SAN FRANCISCO -- The state's largest salmon run is suffering an "unprecedented collapse," part of a broader decline throughout the West that has scientists vexed and will likely trigger severe fishing restrictions, according to federal fishery regulators.

The number of chinook or "king" salmon returning from the Pacific Ocean to spawn in the Sacramento River and its tributaries this past fall dropped 67 percent from a year earlier, according to an internal memo to members of the Pacific Fishery Management Council that was obtained by The Associated Press.

The Central Valley salmon population has fallen by more than 88 percent from its all-time high five years ago, when salmon restoration efforts in the Sacramento watershed were being touted as a wildlife management success story.

But recent years have seen salmon populations steadily dwindle in the Sacramento and many other western rivers, and scientists are trying to understand why.

Some believe it's related to changes in the ocean linked to global warming. Others blame the troubles in California on increased pumping of freshwater from the San Joaquin-Sacramento River Delta.

In his e-mail to members of the fishery management council, executive director Donald McIsaac offered "an early alert to what at this point appears to be an unprecedented collapse in the abundance of adult California Central Valley ... fall Chinook salmon stocks."

"The magnitude of the low abundance ... is such that the opening of all marine and freshwater fisheries impacting this important salmon stock will be questioned," he said.

Only about 90,000 returning adult salmon were counted in the Central Valley in 2007, the second lowest number on record, the memo said. The population was at 277,000 in 2006 and 804,000 five years ago.

It's only the second time in 35 years that the Central Valley has not met the agency's conservation goal of 122,000 to 180,000 returning fish, according to the council, which regulates Pacific Coast fisheries.

More worrisome is that only about 2,000 2-year-old juvenile chinooks returned to the Central Valley last year, by far the lowest number ever counted. On average, about 40,000 juveniles or "jacks" return each year.

The low number of juvenile salmon means this year's runs are likely to be even smaller.

While complete statistics on other key salmon runs won't be available for another two weeks, experts said it looks like a bad year for salmon elsewhere in the West.

Ron Boyce, a salmon program manager for the Oregon Department of Fish and Wildlife, said the Rogue River barely hit its goal of 20,000 fall chinook in 2006 and 2007.

Coastal rivers farther north are in even bigger trouble.

Oregon's Coquille River has seen steadily diminishing returns the past three years, and the Siletz River farther north saw just 500 fish, less than 20 percent of the goal.

"This a large-scale phenomenon affecting chinook stocks and other species coastwide," Boyce said. "It appears for those northern Oregon coast streams, we will not be able to make escapement goals even without any fishing on them."

It is difficult to point to a cause, but the fact that both hatchery and wild fish are showing low returns points to the ocean and estuaries, where salmon spend most of their lives, said Curt Melcher, deputy director of the Oregon Department of Fish and Wildlife.

Last year saw very unusual conditions in the ocean, Boyce said. Southwesterly winds blew all summer, driving warm waters near shore and disrupting the marine food chain.

Some fishermen and environmentalists believe the sharp decline in Central Valley chinook is related to increased water exports from the San Joaquin-Sacramento River Delta, which supplies drinking water to millions of people in drought-stricken Southern California, as well as irrigation for America's most fertile farming region.

"It's time to reduce pumping of delta waters before we destroy the fish and wildlife species we appreciate so much in California," said Mike Sherwood, an attorney for Oakland-based Earthjustice.

Salmon that spawn in Central Valley rivers form the backbone of the West Coast's commercial and recreational salmon fishery and are caught by fisherman from southern California to British Columbia. More than 90 percent of the wild salmon harvested in California originate in the Sacramento River system, officials say.

"Sacramento fish are really what the fishery depends on," said Chuck Tracy, the council's salmon management officer. "When Central Valley fish are low, it gets really hard to catch fish even if you're given the opportunity."

The council plans to meet in Sacramento in March to discuss possible restrictions, including a complete closure of the salmon season that begins in May. Final decisions will be made at its meeting in Seattle in April.

"Even if they have a salmon fishing season, there won't be very much salmon to catch without a strong Central Valley component," said Alan Grover, a biologist with the state Department of Fish and Game.

Duncan MacLean, a Half Moon Bay fisherman who is on a team that advises the fishery council, said he's bracing for hard times.

"It's probably going to be worse than anything we've experienced before," said MacLean, 58, who relies on salmon fishing for up to 70 percent of his income. "It's going to put a lot of us out or business. I don't know how I am going to be paying my bills through the summer."

Dick Pool, who owns Concord-based fishing gear manufacturer Pro-Troll, said the salmon collapse will be felt in fishing communities all along the coast, noting that a recent study found recreational anglers spend more than $2 billion annually in California.

"The impact is going to be huge," said Pool, a former board member of the American Sportfishing Association. "It will take its toll on manufacturing, retailers, wholesalers, fishermen and the charter fleet."

The salmon fishing industry is still reeling from severe limits on West Coast salmon fishing in 2006 to protect dwindling populations on the Klamath River in Northern California and Oregon.

After three years of poor returns, the number of returning Klamath chinook in 2007 exceeded minimums set by federal fishery managers. Preliminary counts showed about 50,000 spawners, though low numbers of juvenile fish indicate there may be poor returns of adult salmon this year.

The precipitous decline of Central Valley chinook marks a dramatic reversal for what's traditionally one of the West Coast's most abundant salmon run. After hitting a record low of 83,000 returning adult salmon in 1992, Sacramento River salmon returns rose steadily over the next decade as the state and federal government spent some $1 billion to restore salmon runs throughout the river system.