[SQUEALING, SOUND OF RATTLING CAGE]

GRABER: This is the sound of one unhappy Nutria. The reddish-brown animal bangs against the walls of the metal trap it had the misfortune to walk into. A Nutria looks like a cross between a large rat and a beaver, complete with a long, flat tail. Mark Sherfy is one of the lead researchers of the Nutria program here at the Blackwater Wildlife Refuge. He crouches next to a couple of colleagues who inspect the animal.

SHERFY: As you can see the animal’s got several adaptations that allow it to be – to successfully colonize an aquatic habitat such as this. You’ll notice the position of the ears are very high up on the head. That keeps them out of the water while the animal is swimming. The webbed hind feet allow the animal to propel itself through the water.

[SQUEALING, RATTLING CAGE]

GRABER: Sherfy and his team are on a small island of brown and green rushes, squishy mud underfoot. This island is one of many here at the 26,000-acre refuge. They’re separated from each other and the wetlands on shore by narrow, lazy waterways. After a half-century of Nutria infestation, these marshes are fast disappearing under the rodents’ sharp, fiery-colored incisors.

SHERFY: The orange teeth that you see are a very distinctive feature. They’re used to gnaw away at the root mat of the marsh that you’re standing on. The below-ground portions of many of the plants that you see are a favored source of forage for the animal. They use the teeth and their front legs to excavate roots and tubers from wetland plants.

GRABER: Sherfy’s co-workers slip a restraining noose over the animal’s head to keep it from biting. They weigh it, tag it, and check its health.

[SQUEALING]

SHERFY: I don’t see any parasites, ticks.

GRABER: Sherfy points out that the very tip of the animal’s tail is gone.

SHERFY: You often see a stub of only a few inches. They seem to be susceptible to frostbite damage this far north.

[SQUEALS]

GRABER: Here in Maryland, we’re at the northern-most reaches of the Nutria’s east coast invasion. They’ve also established themselves throughout Louisiana and the Mississippi delta, and even in California and Oregon – all places where people introduced Nutria in the hopes of establishing a profitable fur trade. Those profits never materialized. Instead, those areas were left with a pest that has almost no predators. Unfortunately, what it does have is an amazing ability to reproduce year-round. A single female can give birth to eight pups, and four months later, when she’s ready to mate again, her offspring are almost sexually mature themselves. Current estimates of the Chesapeake’s Nutria population run as high as 50,000.

SHERFY: We have a pretty good sense for how to go out and trap Nutria. We don’t have a very good sense at all about how to control a population.

GRABER: Great Britain is the only place to have eradicated invasive Nutria entirely. To do so, researchers there studied the animals for years before actually trapping them.

SHERFY: One of the things that they learned there was about dispersal, or movement, of Nutria. And the fact that you had to account for and understand movement patterns of animals locally in order to be able to eradicate a population. The factors that influence movement rates in Great Britain are likely to be different from here. Differences in weather, differences in how habitats are arranged.

GRABER: Hence the study here on the Chesapeake. For the first year and a half, researchers tagged and occasionally attached radio collars to animals at six study sites in and around the Blackwater Refuge. They’ve learned where the animals roam and how reproduction fluctuates with the seasons. Using this information, they’ve designed an intensive harvest program. Harvest is a polite word for trapping and killing the animals, using humane methods approved by the American Veterinary Medical Association. As counter-intuitive as it may seem, harvesting could lead to more Nutria. That’s because there would be less competition for food among the remaining animals. Those animals could be healthier and thus have larger litters. So researchers will perform autopsies on some of the pregnant females to see if that’s true. If this limited harvesting shows good results, the effort will be stepped up throughout the Chesapeake. But for now, all trapped animals are set free. The team has finished studying the Nutria captured today.

MALE: I think that’s it.

[SOUNDS OF CAGE]

GRABER: One assistant takes the noose off the animal and lifts up the back of the cage. The Nutria slips silently back into the water, its death sentence delayed. Sherfy admits that it’s a little frustrating to watch these destructive critters swim away.

SHERFY: We could be taking these animals out of the population. But in the long term we think we’re going to gain more by understanding these animals and applying what we learn from these animals to control an eradication over the long haul.

GRABER: Scientists here also hope to isolate Nutria pheromones that might be effective in luring out some of the more remote animals. Other states are dealing with the Nutria problem in their own way. In Louisiana, for example, the state government there is actively encouraging chefs to create recipes for Nutria meat. Here in Maryland, researchers hope that years down the road they will be rid of Nutria entirely, bringing them one step closer to saving the Chesapeake’s marshes. For Living on Earth, I’m Cynthia Graber.

[MUSIC: Talking Heads "Listening Wind" Remain in Light]