An invasive South American rodent has returned to the Bay Area — and it may be too late to eradicate it

Strolling atop levees at Grizzly Ranch in the Suisun Marsh, south of Fairfield, wildlife biologist Robert Eddings stops in front of a brackish pond. Unlike most of the surrounding wetland, this pond stays flooded all year. Its lush green cattails make it a haven for waterfowl and an irresistible feast for invasive nutria — housecat-sized, semi-aquatic rodents with bright orange teeth.

“They’re out there somewhere,” Eddings said.

Indeed, this pond is where the first nutria at Grizzly Ranch were found in 2023. Native to South America, nutria are elusive, burrowing inside of levees during the day and emerging at night to devour marsh plants.

“They cut cattails at the bottom and lay them over, like you took an edge trimmer [to them],” Eddings said.

The animals’ destructive eating habits, combined with the fact that their burrows can weaken levees, means that nutria pose a serious threat to this precious remnant of California’s once-extensive wetlands.

That’s why wildlife biologists and landowners across the state are on the lookout for nutria, reporting all sightings to the California Department of Fish and Wildlife, which has a unit dedicated to their eradication. Since 2017, the Nutria Response Team has caught and killed more than 5,000 animals — and the stakes are high. The rodents reproduce quickly, and as their population grows, eradication gets more difficult. If nutria gain a solid foothold, minimizing damage to the state’s wetlands could cost California millions of dollars each year.

“We’re seeing the infestation grow,” said Krysten Kellum, the agency’s Information Officer in the Bay Delta Region and Central Region.

Nutria were introduced to California for the fur trade in the early 1900s. The rodents occasionally escaped from farms, establishing wild populations. By the 1940s, the fur industry had collapsed, and many nutria were released. But the animals were declared eradicated from the state in the 1970s; even as nutria destroyed wetlands in Louisiana and Maryland, they seemed to have disappeared from California.

Now they’re back.

In 2017, a pregnant nutria was found in a privately owned piece of wetland in Merced County. Since then, DFW staff have been working seven days a week to find, trap and kill nutria all over the San Joaquin Valley — and more recently in the Bay Area.

Eddings, regional manager of the California Waterfowl Association in Suisun Marsh and Bay Delta, meticulously manages the marsh at Grizzly Ranch, flooding and draining it on a seasonal schedule while keeping invasive plants under control. In November, the recently flooded wetland buzzes with mosquitoes. Fish swim through channels that crisscross the landscape, and ducks float across the still surfaces of ponds.

Suisun Marsh is a throwback to California as it was hundreds of years ago. Much of the Bay Area and Central Valley were once vast wetlands. They helped protect the state against drought and floods, and provided homes for wildlife. Now, over 95 percent of the state’s wetlands have been developed, many of them repurposed for agriculture. Preserving what remains is a priority for conservation managers like Eddings.

Eddings maintains the marsh’s tall grasses and cattails, which provide shelter for breeding birds. Nutria threaten that critical habitat by gnawing on cattails’ fleshy bases, killing the entire plant to consume just a small part of it. Funded by the state legislature, the U.S. Fish and Wildlife Service, and the local Delta Conservancy, the Department of Fish and Wildlife has removed more than 480 nutria from the 116,000-acre Suisun Marsh since they were first spotted here in May of last year.

Motion-activated wildlife cameras installed by the agency can be seen at locations throughout Grizzly Ranch, pointing at likely nutria habitat. There are 1,500 such cameras active across the state. Throughout California, state workers construct and monitor platforms made out of plants in wetland ponds. The platforms mimic islands built by nutria, atop of which they feed, groom and loaf around. When the invaders are spotted, wildlife managers set traps and euthanize the animals.

California even hopes to make use of trained dogs that can sniff out nutria scat. Sniffer dogs were a part of Matyland’s successful Chesapeake Bay Nutria Eradication Project, which stretched over two decades, starting in 2002 and finally declaring victory in 2022. No nutria have been captured in Chesapeake Bay in over nine years.

“It was exciting. And then it was a slog,” said Jonathan McKnight, a biologist with the Maryland Department of Natural Resources who was closely involved with the project. “Finding a nutria in a marsh is a little bit of a needle in a haystack situation.”

To eliminate nutria from Maryland’s 250,000-acre section of the Chesapeake Bay, the eradication team divided the land into a grid with 40-acre cells. Starting in the middle, they trapped nutria on one grid cell at a time. “We made sure we got every single nutria out of those areas and then moved on,” McKnight said.

California’s methods are informed by what worked for the Maryland team. But even with thorough surveillance and trapping, it’s hard to know whether eradication has been successful. Maryland didn’t dare to announce success until four years had gone by without any signs of nutria. “You can’t prove something isn’t there,” McKnight said. “All you can do is make the odds approach zero.”

Maryland proved that eradication is possible, but California faces an uphill battle. The Maryland project covered a much more limited area. Even with 50 workers from the state’s Nutria Response Team on the ground, finding and killing every last creature across the Central Valley and Bay Area could take years.

California is home to myriad non-native and invasive species, from eucalyptus trees to grass carp. Most attempts to eradicate these species fail, and turn into long-term management efforts, Eddings said.

“Once something is this widespread, I don’t think eradication is likely,” Eddings said. “We’ll see what happens.”