Scientists are scratching their heads this year as fish return to the Yuba River and Central Valley in mysterious ways.

"This is an interesting year all around for salmon," said James Navicky environmental scientist for California Department of Fish and Game.

This year, fall run Chinook salmon returned later, and in lower numbers above Daguerre Point Dam on the Lower Yuba River.

But fishermen have noticed a new phenomenon - a large number of salmon spawning below the dam and electronic surveying equipment.

"These fish have decided there's no need to go up river," said Navicky. Less than 400 salmon have been tracked electronically as they passed the survey site at the dam, Navicky said. That's similar to November numbers in 2004, but down from last year which soared at 3,457.

"You can see the kind of variability," Navicky said.

Scientists won't have a final tally of fall spawners above and below the dam until the end of the year when they finish collecting data from carcass surveys, Navicky said.

While fall counts of salmon are down throughout the Central Valley, healthy comebacks of steelhead, spring run Chinook and late fall run Chinook are surprising scientists.

Last year, only 150 steelhead returned up river while more than 500 of the lively fish came back this year.

The overlooked late fall run Chinook - the largest growing king salmon in the Central Valley at over 3 feet long and weighing more than 20 pounds - returned in large numbers in February.

"The late fall run have rebounded ecstatically. It's really exciting to see them return in numbers that they did," said Navicky.

In February, 113 fish returned, compared to 22 in 2006, 5 in 2005 and 16 in 2004.

The spring run Chinook, considered threatened by extinction in California, also showed up vigorously.

In May, 153 fish returned, compared to 2 in 2006, 113 in 2005 and 53 in 2004.

This year, several months separated the returns of spring and fall salmon, mimicking distinctive independent runs closer to historical ones, Navicky said.

"From a science perspective, that fits what we want to see," Navicky said.

The Sacramento pike minnow, which evolved with, and follows the same habitat as salmon, has also come back in higher numbers this year. As of early November, 361 fish were counted, compared to 141 in 2006, 69 in 2005 and 339 in 2004.

Scientists say huge swings in fish populations from year to year are not uncommon.

This year has been one of the driest in recent memory, and there is talk of La Nina - excessive warm waters coming up the California coast, that could postpone the three year old salmon from leaving the ocean and returning to their birth place until next year.

"Our rivers are low throughout the Central Valley, that could have something to do with it," Navicky said.

The change in fish migration patterns means fly fishermen have to rethink their tactics for catching steelhead and trout - fish that typically follow the salmon run and feed on salmon eggs.

"We have to fish on them in different ways," said Tony Dumont, owner of Nevada City Anglers.

Instead of using egg patterns - flies that look like salmon roe - anglers are using nymph patterns to mock the larval stages of insects.

"It's a little tougher," Dumont said.

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To contact Staff Writer Laura Brown, e-mail lbrown@theunion.com or call 477-4230.