Flooded farmland helps baby salmon

River Partners Restoration biologists Jeff Holt, right, and Trevor Meadows net the fall-run chinook salmon out of the fish pens where they were raised on the floodplain at Dos Rios Ranch near Modesto on March 6.
River Partners Restoration biologists Jeff Holt, right, and Trevor Meadows net the fall-run chinook salmon out of the fish pens where they were raised on the floodplain at Dos Rios Ranch near Modesto on March 6. River Partners Restoration

Researchers flooded 2 acres of riverside farmland near Modesto last month to see if it might do baby salmon some good.

They mimicked a natural floodplain at Dos Rios Ranch, where the Tuolumne and San Joaquin rivers join, with the hope it could provide waterborne insects and other food the fish need before heading out to the Pacific Ocean.

Judging by the rapid growth – the 120 salmon tripled their weight on average over three weeks – the experiment succeeded.

“The floodplain is really like a bug buffet that allows the fish to pack a lunch for the long journey down to the ocean,” principal researcher Jacob Katz said Monday.

The research, done at Dos Rios and five other sites in the Central Valley, is part of the long effort to rebuild chinook salmon populations. They have struggled for decades because of dams blocking part of their spawning grounds, reduced flows on the rivers, non-native predators and other factors.

The region’s farms and cities could be forced to cut back on their river diversions if the salmon do not recover through habitat restoration and other measures.

Katz is the Central California director for California Trout, which is based in San Francisco and works to restore wild trout, steelhead and salmon. It is one of several partners in the project, which started four years ago at Knaggs Ranch, a rice farm in Yolo County, and expanded to the other sites this year.

The U.S. Bureau of Reclamation provided $350,000 toward the 2015 cost. The California Department of Water Resources added $150,000. An additional $75,000 came from Knaggs Ranch. The effort also involves River Partners, a Chico-based restoration group with a branch office in Modesto, and the UC Davis Center for Watershed Sciences.

The fish for the project came from a hatchery along the Feather River. They were put to death afterward because researchers did not want them mixing with wild fish that return to their native streams after two to five years at sea, Katz said. The long-term goal is to create larger rearing areas for wild salmon.

The main work was at Knaggs Ranch, where about 45,000 salmon were placed in rice fields along a Sacramento River flood channel. Two smaller sites on this channel also were used, along with the Sutter Bypass, north of Sacramento, and the Cosumnes River to the south.

Dos Rios Ranch, which totals 16,003 acres, was set aside in 2012 to provide habitat, as well as to help manage river flows in years that are much wetter than 2015. The experiment involved a minor amount of water, which went back into the river, Katz said.

The site, which sits amid wheat being grown for dairy cows, was flooded to about shin depth. Researchers set up a few 30-square-meter pens so they could monitor the salmon as they ate. Mayflies, stoneflies and water fleas are typical foods.

The average salmon weighed 1.26 grams at the Feb. 5 start of the experiment and 3.77 grams when it ended Feb. 26. The average length went from 48.6 to 65.8 millimeters.

Those numbers might seem tiny to anglers who have hauled in record-breaking fish, but for researchers trying to help salmon through a crucial stage of life, they are huge.

“The juvenile salmon in this experiment grow tremendously when given access to food-rich agricultural floodplain habitats,” said Carson Jeffres, a researcher at the UC Davis center, in a news release.

Katz said natural floodplains have been cut off by the levees that line many river stretches in the Central Valley. Breaching them at certain times and places allows the water to slow down and spread out, exposing it to sunlight that encourages the algae that feed the insects that in turn feed the fish.

Dos Rios was purchased from prominent farmer Bill Lyons Jr. and his family. They also have been involved in efforts to rebuild waterfowl populations in the area, as have rice growers in the Sacramento Valley.

The salmon effort is known as the Nigiri Project, named for a type of sushi served on a wedge of rice.

John Brennan of Robbins Rice Co., owner of Knaggs Ranch, said the research “is showing that farms can also support threatened salmon if we manage our fields and flows in the right ways.”

Bee staff writer John Holland can be reached at jholland@modbee.com or (209) 578-2385.