Just six years ago, the nutria were so thick in Blackwater National Wildlife Refuge that when hunters rode down the tidal guts of the preserve, the rodents were "jumping off the edge like frogs into a ditch, one after another," said Steve Kendrot, the federal wildlife scientist in charge of wiping the invasive species out of the Delmarva Peninsula.

Now, only a few stragglers remain in Blackwater's 27,000 acres after a relentless hunt that killed almost 12,000 of the orange-toothed herbivores.

For decades, the nutria, renegades from a failed 1940s fur farm, had been eating the marsh to death. Their destructive group dietary habits undercut the root mat that holds marshes together. They are blamed, in part, for the loss of 8,000 acres of marsh that have slipped under the waters of what is now Blackwater Lake.

The state-federal Chesapeake Bay Program considers nutria to be one of the six most harmful aquatic invaders of the Bay region because of their destruction of marshes on public and private property across the Delmarva.

The U.S. Department of Agriculture's Animal and Plant Health Inspection Service's Maryland Nutria Project, which Kendrot heads up, was put in place to eliminate the pests, a goal he said he hopes achieve in the next five years.

"This is the only large-scale eradication program in the country and only the second one in the world," Kendrot said. He said England ran a successful program in the 1980s and rooted out a feral nutria population from its marshes.

He said his technicians scored a major victory in February when the owner of 600 acres of marsh east of Blackwater, near Cambridge, MD, allowed them on his property for the first time.

"We are very happy to have worked with that landowner and have removed more than 100 nutria," Kendrot said. "We are eager to see how that will affect our future discoveries in the surrounding areas."

He said that only a few other landowners have been reluctant to let the federal agents on their land and their properties are smaller parcels. So far, the hunt has expanded onto more than 350 privately owned tracts throughout the tidal marshes of the Eastern Shore.

Last year, hunters covered 11,000 acres of Deale Island in Somerset County, MD, and killed a thousand nutria.

While the invasion of the Delmarva is serious enough to send 15 full-time trappers into the marshes with a mission to kill, it is relatively small on the national and world scale.

Nutria, a native of South America, once thought to have prized fur, have been introduced into 17 states and every continent except Australia and Antarctica, according to government studies.

More than 20 million nutria are believed to inhabit the swamps and bayous of Louisiana and neighboring Gulf States. The numbers are so large and the wetlands are so vast that an attempt to deal with the problem there has not been considered.

But the Delmarva offered scientists an attainable goal.

"The Delmarva is sort of a biological island," Kendrot said. "There are no known nutria north of the Delmarva and the Chesapeake Bay is a pretty formidable barrier against re-population from the western shore."

The nutria, a rather benign rodent that looks like a large muskrat with bad teeth, has adapted to the Eastern Shore with great success because of its incredible fertility rate. A female nutria becomes sexually active at four months. Its gestation period is four months and it delivers a litter of four to five young. She goes into heat immediately after giving birth and becomes pregnant within 24 to 48 hours. When a nutria is 8 months old, females from her first litter are entering the reproductive cycle. Their population, given the right climate, can expand exponentially.

By the late 1990s, it was clear that Blackwater specifically, and the Great Marshes of Dorchester County in general, were in danger and the nutria was a major factor in the decline.

"People who have been around for decades say that a crew of three guys could go into the marsh and kill 500 to 600 in a single day," Kendrot said.

Dixie L. Birch, the supervisory wildlife biologist for the U.S. Fish and Wildlife Service at Blackwater, said that the preserve has been losing 100 acres a year to a combination of nutria, rising seawater and subsidence. If current trends are not reversed, she said, a good portion of the marsh would be lost in 10-15 years.

In 2002, the USDA, working with the Fish and Wildlife Service and the state of Maryland, put together a plan to stop the nutria.

Using traps, guns and detection dogs, the nutria "technicians" work some of the most difficult terrain on the Delmarva. The mud of marshes is extremely soft and often impassable on foot. Much of the outdoor work has to be done in the winter when the marsh is frozen. The technicians are equipped with GPS receivers to help them find their way. An overlay of their tracks on a map of the area shows there are very few places they missed.

"We have covered tens of thousands of miles on foot and in boats," Kendrot said. "There is a phenomenal amount of physical labor involved. The success of this project rides on the shoulders of those guys who are willing to pull on the waders and get into the marsh."

As the hunters eliminate the larger colonies of nutria, they rely heavily on their dogs to find the holdouts.

"We might get 99 out of a 100 in an area with traps, but getting that last one could take weeks," Kendrot said. "We can go in with a dog and if the conditions are right, catch it in 15 minutes."

As the search for the remnants gets more difficult, new measures are being considered. Studies are under way to implant a sterile female with a transmitter and release her back into the wild.

"Nutria are gregarious and they tend to seek each other out when their numbers are reduced," he said. "If we can insert an animal in there that poses no risk to starting a new population, they will search out other nutria. That would enable us to come in behind them and track them down."

An added benefit to nutria-free marshes is the return of the native muskrat population.

"When we remove the nutria, almost immediately we see an increase in muskrat signs," Kendrot said. "Within the next year, there is a drastic increase in the number of muskrat houses on the marsh."

By all measures, the nutria program is accomplishing its goals, but Kendrot is quick to point out that it can't replace lost marsh.

"Removing nutria is critical to preserving what is left," he said. "We can't expect to retain the remaining marsh with an injurious species like the nutria continuing to do what it was doing. That doesn't mean that removing the nutria is protecting the marsh for all eternity."

He said that a nutria-free marsh is much more resilient than an infested marsh.

"The hope is that by protecting the remaining marsh...it will remain long enough to allow new marsh to be created."