Slippery Business

Scientists race to understand the reproductive biology of freshwater eels.

lifecycle of freshwater eels

Diagram shows the life cycle of freshwater eels (not shown to scale). Hatched in the western Sargasso Sea, larvae of American eels, Anguilla rostrata, and European eels, A. anguilla, ride currents toward North America. There, American eels enter coastal estuaries (orange arrows on map show approximate routes). European eels continue onward to Europe and North Africa (yellow arrows). After spending years in rivers, lakes, or sheltered coastal waters, mature eels return to their birthplace to spawn and die. Melisa Beveridge


As they near land, having reached about two and a half inches in length, the leptocephali metamorphose into “glass eels,” their bodies adapting to freshwater and rounding out in cross section (see diagram). At that stage hundreds of millions of juvenile eels are netted—legally and illegally—out of estuaries each year, mostly to supply the Asian aquaculture industry. Stronger swimmers now, many glass eels that escape the nets enter a river and swim upstream in search of a freshwater body to call home. (Some stay behind in coastal waters.) As the glass eels grow they develop pigmentation, at which point they are called “elvers.” Once they exceed six inches in length and darken to their characteristic muddy yellow-brown-green color, they are called “yellow eels.” When they find a home they like—some females forge as far inland as South Dakota—they stay there for years, passing their days resting in the mud and their nights hunting.

When they are somewhere between five and twenty-five years old, having grown as long as five feet, yellow eels mature sexually and enter the “silver-eel” phase of their lives. Their backs darken and their bellies whiten for oceanic camouflage from predators above and below; their eyes enlarge and change pigmentation for improved vision deep in the ocean. They head back downriver, on a long return swim to the Sargasso. Their digestive systems atrophy, and they fast during the entire trip. They have no need to hunt; they have only to swim, arriving with just enough energy left to spawn before they die.

That description of an eel’s life cycle has been accepted since 1923, when a Danish researcher, Johannes Schmidt, published data from eighteen years of trawling for eel larvae in the Atlantic Ocean. He showed that larvae caught in the Sargasso Sea were so small that they must have hatched nearby. But though adult eels are frequently taken during their downriver journeys to the sea, none have ever been captured in the Atlantic, and it was not until 2008 that the first were found in any ocean. That’s when a crew of researchers from Japan netted three freshwater eels—two Japanese eels and a giant mottled eel (A. marmorata)—about eighty miles off a seamount in the western North Pacific. Dissection revealed the eels to be sexually mature males, and a few days later a different ship found recently hatched larvae nearby. Scientists had been zeroing in on the area as the Japanese eels’ spawning grounds, and here, at last, was proof.

The trawling expedition’s leader was Seinen Chow, a marine biologist with the National Research Institute of Fisheries Science in the city of Yokosuka. Dubious about the project’s chances of finding an eel, he’d initially declined to participate. “I thought it would be a waste of time,” he says. The team dutifully developed protocols for what to do if they did capture an eel, but they were thoroughly surprised when, on the tenth night of trawling, they actually netted one. “It was panic on board,” says Chow. “We forgot all our protocols for the moment.” The crew couldn’t do anything at first but stare at the eel.

In June 2009, the same team netted another eight Japanese eels, males and females, in three weeks of trawling. The eels are thought to swim deep below the surface, at some 600 yards, during the day and to rise quickly at sunset to 200 yards. Indeed, Chow’s team netted their eels at night.

Starting in 1974, scientists have made numerous attempts to track silver American and European eels on their way to the Sargasso—without much success. “Pop-up satellite tags” are the latest in tracking technology. Would-be eel trackers can attach one to an anesthetized eel with a wire pierced through the animal’s dorsal muscle. The eel is released, and after a certain pre-programmed time, or at a certain depth, the tag pops off. It then floats to the surface, where it transmits stored data about its journey with the eel to a satellite.

In October and November 2008, scientists from the Eeliad project, a consortium of European research groups, released forty satellite-tagged silver A. anguilla eels in Ireland’s Galway Bay, and forty more from the mouth of the Loire River in France. Eighty more are scheduled for release in autumn 2010. Additional eels are being let go with older-style data-storage tags implanted in their bodies. Eventually, the researchers hope, the tags will provide information about migratory behavior that will be useful not only in capturing elusive adults on the spawning grounds, but also in managing stocks, which are in trouble.

By July 2009, sixty-two tags had floated to the surface and transmitted data, the farthest from about 950 miles off the Irish coast, well short of the Sargasso Sea. Still, with funding secured through 2012, the Eeliad’s director, David Righton, a marine biologist with the United Kingdom’s Centre for Environment, Fisheries and Aquaculture Science in Lowestoft, England, is optimistic the project will succeed. “We didn’t expect to reach the Sargasso the first time,” he says.

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