Look Out for Nutria

New neighbors may be moving into our houseboat community, but you wouldn't want to invite them to dinner. The newcomers are nutria, which look like "the biggest rat you'll ever see," according to Charles Easterberg of the UW's Department of Environmental and Occupational Health and Sciences.

Fur farmers imported the rodents from their native Argentina, Chile, and four neighboring South American countries to the United States, introducing them first to California in 1899 and then to Louisiana and Washington State in the 1930's. At the collapse of the fur market, discouraged farmers released their stock into the environment, and the population eventually declined. However, having no natural predators in this area, nutria resurfaced here roughly two years ago. They've been sighted in Lake Washington along the Laurelhurst and Madison Park shoreline near the Seattle Tennis Club, in Union Bay and Portage Bay, and along the University of Washington's shoreline from the Conibear Shellhouse to the fish hatchery. Sightings have also been reported in Seward Park, Juanita Bay, Yarrow Point, Bothell, and along the shoreline just north of Magnuson Park. Nutria activity in Lake Union appears to be minimal, although a new houseboat resident "off Fairview on Lake Union" made a confirmed sighting and described its "regular brazen presence" on her neighbor's dock.

Why should we be concerned?

Destruction of marshland habitat for fish, birds, and other wildlife can be extreme where large populations of nutria thrive, for they are voracious consumers of marsh vegetation. Eating 25% of their weight daily in the form of roots, rhizomes, and plant stems, nutria carve up floating vegetation in their quest for food and burrow into shorelines to build their dens. Their appetite for wetland vegetation destroys fish habitat, pushes out muskrats, and ravages the nesting areas of ducks, grebes, and other water birds. Their burrowing undercuts sidewalks and roadways along Lake Washington Boulevard and is currently carving up the marshland near the Montlake Playfield in the south end of Portage Bay.  Individuals who have encountered nutria feeding on their shoreside gardens report that they are not afraid of humans, and a recent article by Lisa Stiffler of the Seattle P-I states that they are also unafraid of pets and may carry disease affecting both humans and domestic animals.

Individuals and agencies concerned about the health and survival of wetlands, shore birds and other water animals also note the troubling rate of nutria reproduction and are determined to act in order to control the population. Female nutria can produce litters as early as three to nine months of age and, with a 130-day gestation period, bear three litters of 1-13 young in a little more than a year's time; those who have just given birth can breed again the following day. In just twenty years the prolific and voracious nutria destroyed 70% of the marshland in Chesapeake Bay, and, closer to home, Portland has been dealing with a major infestation. Portlanders worked with the USDA early enough, however, to at least control the damage in their area. A partnership of private, governmental, and non-profit agencies in Skagit County also took quick action and reduced the population there by 95%.

How can I spot a nutria?

The nutria shares a number of physical characteristics with fellow wetland dwellers the native muskrat and beaver. All three have thick coats of underfur covered by long, glossy guard hairs in shades of brown, and both beaver and nutria have large orange incisors. Beaver, nutria, and muskrat also share a similar profile with their humped back, short legs, and long tail and look most alike while swimming.

However, the three rodents are very different in size - the beaver being the largest of the three, the nutria next in size, and the muskrat by far the smallest. Length and weight statistics differ according to various sources of information, but it's easiest to remember that, on average, adult muskrats are 2 feet long including the tail and weighs 3 pounds; nutria are 3 feet long and weigh approximately 15 pounds, and beavers are the largest and heaviest of the three at 4 feet of overall length and 45 pounds. The nutria also stands out for its muzzle of coarse white hairs and prominent long white whiskers, patches of lighter fur below its ears, and its long, nearly hairless, scaly, round tail. The muskrat's tail is also scaly but is flattened vertically, and the beaver has a characteristic wide flat tail. While swimming, both muskrat and beaver use their flattened tail as a rudder even though the beaver's tail almost never shows while swimming, while the top edge of the muskrat's flattened tail moving from side to side is often seen. However, "a round tail held out of the water or swaying back and forth on top of the water," belongs to a nutria, says Carol Frishmann, who maintains a website dedicated to pets and wildlife called This Wildlife.

What's being done?