Decades of experience have proved that Sacramento Valley rice farmers can use their fields to grow healthy ducks. Now, research under way in the Yolo Bypass aims to find out if they can grow salmon, too.

On Tuesday, researchers from UC Davis, the California Department of Water Resources and a nonprofit fisheries group released 50,000 juvenile salmon into a 20-acre rice field north of Woodland.

The goal is to find out whether a rice field, flooded between harvests to degrade stubble and provide waterfowl habitat, can stand in for the wetlands that once carpeted the Central Valley and served as a massive nursery for juvenile salmon.

At stake is not just the survival of Central Valley salmon, but also the future of rice growing in the Yolo Bypass and a water supply that serves 25 million Californians.

"This area you see here is really a lab experiment writ large," Jacob Katz, a fishery biologist at the Cal Trout nonprofit and the project leader, said as the 2-inch salmon were removed from a trailer, weighed and sorted into tanks. "What we're really trying to demonstrate is you can have both fishery benefits and agriculture on the same parcel. All you need to do is get out here and work out the protocols," he said.

The Yolo Bypass, an expanse of farmland and natural habitat that stretches from Sacramento to Davis, was created a century ago to divert floods from the city of Sacramento. When the Sacramento River swells in wet winters, it flows over Fremont Weir, diverting most of the floodwaters into the 58,000-acre bypass, which is farmed the rest of the year.

Scientists have known for more than a decade that salmon grow faster and bigger when they are able to access the Yolo Bypass during floods. That's because flooding triggers massive insect blooms that provide great fish food. The bypass also gives the small fish a refuge from bigger predators in the mainstem of the river, such as non-native striped bass.

Trouble is, the bypass usually doesn't flood often enough – or long enough – to play that role for salmon. And if the bypass floods too long – into April or May – it conflicts with the rice planting cycle.

The experiment launched Tuesday aims to find the optimal duration of flooding that can benefit salmon while avoiding conflict with rice planting. Researchers will also try to determine an ideal flooding depth, and whether rice stubble, weedy ground or bare ground is best for flooded habitat.

The research began last year on just 5 acres using 10,000 hatchery-born chinook salmon on a farm property owned by Knaggs Ranch LLC. This year's project scales that up to 50,000 fish on 20 acres. It also adds wild-born salmon to determine if they grow and survive differently.

Some of the fish are implanted with electronic tags that will allow scientists to track their growth rates in real-time as they swim about the flooded fields for four to six weeks. The technology is similar to the FasTrak transponders that automatically charge car commuters a toll when they cross a bridge.

At the end of the study, the fish will be released back into the Sacramento River.

Only about 5 percent of the floodplain habitat that once carpeted the Central Valley remains today. That is because people walled the rivers off with levees to grow food and build cities.

Scientists have known for 30 years that connecting rivers back to these floodplains may be the most important thing humans can do to restore salmon populations. Research has shown that salmon raised on floodplains grow faster and bigger, feed more successfully when they enter the ocean, and are more likely to return as spawning adults three or four years later.

Making more of this habitat is essential to restoring salmon runs, which have dwindled to a fraction of historic levels because of water diversions and habitat loss. The Endangered Species Act, which protects some salmon runs, also has imposed deadlines to begin restoring floodplain habitat.

But restoring habitat is difficult, Katz said, because it has to fit into the realities on the landscape, which includes farming.

"What we're trying to do is create a new ecological model for agriculture that works," Katz said.

Something similar happened in the early 1990s when air-quality regulators and the farming community recognized that burning rice stubble after harvest posed a public health threat. The practice was replaced by flooding fields instead, which rots the stubble and has the side-benefit of creating winter waterfowl habitat when the fields aren't being cultivated.

Part of the research under way at UC Davis includes studying different soil samples to determine which type produces the most bugs when flooded. The bugs hatch from eggs and larvae deposited in the soil by a previous generation of insect life.

"There are hundreds, if not thousands, of insects that are coming out of each 9-by-6-inch block of earth," said Carson Jeffres, a fish ecologist at the UC Davis Center for Watershed Sciences.

Knaggs Ranch manager John Brennan said the owners wanted to participate in the study to find another benefit that rice farms can provide in their offseason, in much the same way that some ski resorts serve mountain bikers in the summer.

Early results, he said, show that young salmon fatten up just fine on flooded rice stubble. If the timing, depth and duration of that flooding can work for rice farmers, the partnership will be a success.

"Everybody who is dependent on water should be making some strides toward trying to solve some of these fish issues," Brennan said.

One of the big threats on the table is a proposal to modify Fremont Weir so the Yolo Bypass floods more often, and for longer periods. This proposal is contained in the Bay Delta Conservation Plan, the same project that proposes to build two massive tunnels to divert the Sacramento River across the Delta.

The proposal alarms both farmers and flood-control officials. If the bypass floods too long, the fields may not be ready for planting at the optimal time in spring. If the fields can't be planted, they will eventually sprout trees and shrubs, which could reduce the capacity of the bypass to absorb floods.

The partners in the Knaggs Ranch study hope to find a compromise that works better for all these functions already provided by the bypass.

"We're trying to see if there are different ways to manage the land to make it as fish-friendly as possible," said Ted Sommer, an ecologist and program manager at the state Department of Water Resources. "I really hope we can find a way to keep agriculture as robust as possible."

© Copyright The Sacramento Bee. All rights reserved.

• Read more articles by Matt Weiser

Order Reprint