Study: Flooded Rice Fields Good Nurseries for Baby Salmon

UC Davis undergraduate student Emma Cox works to transfer baby salmon to pens for a salmon-rearing study. | Photo: Gregory Urquiaga, UC Davis

Salmon and rice go well together on the dinner table, and it turns out they may be good partners in the watershed as well. That's according to a recent set of studies by researchers at UC Davis, who looked at how well young salmon fare in flooded rice fields. Fisheries biologists with UC Davis are hoping rice fields in the Sacramento River Delta can help promote recovery of California's struggling Chinook salmon runs. So far the results seem to be promising.

The studies took place in the Yolo Bypass area, familiar to Interstate 80 travelers as that long stretch of floodplain west of Sacramento. In February, about 4,500 juvenile Chinook salmon were released into nine simulated flooded rice fields, and their daily growth charted. The idea was to see if rice fields might offer a suitable feeding grounds for baby salmon. researchers also looked at how fish move between different agricultural habitats, and at ways to change flooding schedules to benefit fish.

The most exciting result: juvenile fish in the rice fields grew at record rates, adding up to an inch in length every two weeks during their biggest growth spurts. That's faster than any salmon growth previously recorded in the state of California.

Chinook salmon, Oncorhynchus tshawytscha, are the largest species of Pacific salmon. California's other salmon species, the coho and steelhead, prefer short, steep coastal streams for spawning. The larger Chinook tend to prefer larger main-stem rivers such as the Klamath, the Russian River, and the Sacramento-San Joaquin system. Think of coho and steelhead as sports cars suited to twisty mountain roads, and the Chinook as the 18-wheeler that opts for Interstate 5.

Different populations of salmon can use the same river for spawning, separated only by the time of year they enter the river. These populations are called "runs." Different runs of salmon are treated as species for purposes of management under both the state and federal Endangered Species Acts. Winter-run Chinook in the Sacramento were declared endangered by the state in 1989 and the feds in 1994, while the fall and late-fall runs throughout the Central Valley have been designated Species of Concern by the feds, a possible precursor to listing. Central Valley spring-run Chinook have been listed as threatened by both the state and feds since 1999.

Part of the reason for the decline in the Sacramento River runs is that the watershed's Chinook salmon evolved in a much-different river system. Before Europeans arrived in California and started engineering the river, the lower Sacramento had extensive seasonal floodplain wetlands that offered both shelter and food, the latter mainly in the form of zooplankton, as well as insects and other invertebrates. A century and a half of building dams and dikes have drastically altered the watershed; more than 95 percent of the Central Valley's former floodplain wetland habitat has been altered, making it less likely that young fish will make it out to sea in shape to survive the rigors of the open ocean.

So replacing some of the juvenile rearing salmon habitat we lost when those floodplains were diked, dammed, or developed is potentially a good thing for the Chinook. And according to UC Davis researchers, using the Yolo Bypass' rice fields might be a big help for the fish. As long as there was fresh water flowing, fish were equally happy in rice stubble, plowed, or fallowed fields, and just basically chowed down on the available food, gorging on zooplankton. (It seems that young Chinook are especially fond of cladocerans, also called "water fleas.")

The result: fatter fish faster.

Before and after weight gain shots | Photo: Carson Jeffres/UC Davis

Here's some B-roll describing the studies produced by UC Davis. Veteran fisheries biologist Peter Moyle, who may know more about California's chinook than any other living person, sums up the issue in three words: "Salmon need floodplains."

One unanticipated result: fish mortality was higher in 2013 than in similar studies in 2012. There's a likely reason for this: 2013 was dry, and more fish-eating birds flocked to the Yolo Bypass when their other habitats dried up. That's a loss for the salmon, but a win for other wildlife.

"We're finding that land managers and regulatory agencies can use these agricultural fields to mimic natural processes," said Carson Jeffres of UC Davis's Center for Watershed Sciences. "We still have some things to learn, but this report is a big step in understanding that."

Among the funders and cooperating agencies supporting the study were Cal Marsh and Farm Ventures, Knaggs Ranch, California Trout, Metropolitan Water District of Southern California, and the U.S. Bureau of Reclamation.