DAVID STEINER FIRST NOTICED THEM at his Suisun Marsh duck club, Joice Island Mallard Farms, a couple of years ago.
“You’d see this animal swimming with just its head above the water,” Steiner said. “At first glance they could’ve been beavers or muskrats.”
Then Steiner and his fellow club members started spotting them rooting around on levee banks, and they knew instantly what they were: nutria, invasive semi-aquatic rodents originally from South America that are often portrayed luridly in the popular press as giant voracious swamp rats.
Nutria aren’t rats – but they’re no more welcome in California’s wetlands than Norway or black rats are in California’s cities and suburbs. For one thing, nutria are distinctly rat-like in appearance, with long naked tails and vivid orange buck teeth. And they are big – up to 20 pounds. They can consume 25% of their body weight in vegetation daily and despoil up to 10 times that quantity. They’re vectors for a variety of diseases and parasites, and they burrow incessantly, posing a significant risk to levees.
“We’ve had a few sightings in the eastern part of the county,” said Matthew Slattengren, the agricultural commissioner for Contra Costa County. “While I haven’t heard of much crop damage yet, I’d be very concerned if their population expanded to any significant degree. Whenever I think of nutria, that line from the Princess Bride comes to mind – ‘rodents of unusual size.’ They’ll eat corn and rice, ruin a lot of what they don’t eat, and strip the bark from fruit trees. We really don’t need 20-pound invasive rodents attacking our crops in this county. We have enough problems with ground squirrels.”
Nutria have thick, lush pelts, and they were first imported to California for the fur trade in 1899. Some escapees from the fur farms were discovered in the 1940s and 1950s, but they were reportedly eradicated by the 1970s. Then a pregnant female was discovered and killed in a privately-owned wetland in Merced County in March 2017.
California Department of Fish and Wildlife (CDFW) spokesman Steve Gonzalez stated in a recent email that his agency conducted surveys of the area and removed 19 nutria from the nearby state-managed Grasslands Ecological Area by the following summer. But by 2018, nutria colonies were reported in San Joaquin, Stanislaus, Merced, Tuolumne, and Fresno counties.
Edwin Grosholz, a Distinguished Professor Emeritus in the Department of Environmental Sciences at the University of California, Davis, and an authority on invasive aquatic species, said it’s impossible at this point to determine if the animals discovered in 2017 are merely holdovers from the populations that existed in the 1950s and 1960s or represent a new introduction
“To determine that you’d have to have genetic data from both populations and compare them, and that hasn’t been done or isn’t possible due to a lack of reliable samples,” he said.
In any case, the California Department of Fish and Wildlife implemented an aggressive trapping program shortly after the hefty rodents were discovered – or rediscovered. Agency staffers have trapped thousands over the past seven years, but the doughty animals have maintained a steady, seemingly inexorable expansion in range: north to the Suisun Marsh and perhaps beyond, east up the drainages of at least two rivers that feed into the San Joaquin Valley.
“Fish and Wildlife has been very responsive in trapping them on our property,” said Steiner. “At one point we had three teams of three people each trapping out here. I know they took at least 20 off this property alone. Plus, CDFW has land bordering our club and there are other clubs adjacent to us, so I don’t know how many they’ve trapped there. But I assume it’s a high number.”
A costly battle
CDFW ‘s Gonzalez said the agency is working to trap and eradicate nutria in areas with known populations.
“[We use] game cameras to identify locations nutria are inhabiting,” Gonzalez stated. “Then CDFW staff set traps there. Traps are checked each morning and evening. We have staff with boots on the ground 365 days a year. Currently, there is a small window of opportunity to successfully eradicate the nutria population in California.”
That task increasingly looks Herculean in scope. Nutria now inhabit around 20 states, and only one – Maryland – appears to have successfully eliminated them. Biologists there employed a variety of strategies, including intensive surveys, releasing sterilized animals fitted with radio tracking devices to locate colonies, dogs trained to sniff out populations, and both traps and snares. But California isn’t Maryland, said Grosholz.
“Yes, Maryland eradicated them, but at a huge expense,” Grosholz said. “And compared to California, it’s a very small state. Really, their problem was trivial compared to ours, given there are so many places in California nutria either inhabit already or can inhabit. Just consider where they are now – from [the mid] San Joaquin Valley to Suisun Marsh. It’s a huge area.”
Grosholz said nutria pose an especially dangerous threat to the levees of the Sacramento/San Joaquin Delta, given they are already in a “destabilized” state.
“They could really accelerate the end-game (of widespread levee collapse),” he said.
Grosholz added the problem is further compounded by CDFW’s limited budget and an onslaught of other invasive species – most particularly the golden mussel, a small bivalve that reproduces rapidly and prolifically and could gum up the pumps and pipes of the State Water Project and the federal Central Valley Project, the two huge public works complexes that transport water from the Sacramento/San Joaquin Delta to Southern California cities and farms.
“The risk golden mussels pose to California’s water distribution system is enormous, and I think DFW is increasingly focused on that,” said Grosholz. “The agency has limited budgets, and they can only do what they are funded to do. If we want an expanded response to invasive species – including nutria – then the state legislature must provide the funds.”
Even the agencies invested with regulatory powers over invasive species are split on how to best deal with nutria. The U. S. Fish and Wildlife Service launched a “Save a Swamp, Sauté a Nutria” campaign earlier this year, noting their meat “…has been compared to rabbit or even the dark meat of a turkey.” But the California Department of Fish and Wildlife has condemned that idea, noting swimming nutria are easy to mistake for protected or regulated species such as otter and beaver.
‘A more reasonable hope’
In the end, Californians may simply have to learn to live with the massive “swamp rats” now inhabiting their wetlands.
“It’s not as though they’re easy to control,” said Steven Chappell, the executive director of the Suisun Resource Conservation District. Chappell’s remit includes thousands of acres of wetlands, including Steiner’s duck club.
“In Suisun, nutria habitat goes up about 2,000 percent each year early in the fall when the seasonal marshes are flooded prior to duck season,” Chappell said. “At that point, finding and detecting them becomes very difficult. The only time that DFW can do effective trapping is when the marshes are drawn down in summer and the nutria are restricted to canals and limited wetland areas.”
And that’s just in the freshwater habitats, said Chappell.
“The Suisun district also includes broad expanses of tidal marsh outside the levees, and nutria do very well there year-round,” said Chapell, “so I really don’t think eradication is likely at this point. Ongoing management is probably a more reasonable hope.”
Update Note: Subsequent to the publication of this article, a CDFW spokesperson said the agency’s own genetics research into historical and contemporary samples of nutria populations suggests contemporary California nutria were likely introduced from Oregon in recent years.