The Sacramento-San Joaquin River Delta ecosystem is in its worst-ever crisis as Central Valley salmon and Delta fish populations move closer and closer to extinction.
The Delta Smelt has become virtually extinct in the wild, due to massive water exports to agribusiness and Southern California water agencies through the state and federal water projects, combined with toxics, invasive species and water pollution. The California Department of Fish and Wildlife caught zero Delta smelt in its annual Fall Midwater Trawl survey in 2024, despite the stocking of tens of thousands of hatchery-raised smelt over the past few years.
The Sacramento perch, the only native member of the sunfish family west of the Rocky Mountains, became extinct in its native habitat in the Delta by the 1970s. Fortunately, the unique fish is thriving in Pyramid Lake in Nevada and Crowley Lake in California and the CDFW recently introduced the sunfish to a pond in Sacramento.
But one creature that has increased in numbers in the Delta is the nutria, a giant swamp rat that causes massive damage to crops, levees, and the Delta.
On Jan. 30, on the heels of the most captures in a single year, Rep. Josh Harder (Democrat CA-09) and Rep. David Valadao (Republican CA-22) introduced bipartisan legislation to respond to the “exploding infestation” of nutria. The Nutria Eradication and Control Reauthorization Act would continue funding efforts to eradicate these invasive rodents.
“In 2020, Harder worked with President Trump to bring money to California to fight the nacho cheese toothed swamp rats; however, this funding is set to expire this year,” according to a press release from Harder’s Office. “This comes as these 40 pound rodents – which eat 25% of their body weight every day and can reproduce 200 offspring every year – have expanded their territory across the Valley.”
In 2023, 78% of nutria found were in newly infested areas including the northwestern Sacramento-San Joaquin Delta and throughout the San Joaquin River corridor in Fresno County, Harder noted.
Harder: ‘We’re in a war with giant swamp rats
“We’re in a war with giant swamp rats, and our farms, levees, and waterways will pay the price if we lose,” said Rep. Harder. “We have to keep our foot on the gas to drive these pests out of the Valley for good. I’ll work with anyone to ensure our farmers, water infrastructure, and Delta are no longer plagued by the swamp rat menace.”
“Nutria are a devastating threat to California’s farmland, levees, and waterways, and in recent years, their population has gotten out-of-control,” echoed Rep. Valadao. “This invasive species not only destroys crops—they weaken our flood protection and wreak havoc on the San Joaquin Valley’s ecosystem. As these pests continue to spread, it is important the federal government takes decisive action to stop them, and I’m proud to help lead the effort to eliminate these destructive rodents from California for good.”
In 2024, more than 1,300 nutria were captured in California, the most in a single year and more than the last two years combined. “Funding for these eradication efforts is critical to ensure that the nutria infestation does not continue to spread,” added Harder.
Nutria first introduced to California in 1899
Nutria have a surprisingly long history in California. “Nutria were originally introduced to the U.S. (Elizabeth Lake, California) for the fur-trade in 1899, but failed to reproduce,” according to the CDFW.
“Subsequent introductions were successful, as records indicate nutria were present in the Central Valley and South Coast of California in the 1940s and 50s, but nutria were later declared eradicated from the state in the 1970s. In 2017, a reproducing population of nutria was discovered in California’s San Joaquin Valley,” the agency stated.
The timeline of significant nutria detections and CDFW's response are detailed below.
- March 2017: the first nutria (a pregnant female) is incidentally taken in a private wetland in Gustine (Merced County).
- Summer 2017: CDFW conducts surveys in the area to evaluate the extent of the potential infestation; 19 additional nutria are taken in the managed wetlands of the Grasslands Ecological Area.
- February 2018: CDFW institutes an Incident Command System (ICS), partially redirecting staff to implement eradication efforts. The ICS operates in collaboration with USFWS, CDFA, DWR, and through supplemental contracts with USDA-Wildlife Services.
