MILLIONS of sockeye salmon expected to reach the Fraser River on Canada's Pacific Coast this month have vanished, devastating the local fishery, officials say.

According to the nation's Department of Fisheries and Oceans, between six to 10 million sockeye were projected to return to the river this month.

But the official count is now just 600,000 for the "summer run" - by far the largest of four salmon groupings that return to area lakes and rivers each year from June to late August. Where the other fish went remains a mystery.

The Globe and Mail newspaper cited fishermen who today said the situation was "shocking", a "catastrophe" and a "crisis", while public broadcaster CBC said this could end up being the worst year ever for the Pacific salmon fishery.

A record number of smolts were born in the Fraser watershed in 2005 and migrated to the ocean, and were expected this month to return en masse to spawn.

"It's a bit of a mystery," Watershed Watch Salmon Society fish biologist Stan Proboszcz said.

Officials and ecologists speculated they could have been affected by warmer ocean temperatures, fewer food sources or more prey. Others suggested juvenile salmon may have contracted sea lice or other infections from some 30 fish farms in the Strait of Georgia as they migrated out to sea.

Fisheries officials may have also erred in their complex forecasting calculations, or the fish could just be late arriving, although the latter is very unlikely, said Mr Proboszcz.

"Honestly, we don't know what happens to them when they go out into the ocean," he said. "There's a myriad of factors that could explain what's going on."

Regardless, this outcome is "quite shocking", he said.

Department spokeswoman Lara Sloan said the Fraser River commercial sockeye fishery has not opened as a result of the drop in fish stocks, and a parallel aboriginal fishery scaled back its catch this season to just five percent of its usual take. Moreover, no recreational fishing has been allowed.

Ms Sloan also declined to try to pinpoint the specific reason for the collapse in sockeye salmon stocks.

"There are a lot of variations in the ocean," she said. "They're all interconnected, so it's impossible to point to one reason for this happening."

"So far, they're not coming back in the numbers we expected, but we will continue to look for them," she said.

Meanwhile, pink and Chum salmon are still due to arrive around the end of August through October. So far there is no indication they have been affected.

Chinook salmon are also returning to spawn in the region, but they have been a "conservation concern" for several years, and their numbers remain low.

Originally published as Millions of salmon go missing