In 1970, a fisheries attaché in Tokyo sent a memo asking if the state of Maine had enough elvers to warrant a commercial fishery. The task fell to Sheldon, a state employee with a newly minted degree in wildlife management from the University of Maine at Orono. His boss told him, "Bill, go out and find out if we got any."

Over the next two springs, 1971 and 1972, Sheldon found transparent elvers — "glass eels," as he called them (he and many others use the terms interchangeably) — swimming up every stream he visited: the Kennebec, the Penobscot, the Pleasant, and the St. Croix. The transparent fish shimmied over waterfalls, rocks, fishways, and straight up the face of hydroelectric dams. His report, "Elvers in Maine: Techniques of Locating, Catching and Holding," describes the basic life cycle: In November, orphaned at birth in the mysterious depths of the Sargasso Sea at the heart of the Bermuda Triangle, eels begin their elusive migration as transparent ribbons "shaped like willow leaves." Known as leptocephali, literally "slim-headed creatures," they float up the East Coast on the North Atlantic Drift and — with no homing instinct — they're blown inland. They wriggle toward the smell of freshwater in Florida, up the East Coast all the way to Maine and Greenland, or to the coastal waters of Haiti and Venezuela if the currents carry the larvae southward. In freshwater, translucent glass eels develop into pigmented, serpentine elvers.

Eels live for up to 20 years, and the American species, Anguilla rostrata, once inhabited nearly every body of freshwater east of the Mississippi. Sheldon didn't say then — and no one can really say to this day — why eel migration is the reverse of most other fish and why at the end of their life the entire population heads back to their birthplace in the Sargasso Sea where they have one last orgiastic night before dying. "Wherever eels spawn," he wrote, "it seems likely that the entire North American population could be sustained by the spawning of adults from only a few of the freshwater population."

A mature female pumped out as many as 22 million larvae, and where brackish tidewater met freshwater, Sheldon began catching them in a homemade Sheldon's elver trap, a shoebox-sized cage cobbled together with wood and a window screen with a garden hose for a carrying handle. He sent one batch to Japan just to see if they made it alive. They made it, but Japanese buyers preferred the Japanese eel, Anguilla japonica. A pound of American eels were worth $30, so Sheldon went into the lobster business. By the late 1970s, a Washington Post headline read, "Japan Interested in Maine Trash Fish." ("For a while," the Post story said, an eel fisherman "was getting $300 per pound… But the bottom fell out after only a few months." One guy kept elvers in a wooden water tower in an attempt to stockpile but he too was left with nothing when the market crashed.) Former North Carolina Sen. Jesse Helms accused Japan of manipulating prices in 1978; a buyer in San Francisco told the Wall Street Journal that Americans simply didn't know how to handle their eels.

Sheldon began chasing the elver runs, traveling to Florida in January and fishing right up the East Coast. It was legal in all the coastal states. Elvers reached New Jersey in February, and when they hit Maine in March and April, he came home. In 1995, a shortage of Japanese eels sent prices from $55 a pound to $300, and anyone with a mesh net began staking out claims. Police reports began filtering in that year: Fishermen pushed each other into streams, fistfights broke out, and gas tanks, some said, were being filled with bleach. Property owners lobbied to keep the working-class trespassers off their waterfronts. Those selling adult eels for bait wanted to put a stop to catching the baby elvers. Sheldon argued the high natural mortality rate meant any elvers he caught might be eaten by predators anyway. By the late 1990s, Florida, Connecticut, and New Jersey banned elver fishing. One 1999 Bangor Daily News editorial said fishermen could not be blamed for an apparent decline in eel populations; these laws were motivated more by snobbery than biology.