PRINCESS ANNE, Md. — As the sun climbed over the Manokin River, the wake from Daniel Dawson and K.C. Kerr’s skiff sent a great blue heron aloft. Swallows darted over the cordgrass and bulrushes, and an osprey circled overhead.

The eyes of the two men were trained not on the sky but on the muddy riverbank. Their quarry was the nutria, a terrestrial — and highly unwelcome — denizen of the vast network of rivers, estuaries and marshland that drain into the Chesapeake Bay.

“I’m pretty sure there was one up there in the last day or two,” said Dawson, combing through the grass in mud-encrusted waders on a recent expedition. “We’ve got one that comes up here and just grazes.”

A web-footed rodent akin to muskrats and beavers, the nutria has a voracious appetite that wreaks havoc on the bay’s ecosystem.

Nutria colonies once riddled the Blackwater National Wildlife Refuge, an ecologically fragile reserve on the Delmarva Peninsula, an isthmus between the bay and the Atlantic Ocean.

The peninsula, nearly 200 miles long, includes parts of Maryland, Delaware and a sliver of Virginia. Over the past decade, trappers with a federal eradication program have come tantalizingly close to eliminating them, making Delmarva a closely watched example for other nutria-plagued regions, including the Gulf Coast and the Pacific Northwest.

But the creatures have proven to be a cunning and resilient adversary — despite the program’s undisputed success, nutria have unexpectedly turned up recently in pockets along rivers like the Manokin, a distant pond in Delaware, and a marsh near the Virginia border where they had not been spotted for a decade.

Wildlife officials in Delaware are watching with worry, as are those in Virginia who are unnerved by a recent report of nutria close to their sliver of the Delmarva. There are also growing colonies on the coast near Virginia Beach that could migrate to ecologically sensitive areas, said Michael Fies, wildlife research biologist with the Virginia Department of Game and Inland Fisheries.

Tactics are evolving accordingly, turning the peninsula into a testing ground for techniques to combat the nocturnal nutria. Infrared cameras record their nighttime movements and how they react to lures concocted from scent glands. Dogs are being trained to sniff out “nutes,” as the team calls them.

New technology is supplementing time-tested trapping methods. Surgeons have implanted radio transmitters into captive “Judas” nutria to lead them to new colonies. Trackers like Dawson carefully collect fur from “hair snares” for lab analysis. Satellite positioning marks the location of every dropping and footprint found. The head of the team, Stephen Kendrot, jokingly calls their headquarters “the war room.” The project has yielded some startling results, among them that nutria tracked with radio collars can travel miles in one night. Their tenacity has earned grudging admiration from the men and women prowling Delmarva waterways in pursuit.

“They’re a pretty amazing species, actually. I’ve developed a lot of respect for them over the years,” said Kendrot, the eradication project’s supervisory wildlife biologist.

Though unwelcome today, nutria did not arrive on their own. Native to South America, they were brought to the United States for fur farming in the 19th century.

The market never evolved, fur farms shut down and some farmers released the animals into the wild. Because they do not hibernate or burrow, they perished in colder states but have flourished in areas with hospitable climates.

Females can bear three litters a year with as many as 12 young, which quickly reach sexual maturity. Colonies flourish in salty water and fresh, and can make a home in a storm culvert or next to a parking lot.

Nutria are now believed to be in 17 or more states. They are endemic throughout the Gulf Coast, and there are pockets in Oregon, Washington and other states. Louisiana’s population, once estimated to be the largest at 20 million, has fallen after a bounty program for their pelts was instituted.

A 2004 study commissioned by Maryland found that if the population were left unchecked, nutria damage to the bay could eventually cost the state around $37 million each year in lost economic activity.

Then the eradication project, which is financed by the U.S. Fish and Wildlife Service but carried out by a division of the Department of Agriculture, went into high gear. In the first year, trappers killed thousands through the program, the only one in the country with a goal of eliminating nutria.