Millions of sockeye salmon have disappeared mysteriously from a river on Canada's Pacific Coast that was once known as the world's most fertile spawning ground for sockeye.

Up to 10.6 million bright-red sockeye salmon were expected to return to spawn this summer on the Fraser River, which empties into the Pacific ocean near Vancouver, British Columbia. The latest estimates say fewer than one million have returned.

The Canadian government has closed the river to commercial and recreational sockeye fishing for the third straight year, hitting the livelihood of nearby Indian reserves.

"It's quite the shocking drop," said Stan Proboszcz, fisheries biologist at the Watershed Watch Salmon Society. "No one's exactly sure what happened to these fish."

Salmon are born in fresh water before migrating to oceans to feed. They return as adults to the same rivers to spawn.

Several theories have been put forward to try to explain the sockeye's disappearance:

• Climate change may have reduced food supply for salmon in the ocean.

• The commercial fish farms that the young Fraser River salmon pass en route to the ocean may have infected them with sea lice, a marine parasite.

• The rising temperature of the river may have weakened the fish.

The Canadian government doesn't know what's killing the fish, but believes the sockeye are dying off in the ocean, not in fresh water, based on healthy out-migrations, said Jeff Grout, regional resource manager of salmon for the Department of Fisheries and Oceans.

It's too soon to know yet how widespread salmon losses are in the Pacific salmon fishery, but British Columbia's northern Skeena River has also seen lower-than-expected returns this year, Grout said.

Signs are more positive for other salmon species such as chinook, pink and coho, he said.

The reduced salmon return affects the environment around the Fraser River, Proboszcz said. After spawning, adult salmon die, creating a food source for bears and eagles and adding nutrients for plants.