In California’s Central Valley, research on an innovative approach for converting riverside habitats into multi-use areas offers a new way for fish and farmers to coexist. But will the toxic legacy of the Gold Rush haunt efforts to find common ground on these highly contested lands?
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For most of us, our first exposure to salmon comes in the form of orange slabs of flesh on a bed of ice. However, salmonids (a term that refers to salmon, trout, and their relatives) are an extraordinarily diverse and iconic group of fishes. Whether found in the depths of cold lakes or flinging themselves upstream in fast-moving rivers, these creatures have evolved remarkable strategies for surviving in challenging environments.
Despite their ability to live in diverse landscapes and cope with dramatic seasonal changes in their environment, many populations of salmonid fishes are in severe decline. A recent report published in the journal Environmental Biology of Fish warns of the impending extinction of 78% of native California salmonid species within the next century if current trends continue. Because three quarters of these species are found only in California, their disappearance would represent global extinction.
While the causes of decline are complex, the physical alteration of California’s aquatic habitats is thought to be a key threat to native fish populations. Historically, rivers carried rainwater and snowmelt from the Sierra Nevada each year, causing frequent flooding throughout the Central Valley. These seasonally inundated “floodplain” habitats acted as nurseries for young salmon during their migration to the sea. However, in the mid 1800s, agriculture gained a foothold in the region and farmers sought to protect their crops from the floodwaters by lining the riverbanks with earthen levees.
“The Central Valley was once an ephemeral winter floodplain spread over millions of acres, but the vast majority of that is now gone,” said Jacob Katz, a research biologist with California Trout.
Although the construction of these levees has provided a powerful way to physically alter the landscape, many are now in danger of collapsing. To alleviate flood risk, the California Department of Water Resources has proposed a flood prevention plan that would expand the floodplain and move the levees farther from the edge of the river. The aim of the proposed plan is not only to help reduce flood risk in Sacramento, but also to restore aquatic habitats. Some farmers, however, have pushed back against plans that they say disproportionately impact the rural agricultural economy.
“In recent years, we’ve witnessed farmers lose access to water for the restoration of San Joaquin River salmon runs, a goal that has yet to be met and likely never will,” said sixth generation farmer Ryan Schohr. “Sacramento defines conservation as taking water from farmers and ranchers and giving it to environmentalists and Southern California developers.”
In an era of “fish vs. farmer” disputes, the need for solutions that reconcile fish conservation goals and agriculture has arguably never been more important. Enter the Nigiri project, a clever name for a simple idea: how can we manage existing rice fields in California’s Central Valley to function as salmon habitat during the winter?
The project is ongoing, but the preliminary results are promising. Experimental rice fields support high densities of aquatic invertebrates including phytoplankton and zooplankton, providing a food-rich habitat for young Chinook salmon. In fact, juveniles reared in rice fields in 2013 showed the highest average growth rates ever documented for the species in California, earning them the nickname “floodplain fatties.”
“We can look at how the Sacramento Valley used to function and realize that we’re never going back,” said Katz, a lead researcher of the study. “This is now some of the most productive agricultural land on the planet. However, if we mimic natural flood patterns, we can produce this incredible aquatic productivity without affecting agricultural production.”
These results illustrate a way that farmers and conservationists may cooperate and benefit simultaneously. But the story doesn’t end there. In this case, damage from historical land-use practices in California provides an additional wrinkle of complexity.
In the Central Valley, efforts to create multi-use floodplains must exist on landscapes with a legacy of mercury contamination. As a result of decades of mercury mining in Northern California’s Coast Range and gold mining in the Sierra Nevada, the Central Valley watershed is highly contaminated with mercury. Although much of this mercury is buried in the sediment, fluctuating water levels in shallowly flooded wetlands, including rice fields, can lead to its release into the water. Microbial reactions then convert mercury to methylmercury, a neurotoxin that has been shown to cause brain damage and behavioral alterations in Atlantic salmon.
According to some researchers, however, the rapid growth rate of salmon reared on rice fields might be the “silver lining” of this issue.
“One way to reduce mercury concentrations, not necessarily exposure, but concentrations in fish is to promote certain circumstances where they have very rapid growth rates,” said Collin Eagles-Smith, a USGS research ecologist. In other words, a faster-growing fish may be exposed to the same amount of mercury over the course of its lifetime, but its higher overall weight could translate into a smaller concentration per gram of tissue.
While it remains unclear whether methylmercury is impacting fish health in the Central Valley, the negative impacts of the compound as it travels up the food chain to fish-eating birds and mammals are better understood. Mercury has been found to have a host of impacts on birds living in the San Francisco Bay area, from impaired stress hormone responses in chicks to reduced nesting success.
“Essentially, our preliminary data suggests that mercury can lead to reproductive impairment in the birds, which makes them more likely to abandon their nests,” says Eagles-Smith. “There’s still a great deal to learn, however. Rather than being alarmist, we have to be honest about the information we have available so we can create a reasonable risk estimate.”
Landscapes in California are fraught not only with present day conflicts over environmental resources, but also with the vestiges of past land-use practices. The burden of history appears to slow the progress of much-needed reconciliation projects at times, particularly when the ticking of the extinction clock seems to grow louder each day. However, it also offers a cautionary tale to willing listeners. The imprint of human activity, even that which is decades old, has important consequences as we work to restore healthy functioning ecosystems. In the end, balancing cautious, knowledge-based progress with the urgency of present day environmental issues is likely to remain one of the key challenges as we move forward.
Additional information about the Nigiri Project may be found on the UC Davis Center for Watershed Sciences website