SAN FRANCISCO — The number of chinook salmon returning to California's Central Valley has reached a near-record low, pointing to an "unprecedented collapse" that could lead to severe restrictions on West Coast salmon fishing this year, according to federal fishery regulators.

The sharp drop in chinook, or "king," salmon returning from the Pacific Ocean to spawn in the Sacramento River and its tributaries last fall is part of broader decline in wild salmon runs in rivers across the West.

The population dropped more than 88 percent from its all-time high five years ago, according to an internal memo sent to members of the Pacific Fishery Management Council and obtained by The Associated Press.

Regulators are still trying to understand the reasons for the shrinking number of spawners; some scientists believe it could be related to changes in the ocean linked to global warming.

Some fishermen and environmentalists believe the sharp decline is related to increased water exports from the San Joaquin-Sacramento River Delta, which supplies drinking water to millions of people in dry 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 Earthjustice.

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.

In an e-mail to council members, Donald McIsaac, the agency's executive director, said he wanted to give them "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.

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 — used to predict returns of adult spawners in the coming season — 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.

Salmon that spawn in Central Valley rivers form the backbone of the West Coast's commercial and recreational salmon fishery and are caught by fishermen from Southern California to British Columbia.

"Sacramento fish are really what the fishery depends on," said Chuck Tracy, the council's salmon management officer.

Not long ago, salmon restoration efforts in the Sacramento watershed were being touted as a wildlife management success story. But recent years have seen populations dwindle in many Western rivers, and scientists are trying to understand why.

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 in April.

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 as much as 70 percent of his income. "It's going to put a lot of us out or business."