A fisheries research project using winter-flooded California rice fields as a nursery for juvenile Chinook salmon is being hailed for its potential to reverse the drastic decline of this economically and culturally important fish over the past century.
The Nigiri Project, which has been ongoing since 2011, takes its name from the sushi-like marriage of fish and rice. Nigiri sushi is a Japanese dish made with sushi rice hand formed into a small clump and capped with a layer of fish.
The project is a collaborative effort of the University of California, Davis Center for Watershed Sciences, the California Department of Water Resources and California Trout, a 50-year-old nonprofit organization whose mission is to protect and restore wild trout, steelhead, salmon and their waters throughout California.
The largest reason for reduced salmon populations is construction of dams and levees in the valley, which protect people and acres of farmland from flooding but cut off fish habitat, said Carson Jeffres, senior researcher and field and lab director at the UC Davis Center for Watershed Sciences, and an author of the study.
“They have cut off 90% of habitat,” he explained. “Fish were cut off from spawning habitat and those who did spawn didn’t have enough food and were growing very slowly.”
Chinook were chosen for the study because of their economic and cultural importance for commercial and subsistence use, he said.
The latest update on the Nigiri project, published in late February in the peer-reviewed open access scientific journal PLOS One, notes that juvenile hatchery-sourced fall run Chinook salmon stocked in rice fields were two to five times greater in size than concurrently and previously observed growth rates in the adjacent Sacramento River.
Researchers found that despite coinciding with the most extreme drought in California’s recorded history which elevated water temperatures and reduced the regional extent of adjacent flood habitats which concentrated avian predators, that zooplankton in these winter-flooded rice fields were 53 to 150 times more abundant than those sampled concurrently in the adjacent Sacramento River channel.
When some 50,000 juvenile Chinook salmon were introduced into the winter flooded rice fields, “we thought we were sending them to their death,” Jeffres remarked. “It turned out those fish grew faster than we had ever seen juvenile salmon grow before. After the first year we realized they were thriving that that led us to this project.”
Historically, the whole Central Valley was a flood plain marsh, but years of development have greatly reduced nourishing salmon habitat. Now the goal is to be able to spend hundreds of millions of dollars to reincorporate habitat for the wild fish to use.
“We want more of the wild fish to come back where they are supposed to,” Jeffres explained. “This is one of the few places where farming and fishing come together. The fish are out there from December through March, when it is not warm enough yet for rice. It works out nicely that way.”
The Central Valley of California has about 500,000 acres of rice fields, of which Jeffres estimates some 40,000 acres are available for projects like this. The juvenile salmon are reared in net pens, released into the ocean and return two to six years later, but on average at three years old, in the 18-to-25-pound range.
Rice farmers are pretty amendable to use the rice fields for Chinook habitat because they see the future of it as being better for them as a water security issue, Jeffres said. Consideration is being given to doing this on an annual basis.
“They are working with the California Rice Commission, which provides rice farmers with incentives, so they can get paid for that, but it’s not something that happens without effort,” he said. “There is an actual cost that goes into it.”
The role of the university is to provide science to all, for them to have sound science to have good decisions for management.”
The report concludes that the abundance of food resources and exceptionally high growth rates observe during these experiments illustrate the potential benefits of using existing agricultural infrastructure to approximate the floodplain wetland physical condition and hydrologic patters under which Chinook salmon evolved and to which they are adapted.