- September 2018: the first reproducing population of nutria within the legal Sacramento-San Joaquin Delta boundary is discovered south of Lathrop (San Joaquin County).
- May 2019: an individual, dispersing nutria is detected on camera near Rough and Ready Island, approximately 16 miles north of the nearest known population and previous detections.
- July 2019: the state legislature provides CDFW with funding and positions to create a Nutria Eradication Program. With additional grant funding from the USFWS, Wildlife Conservation Board, and Delta Conservancy, CDFW transitions out of the ICS and begins full-scale operations with fully-dedicated staff and resources.
- October 2022: a family group of nutria is detected on lower Sherman Island (Sacramento County), with 34 nutria subsequently removed between then and May 2024.
- May 2023: nutria are detected near southern Grizzly Island, with 513 animals subsequently taken from the broader Suisun Marsh area (Solano County) as of February 11, 2025.
- 2024: nutria are detected up the San Joaquin River in Fresno County, to Friant Dam and above Millerton Lake near new Auberry.
- August 2024: Two adult nutria are taken from Dutch Slough (Contra Costa County), across the Sacramento County line and south of Sherman Island. As of February 8, 2025, a total of 18 adult and juvenile nutria had been taken from the area.
“As of February 11, 2025, a total of 5,493 nutria have been taken in California, with additional animals confirmed present, across Merced, Stanislaus, Fresno, Solano, San Joaquin, Fresno, Mariposa, Sacramento, Madera, Contra Costa, and Tuolumne Counties,” the CDFW wrote.
Confirmed detections of nutria and locations of take in California can be viewed in the nutria detection map (PDF)(opens in new tab).
wildlife.ca.gov/…
New invasive species — golden mussel — discovered in Delta
As if the imperiled Sacramento-San Joaquin River Delta didn't already face enough problems, the California Department of Water Resources staff discovered a new invasive species, the golden mussel, in the Sacramento-San Joaquin River Delta while conducting routine operations on Oct. 17, 2024.
The non-native freshwater bivalve (Limnoperna fortunei) was found in the Port of Stockton. Suspected golden mussels have also been identified at O’Neill Forebay in Merced County.
“This discovery is the first known occurrence of golden mussels in North America,” said Steve Gonzalez from the California Department of Fish and Wildlife. “The species poses a significant immediate threat to the ecological health of the Delta and all waters of the state, water conveyance systems, infrastructure and water quality.”
To protect California against the spread of invasive golden mussel, the California Fish and Game Commission at its Dec. 11-12 meeting added golden mussel to the list of species restricted from “live importation, transportation and possession.” The golden mussel is native to China and Southeast Asia and was likely transported across the ocean on large ships, the Commission stated: wildlife.ca.gov/...
“Golden mussels are a bigger threat to California waters than quagga and zebra mussels because they can thrive in a wider range of water quality conditions,” said Jay Rowan, Chief of the California Department of Fish and Wildlife’s Fisheries Branch. “Putting golden mussels on the restricted species list is an important first step toward containment and prevention of spread of this non-native species.”
The emergency listing will reduce the potential for people to introduce the bivalves to other waters of the state, according to the Commission.
“The species poses an immediate threat to the ecological health of the Delta and all waters of the state, water conveyance systems, infrastructure and water quality. The Commission’s action makes it illegal to be in possession or transport golden mussels in California,” the Commission wrote.
Meanwhile, Governor Gavin Newsom has been pushing three projects — the Delta Tunnel, Sites Reservoir and the Voluntary Agreements — that fish advocates say would hasten the extinction of Delta smelt, longfin smelt, Sacramento winter-run and spring-run Chinook populations, Central Valley steelhead and green sturgeon.
And both Trump and Newsom have issued executive orders that intend to increase water diversions to corporate agribusiness from the Delta in a state where there are five times the water rights as there is actual water —what water policy analysts call “paper water.”
