OTTAWA (AFP) - A judicial inquiry is to investigate why millions of Pacific salmon failed to reach Canadian rivers some months ago, devastating the local fishing industry, Canada's prime minister said Thursday.

Prime Minister Stephen Harper made the announcement in the House of Commons, saying this is a "serious matter."

"We are concerned about the low and falling returns of sockeye salmon in British Columbia," he said.

On Friday, International Trade Minister Stockwell Day, the highest ranking MP from westernmost British Columbia province, will "outline the terms of reference for a judicial inquiry as well as the judge who will lead that inquiry," Harper said.

The Department of Fisheries and Oceans had projected that between six and 10 million sockeye salmon would return to the Fraser river in a peak August run. Only a fraction showed up and where the others went remains a mystery.

A record number of salmon smolts were born in the Fraser in 2005 and migrated to the ocean. Nature dictates that most of them should have returned by now to spawn.

Officials and ecologists speculated the salmon could have been affected by warmer ocean temperatures, fewer food sources, or more prey.

Alternately, they may have contracted sea lice or other infections from area fish farms, or the fisheries department's complex forecasts may be flawed.

"Honestly, we don't know what happens to them when they go out into the ocean," Stan Proboszcz, an expert fish biologist from the Watershed Watch Salmon Society, told AFP in August.

"There's a myriad of factors that could explain what's going on," he said.