bound for western Alaska rivers. If those fish had lived, an estimated 78,000 adult fish would have returned to rivers from the Pacific Northwest to Western Alaska.

Efforts to reduce bycatch are not new. In 2006, bycatch rules were adopted allowing the pollock fleet to move from areas where lots of kings were being inadvertently caught, thereby avoiding large-scale fishing closures. Then, 2007 happened, and it was back to the drawing board.

Last April, the North Pacific Fishery Management Council, the organization that manages ocean fish, passed a hard cap on the pollock fishery. Beginning in 2011, the portion of the fleet that participates in the program is allowed 60,000 kings a year.