Slippery Business

Scientists race to understand the reproductive biology of freshwater eels.


Elvers assemble near the base of a Massachusetts river dam. Many die trying to surmount the dam on their way upriver, and again years later, on their return to the sea as adults. Dams have contributed to a global decline in freshwater eels.

Tim Watts

The Eeliad project is a component of the European Union’s response to a dramatic drop in eel populations—and Europe is not alone. Over the past decade eels have declined around the world. Nowhere have falling numbers caused more alarm than in Japan, which has the world’s largest per capita consumption, and where eels represent a $1 billion annual market. The Japanese consume some 200 million pounds of eel a year, most of it prepared as kabayaki—slices of eel that are butterflied, skewered, marinated in a soy-based sweet sauce, steamed, and grilled for a smoky flavor. Not nearly enough A. japonica is captured each year to feed the national appetite, so the Japanese depend on glass eels captured in Europe, or to a lesser extent in North America, and raised to eating size on farms in China or Japan. The EU is placing increasingly stringent regulations on that commerce, however, which lends urgency to research into captive eel reproduction. Recent breakthroughs have been major, but the work has not been easy or rapid.

Japanese researchers made a giant step forward in eel reproduction in 1973, when, building on earlier work, they induced fertility in males and females with hormonal injections and produced larvae from the fertilized eggs. The larvae lived for the five or six days it took them to consume their yolk sacs, and then died. Later researchers tried a variety of diets, from egg yolk to rotifers, but the larvae grew poorly, if at all, and did not live much past two weeks. (Exactly what larvae eat in the wild is not well understood.) It was not until 1998 that a team led by Hideki Tanaka, a fish biologist at the National Research Institute of Aquaculture, discovered that larvae in tanks would consume a slurry with a base of shark-egg powder. The larvae still only survived for thirty days, but they grew to one-third of an inch before dying.

Continual refinement of the diet extended survival and growth to the two-inch, 250-day mark, when larvae transform into glass eels, ready for freshwater. By 2004 Tanaka’s group had blown past that milestone, too, raising yellow eels of an edible size from birth in captivity. But shark-egg powder is not cheap, and with many shark populations in trouble, it is obviously not a workable or responsible commercial solution. In addition, most of the fertilized eggs still do not produce viable larvae. For now, Tanaka says, mass production on a commercial scale remains out of reach.

Although Europeans don’t consume as much eel as the Japanese, they hold the fish in high regard all over the continent: jellied or stewed in the United Kingdom; smoked in Germany, the Netherlands, and Scandinavia; cooked in a red-wine sauce in France; grilled in Italy—the list goes on. The drop in the European eel population has spurred concern and research activity in recent years. Danish researchers, aiming, like the Japanese, for commercial aquaculture production, have induced sexual maturation and spawning in European eels, and have kept the resulting larvae alive for as long as eighteen days, but not without a struggle. The project’s lead researcher, Jonna Tomkiewicz, an aquatic biologist at the Technical University of Denmark in Charlottenlund, told me, “Right now, we’re where the Japanese were thirty years ago. Their research has received a lot of funding because eels form such a large part of their diet.”

Nevertheless, Tomkiewicz is confident that European eels will eventually be mass-produced. The EU and Denmark recently infused several million euros into her project. Should it succeed, the capacity to breed eels would not only be commercially important, but would also help relieve pressure on the wild stock, she says.
In the U.S., researchers at the University of Massachusetts Dartmouth have bred American eels and sustained the larvae for six days. The program is young—just entering its fourth year—and focuses on understanding the eels’ decline, not on aquaculture. That, no doubt, reflects the economic insignificance of the domestic eel market. Some Asian communities carry on a small trade in eels, and a few Italian-Americans still incorporate them into a traditional Christmas Eve supper. But most Americans will live and die without tasting eel flesh. Quite a comedown in popularity for the fish that was a staple of Native American tribes along the East Coast and its rivers, and that virtually saved the Mayflower pilgrims from starvation their first spring in Massachusetts, in 1621. Eel was avidly consumed as a readily available, inexpensive fish in the U.S. until after World War I, when its presence dwindled, then disappeared from the national table—a change often attributed to modern squeamishness about its snakelike appearance and slime coating. It is hard to think of another food that has fallen so far out of favor in just a century.

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