Robert F. Bukaty/AP

A massive, years-long undercover operation has led to arrests and guilty pleas up and down the East Coast of poachers and traffickers who dealt in a slippery, squiggly and valuable commodity: baby eels.

William Sheldon, who runs one of the biggest and oldest eel businesses in Maine, might be forced to give up his truck with the license plate “EELWGN.” In federal court in Virginia this month, a Brooklyn seafood dealer named Tommy Zhou became the 11th person to plead guilty to eel trafficking as part of the sweeping federal investigation known as “Operation Broken Glass.” Zhou declined to comment.

“I’m kind of chuckling now as more and more faces show up in the paper,” said Tim Sheehan, who runs a seafood company so far north in Maine it’s nearly in Canada. “We could be the last dealer standing.”

Maine is home to the only major legal market in the United States for baby eels, known as glass eels or elvers. (There is also a small market in South Carolina.) But sky-high prices for the little wrigglers has led to widespread poaching, as elvers caught farther south are smuggled north.

Tracking illegal eels is a challenge. “Fishermen can sell eels to dealers who can then sell eel to anybody,” said Toni Kerns, director of the Interstate Fisheries Management Program, part of the Atlantic States Marine Fisheries Commission. The commission is a coastal-state compact that sets eel regulations.

“They don’t have to be shipped out of Maine or South Carolina,” Kerns said.

While the eels are not considered endangered in the United States, restrictions have been set to keep the population strong. But eels are caught at night, and Sheehan said unsavory sellers simply find a licensed fisherman waiting at a dealer and offload their elvers.

“They drive down a back road or go behind a building and swap the eels,” he said.

In 2011, because of a policy change and a natural disaster, the American eel market began to soar. A year earlier, the European Union had banned the export of eels because of fears of the European eel’s extinction. That spring, a tsunami devastated Japan’s fishing industry.

Asian customers turned to the United States for baby eels, which are then raised in tanks and turned into barbecue. While historians say eel was likely eaten at the first Thanksgiving, the domestic appetite for eel remains weak.

In 2010, the average price for a pound of elvers sold in Maine was $185.20. A year later, it was $891.49. A year after that it was $1,868.73, and the number of pounds sold nearly tripled. (Experts say there are about 2,500 eels in a pound.) Until 2013, there was no limit on how many elvers could be caught.

Regulators watched with concern. Hydroelectric dams and environmental stresses had already hurt the eel population. While the U.S. Fish and Wildlife Service in 2015 declared the American eel population stable, the International Union for Conservation of Nature deems the population endangered.

The eels, which are born in saltwater and migrate to fresh­water, cannot be reliably bred in captivity. No one has even seen them spawn in the wild. American eels are born near the Bahamas, in a swirl of warm water called the Sargasso Sea. They float toward the East Coast, transforming first into clear little leaves and then tiny, transparent eels. (Elver comes from “eel-fare,” meaning journey.) As they move north, they grow and darken, gaining a dark stripe along their backs and then a dirty-yellow skin. After years or even decades of feeding in freshwater rivers and lakes, they develop black backs and silver bellies. They take on fat. Their eyes expand, while their guts wither. In the fall, under the dark of a new moon, they glide home to the Sargasso Sea to breed and die.

“In 2012, 2013, there were fishermen that would go out of state and fish and bring the eels back here and sell them before the eels got running here,” said Darrell Young, co-director of the Maine Elver Fishermen Association. “It was illegal but legal — as long as they didn’t get caught, it was all right.”

But many of them did get caught, in undercover investigations that began back then and are now coming to fruition. Agents of the U.S. Fish and Wildlife Service posed as fishermen, joining groups that split motel rooms in Massachusetts for illicit elver catches. They posed as dealers, taking elvers they knew were illegally caught. And they posed as sellers offering eels they said came from outside Maine and South Carolina. One man was caught stocking his home in Lowell, Mass., with tanks to store poached elvers.

The undercover operations wound down in 2014, Justice Department officials said, and prosecutorial work began. The guilty pleas over the past six months account for $2.75 million in little eels.

Sheldon has pleaded not guilty. Prosecutors say he was caught buying what he thought were poached eels from undercover agents and was coaching dealers without licenses on how to launder the origins of their elvers. Timothy Lewis, a Maine dealer accused of poaching in Connecticut, Massachusetts, New York and Virginia, also pleaded not guilty.

Court documents mention more than a dozen unnamed co-conspirators who have yet to be charged.

Sheldon, 71, is a key player in Maine’s legal elver market. In 1974, as a young state employee, he wrote a paper on elver fishing. He was one of the first dealers in the state. His attorney noted his not-guilty plea but declined to comment further.

For now, he’s still dealing in eels.

“He can still buy eels until he’s convicted,” Young said. “According to him, he’s done nothing wrong.”

But dealers such as Sheehan fear that if the criminality doesn’t end, regulators — who frequently battle fishermen over how much elver fishing should be allowed — will shut down the industry altogether.

“What the poachers and the shortsighted people don’t understand is that we’re in a real precarious position with this fishery in the state of Maine,” he said. “If we screw up, the fishery’s gone.”