The Associated Press

SACRAMENTO - About half the fall-run chinook salmon returned to the Sacramento River and its tributaries last year, the lowest level since 1992, according to the Pacific Fishery Management Council.

The low count came as a surprise to area fisheries managers because commercial fishing along the Oregon and California coasts was significantly cut back last year in an effort to protect salmon from the northern .

The total population of Sacramento River fall-run chinook was estimated at 435,000, about half its 2005 size.

andquot;We're looking at it and wondering what's going on,andquot; said Allen Grover, a fisheries biologist at Fish and Game who monitors salmon. andquot;I think it is a little bit worrisome.andquot;

One factor could be shifts in the Pacific Ocean jet streams. The National Academy of Sciences last week published a study showing a southern shift in the jet stream had triggered a broad decline in Pacific Ocean food sources.

andquot;If you change the timing of this seasonal cycle, it doesn't provide the food for organisms that depend on plankton, and salmon are one of those ,andquot; said lead author John Barth, an oceanography professor at Oregon State University in Corvallis. andquot;If the food is not there when these organisms are naturally ready to make use of it, they're going to suffer.andquot;

The decline of the Sacramento River's salmon is not severe enough to bring new fishing restrictions because the population is still well above protective ranges. But fisheries managers say they are concerned about the Central Valley stocks.

The fishery council is scheduled to meet in Sacramento next week to draft plans for the commercial salmon season, which starts May 1.