The spring chinook, a mix of wild and hatchery stock, are packed with oil that gives their flesh a rich taste and fuels a remarkable freshwater migration. Some push deep into Washington, while others, listed as threatened under the federal Endangered Species Act, spawn in Snake River drainages as far as 900 miles from ocean-feeding grounds.

The spring chinook run into June and are followed by other runs of Columbia River Basin summer and fall salmon, which are forecast to arrive in numbers large enough to schedule sport and commercial fishing openings.

But this year's dismal dam count underscores the volatility of the spring chinook runs, which have been one of the focal points of a Columbia Basin salmon-restoration effort that has cost billions of dollars during the past decade.

Scientists believe the ocean, where the fish migrate after their freshwater birth, has a big influence on run size.

Just five years ago, more than 400,000 Columbia Basin spring chinook returned from the ocean in the largest run since construction of the Bonneville Dam in 1938.

The fish benefited from strong seasonal ocean upwellings of nutrient-rich cold water that produced an abundance of food for young salmon. Those upwellings continued for the first few years of the new century along with freshwater conditions that also favored salmon.

"The good news is that salmon runs are up," proclaimed President Bush in an August 2003 stopover at Ice Harbor Dam. "And that's really positive, and we just need to keep the momentum up."

But that momentum has faded in recent years as upwellings weakened, and survival rates of salmon declined.

Still, scientists this year forecast a run of about 88,000 spring chinook, which still is about eight times higher than the record low return of 10,197 in 1995.

And sport and commercial fisheries that opened earlier this spring resulted in decent catches of fish.

That prompted biologists to think their forecasts — based on the numbers of immature males that returned to the river last year — were on track.

So where are the fish?

Perhaps their migration has been held off by cool weather and high water in the river and will storm through the dam in numbers that would allow more sport fishing on the spring chinook. (Limited tribal fishing now is under way above Bonneville Dam.)

But such river conditions in years past didn't cause lengthy delays.

"We're absolutely at our wits' end trying to figure out what's going on," said Bill Tweit, a Washington state Department of Fish and Wildlife biologist.

Forecasts also were off last year, when biologists predicted 254,100 fish would reach the Columbia River mouth, and the end run was estimated at 106,000 fish. Last year, that run peaked in May rather than April.

That prompted biologists to investigate what might have happened to the fish. They looked at Canadian fisheries and the take of an expanding sea lion population.

The report concluded that none of those factors could have been responsible for the shortfall and affirmed the most important influence probably was a change in ocean conditions.

Hal Bernton: 206-464-2581 or hbernton@seattletimes.com