The American eel had been good to John Rorabeck. For decades, he guided his workboat along the rivers and lakes of Ontario, catching more than 400 eels a day with little difficulty. He sold everything he caught - and for good prices.

But one day in 1989, that began to change. Rorabeck went to his friend John Casselman, a longtime biology professor at Queen's University who had studied eels most of his career. There's a problem, Rorabeck told him. And it could soon affect the 100 eel fishermen in the region as well as the species.

It took a couple of years for the science to prove Rorabeck was right. Eels have largely disappeared not only from Lake Ontario, where they are an endangered species, but also from most of the Great Lakes. All along the East Coast, their numbers are at all-time lows. Commercial fishing for eels is banned in all of Ontario as well as the upper St. Lawrence River. Quebec still permits eel fishing, but the catch is greatly reduced from what it was just two decades ago.

"American eels are leaving the largest basin of freshwater on the planet, and they are not returning," Casselman said. "These eels are a miner's canary. They are telling us something."

But the exact message is hard to discern. Eels have a complicated life cycle that scientists do not fully understand.

Eels are a catadromous fish, meaning they live in freshwater but breed in the ocean.

They begin life as eggs floating in the currents, and then become glass eels. They take on a pigment when they move into fresh or brackish water. As they mature, their color turns to a dark, olive green.

When eels are sexually mature, they leave their river habitats and head for the Sargasso Sea, where they spawn deep in the water during winter and early spring. Casselman said that no scientist has ever seen this spawning, despite the fact that eels have been a viable commercial species since pre-Colonial times and have been around for more than 125 million years.

Eels have survived continental drift. They've endured tremendous fishing pressure from native peoples, and then had a comeback after the invention of the musket and a preference for wild game instead of eel meat. But the steely eel has proved no match for a variety of obstacles in the Great Lakes, among them dams that block passage, toxins from chemical plants, loss of habitat, changes in the food web and incremental changes in climate and ocean currents. They face similar challenges in the Chesapeake, where the Conowingo Dam on the Susquehanna River has blocked their entry to the East Coast's longest river system.

For two decades, Casselman and Rorebeck, among others, have been sounding the alarm that the Great Lakes may lose the eels altogether if something wasn't done. Casselman estimates that, between 1950 and 2007, the Great Lakes lost 99.2 percent of the eel population. That loss has allowed the round goby, an invasive fish, to get a foothold in Lake Ontario as it has found the vacated eel habitat a nice place in which to live.

The tide may be finally turning. The New York Power Authority recently spent nearly $2 million trying to help eels get over the Robert Moses Dam on the St. Lawrence River.

"It would be a shame to lose our association with this species, because then we will never understand its mysteries," Casselman said. "We had better not lose these important ancient contacts."

Rorabeck, who now works for the Ontario government catching eels for research, said he does not expect he will ever again earn his livelihood as a commercial eel fisherman. Still, he has a great respect for the animal that sustained most of his career, and he hopes efforts in Ontario and elsewhere can bring it back.

"We gotta work harder to preserve it, because if it goes, there's not much hope for the rest of us," Rorabeck said. "With our forefathers, when all else failed, it was the eel that kept us alive."