California is feeling optimistic it’s winning the war on giant, destructive swamp rodents
California may be winning its five-year, $13 million battle with nutria — the 20-pound, orange-toothed swamp rodents that biologists once feared would play hell with wetlands, flood-control levees and the state’s water-delivery system.
“We do absolutely feel like everything is trending in the right direction,” said Valerie Cook, who runs the Department of Fish and Wildlife’s nutria eradication program.
Cook said her team is seeing nutria numbers declining, and they’ve managed to keep them out of the Sacramento-San Joaquin Delta, California’s most important waterway.
Scientists who deal with invasive pests were first alarmed when nutria — a beagle-sized rodent native to the wetlands of South America — were spotted in a private duck-hunting marsh in the spring of 2017 near the farming community of Gustine in Merced County.
A year later, it seemed almost a sure bet the infestation was growing out of control after a nutria was killed on a farm west of Stockton at the very edge of the Delta.
The fear was the nutria would become impossible to contain if they got into the San Joaquin River and, from there, made it to the network of sloughs, river channels and canals inside the nearly 1,200-square-mile Delta, the West Coast’s largest estuary and the hub of the delivery system that supplies water to millions of Californians.
“If they … actually establish breeding populations in the Delta, it’s going to become so large of a job that we don’t know it would be feasible” to eradicate them, Cook said.
Part of the challenge is because nutria are prolific. A female nutria is sexually mature by six months and can have litters of a dozen or more. They can become pregnant within 48 hours after giving birth. They’ve been known to have as many as three litters in just over a year.
Their offspring quickly disperse. The youngsters have been known to strike out 50 miles or more in search of new territory.
In other words, if not quickly eradicated, nutria populations can expand and exponentially explode in size.
Giant swamp rodents a risk to California
That’s what happened in Louisiana, after nutria, first brought to that state in the 1930s, escaped from fur farms. They became so established, Louisiana’s bayous and other wetlands will likely never get rid of them.
Such a scenario would be bad news for California.
Nutria colonies aggressively feed on wetland vegetation — posing a substantial threat to the few remaining wetlands that haven’t been plowed or paved over to build farms and cities.
At the same time, the nutria’s tendency to burrow dens deep into river banks poses a grave risk to the state’s water-supply infrastructure and its flood-control levees. That’s especially worrisome in the Delta, which supplies water to 25 million Californians and millions of acres of Central Valley farmland.
Desperate to ward off a nutria infestation in the Delta, the state and federal governments began funneling money into killing nutria, the bulk of which came from $10 million in grants from the Sacramento-San Joaquin Delta Conservancy.
The cash infusion into the nutria-killing program appears to be paying off.
After being found in Merced County in 2017, nutria were soon discovered across much of the San Joaquin Valley, including in Madera, Mariposa, Fresno and Stanislaus counties.
But so far, the furthest north they’ve been made it has been in San Joaquin County; 110 nutria have been killed there since the first one was found on that farm in 2018.
Statewide, the Department of Fish and Wildlife reports that nearly 3,000 nutria have been trapped and killed by government officials or by landowners, who have permission to shoot them on sight.
State biologists are finding fewer and fewer nutria colonies after their numbers peaked in 2020. That year, 1,239 of them were killed.
Only 703 were killed the following year. So far this year, 195 nutria have been shot or trapped, despite biologists spending a similar amount of time in the field scouting for them.
State wildlife officials have broad legal authority to kill destructive invasive species, and when it comes to nutria, there has been little opposition to doing so from California’s influential animal rights organizations.
How did nutria get to California?
Five years after they arrived, it’s still not clear how nutria got in the San Joaquin Valley in the first place.
Cook said she believes they were intentionally released, but it’s not clear who did it or why. It’s been illegal to import nutria into the state for decades. Before the ban, nutria were brought to the Central Valley in the 1940s and ‘50s or fur farming. They were thought to be eradicated from California by the 1970s.
Cook is convinced the nutria found in 2017 were newly imported — not hidden descendants of the nutria believed to have been wiped out decades earlier.
“Based on the reproductive rates, and the population numbers that we have seen over the last few years,” Cook said, “I don’t in any way believe there was a remnant population here that went undetected for 50 years.”
Biologists have begun studying the DNA taken from the nutria that have been killed so far, and that might provide some clues to where they came from, Cook said.
In the meantime, the work of killing nutria will continue. If biologists can’t find any more, they’ll keep watching, hoping they eliminated every last one.
“After we do take that last animal,” Cook said, “there will be several, several more years of monitoring that goes on before we can be confident that we have gotten them all.”
This story was originally published July 19, 2022 at 5:00 AM with the headline "California is feeling optimistic it’s winning the war on giant, destructive swamp rodents."