The Strange Journey of the American Eel

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The American eel makes an annual pilgrimage from freshwater rivers along the East Coast to the salty Sargasso Sea. How are creatures that spend most of their lives in fresh water able to adapt to the ocean? Biologist Steve Gephard explains.

DEBBIE ELLIOTT, host:

You probably know about the annual return of swallows to Capistrano and the homecoming of monarch butterflies to a certain valley at Mexico. But what about the pilgrimage of the American eel? It travels from rivers along the East Coast back to its ancestral homeland in the Sargasso Sea.

We recently read about this annual rite of passage in the Hartford Courant and it got us wondering about one of those basic science questions we like to answer in Science Out of the Box.

(Soundbite of music)

ELLIOTT: Our question is how do fish that live most of their lives in freshwater suddenly adapt to seawater - the change in environment that would kill most fish species.

Steve Gephard is a fisheries biologist with the Connecticut Department of Fisheries. He monitors eel migrations and we thought he'd be just the man to answer our question. Hello there.

Mr. STEVE GEPHARD (Fisheries Biologist, Department of Fisheries, Connecticut): Hi. How are you?

ELLIOTT: Good. Most fish live in either saltwater or freshwater, but a few species like eel and salmon migrate back and forth. How do they do this?

Mr. GEPHARD: Well, they undergo fairly complex physiological changes that allows their body to change from being, let's say, a freshwater fish and turn into a saltwater fish. There are - the changes in the skin, changes in the kidney, changes in the gills, changes in the lining of the gut and those changes have been evolved in the species over many, many years.

ELLIOTT: Now, there are words that you scientists use to describe these things that I ran across this week - anadromous.

Mr. GEPHARD: Yes. That refers to the fish-like salmon that begin their life in freshwater, go out to saltwater to grow and mature, and then return to the freshwater of origin to spawn. But catadromous describes the American eel's lifestyle. They begin their life in saltwater. They then go into freshwater to grow and mature and then they return to saltwater to spawn. Now if those two terms aren't enough for you, we - now, you have a collective term of diadromous.

ELLIOTT: Migrating between salt and fresh water.

Mr. GEPHARD: For the purposes of reproduction.

ELLIOTT: Let's talk a little more now about the American eel. It lives a really long and rather remarkable life. Will you just take us through the eel's life cycle?

Mr. GEPHARD: It begins in the Sargasso Sea, which is a region of the middle Atlantic, where all eels spawn. And so their life begins as an egg there and transforms into a transparent leaf-like larvae called a leptocephali, and they drift with the gulf stream and other oceanic currents north and all the along the North American coast.

And as they near the coasts, they transform into a transparent eel-like fish that's going to look just like the adult fish except that's about two inches long and it's transparent. And that's known as a glass eel. They then will colonize freshwater rivers and streams and as they start feeding in freshwater they take on the pigments. They become yellowish-green. And now they're called an elver.

And these elvers, then for the next few months, will migrate upstream and colonize just about every freshwater habitat. They will live in this freshwater habitat for varying amount of years, but a lot of the females - they may stay in the freshwater for 20 years, sometimes longer. When they've reached a certain size, they then into a silvery eel and they migrate back to the sea to spawn.

ELLIOTT: So they're, at least, 20 years old before they go out to lay eggs.

Mr. GEPHARD: Certainly the females are. There are some indications that the males may migrate at younger ages.

ELLIOTT: So what happens when they get to the Sargasso Sea and spawn?

Mr. GEPHARD: Well, eels are similar to a Pacific salmon in the sense that they all die after they spawn, that much we know. But there's a lot we don't know because science has yet to actually document the spawning act in the wild. We can track them. We sort of know where and when it happens, but we've not been able to observe it.

ELLIOTT: Why is it important to maintain healthy eel populations?

Mr. GEPHARD: Historically, American eels have been one of the most abundant freshwater fishes in the eastern North America. They're very important to the ecosystem. We tend to forget all of these because we tend to put value on fish depending on whether they are fun to catch on hook and line or whether they are good to eat or good for commerce. They provide food for an awful lot of species, striped bass or marine mammals. And if they are not being eaten, well, they're eating things. And they also are of main source of transportation for a certain species of freshwater mussels.

And frankly, I think there's a lot more that they're important and we're not wise enough to recognize it yet.

ELLIOTT: Are eels really slippery?

Mr. GEPHARD: You better believe it.

(Soundbite of laughter)

Mr. GEPHARD: Yes. They are very slippery. They're - they have a very strong mucus layer in their muscular body. They are very hard to hang on to.

ELLIOTT: Steve Gephard is with the Connecticut Department of Fisheries. Thank you for talking with us.

Mr. GEPHARD: Thank you.

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