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Can the Yolo Bypass floodplain be managed to nurture salmon?

When the Sacramento River runs high after a big storm, floodwaters rush into the Yolo Bypass, which carries them away from California's capital city to the river's downstream delta.

In most years, the floodwaters quickly drain away. But what would happen if the bypass was full of water for longer periods when migrating salmon could use it? Would that provide salmon with the benefits of a natural floodplain and boost the struggling populations of Central Valley Chinook?

A team of researchers conducting experiments in the bypass are coming up with some encouraging results. 

In February, they released thousands of juvenile Chinook salmon from a hatchery in two areas of harvested rice fields that served as bypass research plots.

The team monitored rates of fish growth and survival in the fields, which included some enclosed pens, and found that the salmon thrived -- as long as they weren't snapped up by hungry birds.

The salmon fattened up on high densities of zooplankton. Their average growth rates were the greatest documented in fresh water in California and their overall condition was excellent.

But the survival rate for fish allowed to swim freely around the screened plots was not great -- at most about 29%. Salmon in the pens did better, with survival ranging from 35% to 98%.

Carson Jeffres, field and laboratory director for the Center for Watershed Sciences at UC Davis, said the survival numbers were somewhat misleading because last winter was exceptionally dry.

During the final weeks of the experiment, he said, the research plots were the only part of the bypass that remained flooded, attracting waterfowl eager for a salmon buffet.

In next year's experiment, Jeffres said the fish will be able to swim away as they please, mimicking more natural conditions. 

The research was funded by a variety of interests, including public agencies and water contractors that could use the results in planning habitat restoration in the Sacramento-San Joaquin delta, the hub of the state's water system.

Most of the Central Valley's historical floodplain has been lost to agriculture. The Yolo Bypass, which is farmed under flood easements, is the largest contiguous floodplain remaining in the valley.

The research team, which includes scientists from the nonprofit California Trout, UC Davis and the California Department of Water Resources, is trying to determine if rice fields, which are designed to hold water, could be managed in the bypass to double as salmon habitat between fall harvest and spring planting.

That would entail flooding during more than high-flow periods and retaining the water for three to six weeks.

"This is proving that if you increase the duration of the water, you see benefit," Jeffres said of last winter's results, which are outlined in a recent report.

bettina.boxall@latimes.com

Twitter:@boxall

Copyright © 2014, Los Angeles Times
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