Friday, September 18, 2009

Waterborne rodent invasion coming soon to a beach near you

They’re ugly; they’re voracious; they’re prolific—and they’re here. Large buck-toothed semi-aquatic rodents foraging in the waters of Lake Washington and sometimes venturing onto the shores of Madison Park. They’re a non-native species that could threaten the entire Lake Washington ecosystem if not controlled. And right now, well, let’s just say that they’re certainly not under control.

Unless you live on the Lake or spend a lot of time at the beach or road-end parks you may be unaware of these big rat-like interlopers. They’re called Nutria (species name: Myocastor coypus), and they are much bigger than so-called Norway rats, though smaller than beavers. They already infest Portage Bay, and they are becoming an increasing problem on Lake Washington as well. There are regular sightings of Nutria in Laurelhurst, for example, where eradication efforts earlier this year netted several animals.

Nutria are natives of South America, so what are they doing here? Well, eating up the habitat used by other species; burrowing into and eroding embankments along waterways; and reproducing like crazy. As to why they are here, the answer lies in a failed experiment in raising the critters for their fur. Although Nutria were first brought to the United States in the 1890’s, they were not introduced into the Pacific Northwest until the 1930’s. During and right after the Second World War, when it became evident that raising Nutria for their pelts was no longer economically viable, some animals were probably released into the wild rather than destroyed. Other animals may have escaped captivity even earlier. Nutria were seen in Washington until the 1970’s or early 1980’s and then apparently died out. For some unknown reason (possibly emigration from Oregon), they reappeared in Lake Union and Lake Washington in 2005. And their populations have been growing ever since.

In some parts of the country, Nutria have devastated their adopted homes. On Chesapeake Bay, for example, the rodents’ habit of digging out and feeding on the roots of marsh grasses has caused substantial wetland losses. Millions of dollars are being spent annually on eradication efforts there. As for the Northwest, it’s the Portland area, Skagit County and Portage Bay that seem to have the most clearly documented cases of significant Nutria incursions.

At first sight a Nutria may not seem to be particularly menacing or obnoxious. Its front end is much less rat-like than its back end. Here’s a detailed description so you’ll know one when you see one: an adult Nutria has yellow or reddish brown fur; a dense grey undercoat highlighted with long coarse hairs; yellow or orange teeth; short legs with webbed back feet; and a long rat tail that constitutes about one third of its total length. Nutria weigh between 12 and 15 pounds and are about two feet long. If you do see one on land (as I did a few months ago at a neighborhood road end) don’t mess with it. Those big teeth can be dangerous.

Although the animals are generally nocturnal, there have been recent daytime sightings of Nutria in the waters off Madison Park, on road-end beaches in the area, and along the shoreline of the Seattle Tennis Club. According to Diana Forman of the Portage Bay Floating Homes Association (FHA), Nutria have also been reported along the UofW shoreline from Conibear Shellhouse to the fish hatchery and in many Lake Washington locations, including Magnuson Park, Seward Park, Juanita Bay, Yarrow Point, and Bothell.

So what’s to be done? The most effective and most environmentally correct method is trapping. Earlier this year 165 Nutria were trapped in our area under a program funded by the National Oceanic and Atmospheric Administration (NOAA). Of these, 78% were taken from UW property, and the rest were removed from Portage Bay, Montlake-area wetlands, and Laurelhurst, according to the FHA. Houseboat owners on Portage Bay, along with the Seattle Yacht Club, the Queen City Yacht Club and shoreline property owners, have funded a contract with the USDA Wildlife Services to continue with an eradication program in Portage Bay, an effort which may extend into 2010. As of the end of July, the total number of trapped Nutria had risen to 209, as reported in the Portage Bay FHA’s newsletter.

I spoke with Ken Gruver, Assistant State Director of Wildlife Services, about the Nutria menace in our region. He said “there are already enough Nutria in Lake Washington that they are changing the habitat.” If uncontrolled, they will outcompete other species such as beavers and muskrats, damage or destroy the wetland environments of birds and fish, and undermine property along the lakeshore. Not a good prospect for the Lake.

In addition to trapping, he said, there are also toxicants that can be used on the animals without killing other species. He noted that while Lake Washington is experiencing a marked increase in the number of animals, there is still time to correct the problem before the Nutria invasion gets totally out of hand. He noted that Portland has experienced serious damage to its waterways through the rapid growth of the Nutria population there, which has not yet been controlled. On the other hand, the Nutria problem appears to have been resolved in Skagit County, he said, as the result of eradication efforts there. With regard to our Lake, Gruver said that while the problem may be solvable now, in five years it may be too late.

When will we know that we have a real problem in Madison Park? “When property owners along the shoreline first start to see erosion of their property,” said Gruver. That’s what happened in Laurelhust, he noted. “As soon as people saw the habitat change they began asking what can be done.” There’s nothing like seeing your property drop into the Lake to get your attention.

Yes, it could happen here.

There’s a lot of useful information available on the FHA website about Nutria and their impact on Lake Washington, including resources and details on how to report Nutria sightings. KING-TV has a 2007 news story on Lake Washington Nutria (available on video here), as well as a 2008 update on the Nutria invasion of Laurelhurst and the UW (available here). Finally, there are some great professional Nutria photos available here.

Top photo courtesy of Naturecrusaders. Bottom photo by Milos Andera.


  1. I have a couple of working cairn terriers that would love to dispatch them!

  2. Thanks for an interesting article. I walk along Lake Washington in the Madrona area and have been thrilled to see what I was pretty sure was a beaver, as well as muskrats (both native species, unlike the nutria). Although it is quite difficult to differentiate between beaver and nutria while they're swimming (unless you can see the telltale tail), surely the cottonwood/poplars with chewed trunks (also in the Madrona vicinity) are the work of beavers, not nutria...? Hopefully we can get on top of this problem before they wipe out our native aquatic mammals. Heidi


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