Scientists examining the sudden and widespread collapse of West Coast salmon returns are pointing to the unusual changes in weather patterns that caused the bottom to fall out of the ocean food web in 2005.

NOAA Fisheries Service oceanographer Bill Peterson said Monday the juvenile salmon that left their native rivers and entered the Pacific Ocean in 2005 found little food being transported by the California Current, which flows from the northern Pacific south along the West Coast.

The reason was that the jet stream had shifted to the south, delaying the spring onset of winds out of the north that create a condition known as upwelling, which kick-starts the ocean food web by stirring the water from bottom to top, the agency said.

"If there is no upwelling, there is no phytoplankton growth, no zooplankton growth, and basically you have no food chain that develops, because it all depends on the upwelling," Peterson said from Newport.

"We are not dismissing other potential causes for this year's low salmon returns," Usha Varanasi, NOAA Fisheries Service Northwest Science Center director, said in a statement. "But the widespread pattern of low returns along the West Coast for (both coho and chinook) salmon indicates an environmental anomaly occurred in the California Current in 2005."

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