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In spring, the Sargasso Sea teems with thousands of tiny, transparent, leaf-shaped baby eels. They emerge from the deep waters of the sea, an oval area between the West Indies and the Azores, and drift on ocean currents to the north -- making a 6,000-kilometre migration.

Some stay near the American coast. Others continue their odyssey for as long as two years, growing and changing shape until they reach the St. Lawrence River. But then again, no one has ever seen the American eel spawn. After more than a century of research, they remain a mystery. And their future may be uncertain.

The latest data suggest that the population of the American eel in the Great Lakes-St. Lawrence River region has dropped by more than 90 per cent in five decades. Wildlife managers believe this could not only send that community into collapse but could also launch a devastating ripple through the freshwater ecosystem.

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"The decline is precipitous," says John Casselman, an eel expert and biologist at Queen's University in Kingston, Ont. "We've gone from a situation where, when the Europeans arrived, eels probably made up half of the inshore fish biomass in the upper St. Lawrence River and Lake Ontario."

For generations, eel were plentiful and fished to satiate palettes across the globe. The Iroquois trapped eels with weirs and speared them at night from their canoes. Jacques Cartier described large vats of eels that the inhabitants of Hochelaga (now Montreal) kept to prepare and eat during the cold winters.

Today, the Swedes celebrate the season of alamorkret (eel darkness) with feasts on eel in all of its forms -- smoked, fried, grilled or stuffed -- paired with a glass of schnapps. The Dutch eat it smoked. They're marinated and grilled for one of Japan's national dishes, kabayaki, and feted each October in the Charlevoix region of Quebec.

All that gluttony has contributed to the eels' decline not only in North America but around the world, eel experts say. Japanese juveniles have dropped 90 per cent since 1980. And European fry have plunged 99 per cent -- creating a black-market for baby eels (called elvers) that fetch about 1,000 euros a kilogram.

Eel poaching is also on the rise. While collecting glass eels for a research project in the Netherlands, Willem Dekker was approached by a mugger with a baseball bat -- though he retreated into darkness when he realized the scientist might not be alone.

Although there are some physical and genetic differences between the European eels and American eels, they are all thought to congregate in the Sargasso Sea to spawn.

Scientists have used eel ladders -- passageways that allow eels to swim upstream past river-blocking dams -- to estimate the size of the American population in the Great Lakes region.

At the Moses-Saunders hydroelectric dam on the St. Lawrence River, the number of juvenile eels dropped to 1,000 a day from 25,000 from 1982 to 1995.

"The eels are a concern at both ends of the pipeline," says John Dettmers, a senior fisheries biologist for the Great Lakes Fishery Commission in Ann Arbor, Mich. "The new recruits [into the St. Lawrence]are greatly reduced and the mature eels coming out of the lake to establish future generations are also down."

But the precise cause of the eel's decline remains unclear. Overfishing, chemical contaminants and hydroelectric dams are all thought to have taken a heavy toll on the American eel. Ocean warming may also be contributing to the population collapse.

Part of the ambiguity comes from the eel's mystifying life cycle. The American eel is thought to be a catadromous fish -- one whose life begins and ends at sea, but spends most of its time in freshwater lakes and rivers.

The adult eels congregate in the warm waters of the Sargasso Sea south of Bermuda to spawn, where each female lays as many as 10 to 20 million eggs. Then they die, sink to the sea floor and disintegrate. Or so scientists believe; no one has ever seen an adult eel return to freshwater.

When the eggs hatch, the larvae drift to the Gulf of Mexico, the Atlantic seaboard, the St. Lawrence River and Labrador. Those that reach the estuaries and rivers change into transparent, needle-shaped fish called glass eels, or elvers.

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Some swim farther inland, up small rivers of the East Coast, or along the St. Lawrence to the Great Lakes. The juveniles that swim upstream hit the eel ladder on the Moses-Saunders dam when they are four to six years old.

Scientists recently discovered that the Great Lakes basin contains only the female eels. At the mouth of the St. Lawrence, the female-to-male ratio is one-to-one, says Yves de Lafontaine, a research scientist at Environment Canada's St. Lawrence Centre in Montreal.

"As you progress upstream, the ratio favours the females," he says. "We don't know where the males go."

The female eels remain in freshwater for 10 to 20 years, growing to as much as one metre in length and developing a yellow pigmentation that earns them the name golden eels. As summer fades, these eels leave the inland areas -- travelling under the darkness of night -- to return to the Sargasso Sea to spawn. During the long voyage, they metamorphose into sexually mature silver eels.

They negotiate many hazards en route, including fishermen's weirs and hydroelectric dams, as they drift downstream with the current. Many are killed when they are sucked into the dams' electricity-generating turbines.

The Moses-Saunders dam near Cornwall and the Beauharnois dam near Montreal kill about 40 per cent of the eels -- mainly ready-to-spawn females, Prof. Casselman says, and their crash could affect the population from Lake Ontario to the Gulf of Mexico.

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"The eel decline is a bellwether of larger problems," says Emmett Duffy, a marine scientist at the Virginia Institute of Marine Science and a co-author of a headline-grabbing Science study on ocean species loss published last fall.

As more and more fish vanish, the marine environment becomes increasingly unstable and less likely to recover from stresses such as invasive species, disease outbreaks and algal blooms. The authors of the Science paper predict the collapse of all wild seafood by 2050 if current trends of overfishing and pollution continue.

But adding to the eels' mystery are recent findings by David Cairns, a biologist with the Department of Fisheries and Oceans in Charlottetown. He says the Maritime populations of American eels aren't suffering the same decline as the Great Lakes-St. Lawrence River population.

In fact, by studying their otoliths (or ear-stones), he has found that some American eels spend their entire lives in saltwater, and that their numbers may be quite high. This could affect conservation efforts -- although it does not solve the problem of eels disappearing from freshwater.

That's why -- despite a January decision to keep it from endangered species status in the U.S. -- Fisheries and Oceans Canada is still considering adding the American eel to the Species at Risk Act. And efforts to bring the eel back from the brink and restore the Great Lakes watershed continue.

"We've probably lost seven native species in Lake Ontario," says Rob MacGregor, Lake Ontario manager for the Ministry of Natural Resources. "We don't want to lose an eighth if we can possibly avoid it."

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The Ontario government banned commercial eel fishing in Lake Ontario and the upper St. Lawrence River in 2004, and ended the sport fishing of eels across Ontario.

Last fall, Mr. MacGregor oversaw the release of 50,000 young eels into the St. Lawrence River near the Thousand Islands, a combined effort of the Natural Resources Ministry, Ontario Power Generation and the Ontario Commercial Fisheries Association.

In total, the groups have restocked 144,000 eels. They will grow in Lake Ontario for 10 or 15 years, allowing the ministry to monitor their numbers. "But we don't know if they'll ever go back to the Sargasso Sea," Mr. MacGregor says.

Lake managers in the U.S. and Canada are also working with the Great Lakes Fisheries Commission to develop a bi-national eel-management plan to mitigate eel mortality, but it won't be implemented before 2008, Mr. Dettmers says.

Still, Queen's Prof. Casselman professes subdued optimism for the situation.

"I'm convinced that the species will persist. It came from the ocean, and will go back to the ocean, but it is telling us something about what we are doing to the freshwater environment and maybe to some extent the marine environment and the resources," he says.

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"The eels are sending us a message and I hope that we're reading it."

Hannah Hoag is a freelance writer based in Montreal.

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