Nutria Are Discovered in California but What Are They?
Nutria are large, semi-aquatic rodents that reach up to 2.5 feet in body length, 12- to 18-inch tail length and +20 pounds in weight. Nutria strongly resemble native beaver and muskrat, but are distinguished by their round, sparsely haired tails and white whiskers (see CDFW’s link opens in new windownutria ID guide (PDF)). Both nutria and muskrat often have white muzzles, but muskrats have dark whiskers, nearly triangular (laterally compressed) tails and reach a maximum size of five pounds. Beavers have wide, flattened tails and dark whiskers and reach up to 60 pounds. Other small mammals can sometimes be mistaken for nutria if seen briefly or in low light conditions, including river otters and mink.
The Impact of The Nutria
Through their extensive herbivory and burrowing habits, nutria have devastating impacts on wetland habitats, agriculture, and water conveyance/flood protection infrastructure. Nutria consume up to 25% of their body weight in above- and below-ground plant material each day. Due to their feeding habits, up to 10 times the amount of plant material consumed is destroyed, causing extensive damage to the native plant community, soil structure, and nearby agricultural crops. The loss of plant cover and soil organic matter results in severe erosion of soils, in some cases converting marsh to open water. Further, nutria burrow into banks and levees, creating complex dens that span as far as 6 meters deep and 50 meters into the bank and often cause severe stream bank erosion, increased sedimentation, levee failures, and roadbed collapses.
Habits and Habitat of the Nutria
Nutria can be found anywhere in or near freshwater or estuaries. Thus far, they have been found in cattail and tule marshes, ponds, canals, sloughs, and rivers. All currently known locations are upstream of the Sacramento-San Joaquin Delta, which provides a vast amount of ideal and interconnected habitat for nutria.
Look for nutria and signs of nutria presence in wetlands, canals, rivers, and creeks, along levees and riparian areas, in flooded agricultural fields adjacent to waterways, and in the transition zone between wetland and terrestrial habitat.
Signs of Nutria Presence
Because nutria are wasteful feeders, signs of presence typically include cut, emergent vegetation (e.g. cattails and bulrushes), with only the basal portions eaten and the cut stems left floating, or grazed tops of new growth. Nutria create runs between feeding sites and burrows. Nutria often pile cuttings to create feeding/grooming platforms. Nutria construct burrows with entrances typically below the water line, though changing water levels may reveal openings. Nutria tracks have four visible front toes and, on their hind feet, webbing between four of five toes. Tracks are often accompanied by narrow tail drags.
Nutria Are Now Considered an Invasive Species
The Effort to Eradicate the Nutria in California
CDFW is collaborating with other agencies and local partners to develop the most effective strategy for eradicating nutria from California. As depicted in the “Invasion Curve” figure below, invasive species infestations typically experience a lag phase, while population size and area infested are relatively small, successful eradication is most feasible, and control efforts are most cost-efficient. As time progresses, the population size, area infested, and costs required for control increase exponentially, and the probability of successful eradication is lost. Conceptually, (1) represents where we believe the current extent of the nutria population is in California; eradication is feasible with rapid response; (2) represents the nutria population in the Chesapeake Bay (Delmarva Peninsula) prior to implementation of the link opens in new windowChesapeake Bay Nutria Eradication Project (CBNEP). The CBNEP strategically removed over 14,000 nutria from 2000-2015 and has not detected a nutria since early 2015. (3) represents the link opens in new windownutria population in Louisiana, where population control costs up to $2 million dollars each year for bounty harvests alone.
Nutria burrowing causes extensive damage to water infrastructure, banks, and levees, and creates a hazard for people, livestock, and machine operators. Potential levee and dike failures due to nutria burrowing have serious implications for flood protection, water delivery, and agricultural irrigation in California. Right, Nutria burrow in Tualatin National Wildlife Refuge in Oregon. Photo courtesy of USFWS. Above, extensive burrowing damage by nutria in Oregon. Photo courtesy of Trevor Sheffels, PSU.