Growing with the flow

American eels live many lives as they grow and mature on their migratory journeys.

As the only native freshwater species of eel in this country, the American eel is popular for commercial fisheries and scientific research alike. Though they might not appear special to the untrained eye, there is a lot to learn about these fish!

An American eel in its yellow eel life phase. Chesapeake Bay Program/ Flickr Creative Commons.

Mystery fish

For most of recorded time, it’s been a mystery how eels, a species with no external reproductive features, spawn and reproduce. Where do they all come from?

Sorry to burst your bubble, but American eels do not appear out of thin air. It took a long while to prove that theory wrong, however. Sigmund Freud even tried his hand at explaining how they reproduce.

Until eels reach sexual maturity at around 20 years old, it’s hard to tell male from female without performing an intricate dissection. Even then, you can’t be totally sure, because eels with internal female reproductive features can develop into males when they reach sexual maturity (unless they’re dissected!). We’ll learn more about these sexual differences and how they affect eel behavior later on!

Circle of life

Eels transform through various life stages on their way to adulthood and complete their metamorphosis while migrating hundreds of miles.

Mature, adult American eels can grow quite large during their lifetime. Steve Smith/ USFWS.

Mature eels spawn in the Sargasso Sea, in the Atlantic Ocean. Their eggs hatch into tiny larval creatures called leptocephali (Latin for “flat head”) and float back towards the coast, growing into “glass eels” along the way. Even with Gulf Stream currents, this part of their migration takes years.

On the left is an American eel in the glass eel phase of its life, and on the right are American eels in the elver stage where they are slightly larger and more pigmented. Left to right: Canopic/Flickr Creative Commons and Maryland Fishery Resources Office/USFWS.

As they journey into coastal waters, they become elvers. Elvers are excellent climbers, and eels complete most of their upstream migration during this phase. Swimming against the current and climbing over obstacles becomes more difficult as they grow larger. They continue to mature as they travel upstream, growing and changing color to become “yellow eels.”

When sexually mature and at their largest, they earn the name silver eels. Silver eels typically reside far upstream, and their range spans almost the entire eastern half of the country’s available freshwater. In New York, some silver eels make it as far north as Lake Ontario by traveling through the St. Lawrence River.

Although we said distinguishing between male and female eels is not an easy feat, there is a trick that can help you guess which is which. Female eels tend to be significantly larger than males when fully matured, since they require more body mass and energy as they develop eggs to spawn.

Due to this size distinction, it is assumed that most large, mature eels found very far upstream are likely all female! Smaller male eels tend not to go as far upstream on their migratory journeys.

On the left is an American eel in the yellow eel phase. On the right, you can see a biologist holding a large silver eel. Chesapeake Bay Program/Flickr Creative Commons and Josh Newhard/ USFWS.

Their entire journey inland can take 10–25 years, but they’re not done yet. Mature eels then complete the same trip in reverse, swimming hundreds of miles back to their ancestral breeding grounds in the Sargasso Sea to take their turn at spawning. This is a crucial last leg of their lifetime-long migration.

Until 2015, this part of their journey was the biggest mystery of all, as nobody had witnessed American eels arriving or spawning in the Sargasso Sea.

“You would think that with all the advanced tracking technology we have today, we would know absolutely everything about this species, including spawning locations,” said Service biologist, Arianna Ramirez, who works on aquatic connectivity projects in the State of New York. “But, prior to 2015, the location was only inferred from leptocephali collected in the southern Sargasso Sea over 100 years ago.”

By inserting transmitters in eels and tracking them with satellites, researchers were finally able to map and confirm their migratory route and spawning location. We’re still learning so much about this fascinating fish!

The migration route of a single eel that was captured from the St. Lawrence Estuary (red star) and released downstream (magenta triangle), after being tagged with a satellite transmitter. The black dots show the average daily location for this individual as it made its way all 2,400km (or roughly 1450 miles) to the Sargasso Sea to spawn. (Béguer-pon et al., 2015).

Research on their migratory patterns, spawning behaviors, and populations helps inform fisheries management as well as decisions about development and restoration near rivers and streams. You can learn even more about their migratory journey and some of the roadblocks they face, here.

Colleen Andrews is the Outreach Coordinator with the New York and Long Island Field Offices.