The Amazing American Eel: A journey from the Sargasso Sea

Figure 1: Life cycle of the American Eel.

Figure 1: Life cycle of the American Eel. Image courtesy of Virginia Institute of Marine Science

The American Eel (Anguilla rostrata) may be one of the most underappreciated fish in North America. They have an incredible life, go through many changes and adapt to different environments American Eels can be found from Greenland all the way down to Brazil1. That is quite a large range! They are considered catadromous species; born in saltwater but live out most of their life in freshwater (Figure 1). It’s the exact opposite of salmon which are anadromous. I thought it would be nice to learn about these fascinating fish from their perspective.

Figure 2: Leptocephalus of an eel.

Figure 2: Leptocephalus of an eel. Photo courtesy of Deep Blue

We start our journey in the Sargasso Sea, where both the American Eel and the European Eel breed2. An egg has been laid by a female eel; one of 20-30 million from her alone. It drifts in the warm salt water for a couple weeks and then hatches. Out swims a small baby eel, a Leptocephalus (Figure 2). She’s only 5 cm in length and looks like a transparent willow leaf. She begins swimming towards the coast, but at the mercy of the currents, it will take her a long time.

Figure 3 Glass Eel.

Figure 3: Glass Eel. Photo courtesy of Jordan Colosi

As she swims, she slowly begins her first transformation and upon reaching the Atlantic coast 7-12 months later, she becomes a 2-3 inch glass eel (Figure 3). At this stage she takes on a more eel-like form but is still transparent. This makes her very difficult to see in the deep blue, but this little eel is well on her way to finding a suitable freshwater home.

Figure 4: American Eel Elver.

Figure 4: American Eel Elver. Photo courtesy of Parks Canada

As she grows and moves upstream, she develops a darker brown pigment, and by April to July (a whole year after she was born) she enters the Elver stage (Figure 4). This little Elver is now a feared, 4 inch freshwater predator of insect larvae and gobbles up as many as she can find. In another 12-14 months, she has eaten enough to make it to the Yellow Eel stage (Figure 5).

Figure 5: A Yellow Eel caught during routine monitoring.

Figure 5: A Yellow Eel caught during routine monitoring. Photo courtesy of Ben Lubinski of the Illinois Natural History Survey featuring Chris Maxson holding the eel. The eel was soon returned to the water unharmed.

She will spend the bulk of her life as a Yellow Eel remaining in her river home for anywhere between 3 to 40 years. During this time, she hunts for fish, mollusks, crustaceans, insects, and anything else she can catch. As the sun rises each day, she finds a large rock, fallen log, or some good mud to hide in away from other hungry fish and fishermen.

Figure 6: What I believe is a Silver eel.

Figure 6: What I believe is a Silver eel. Photo curtesy of

Let’s fast forward 20 years. She is now a length of ~ 85 cm (almost 3 feet long) and ready to begin her journey back to the Sargasso Sea to breed. At this point, she stops eating, and her digestive tract begins to degenerate. Her eyes and fins enlarge to aid swimming, and blood vessels around her swim bladder increase in number. Up until now, she has been swimming along the bottom of the river, but now she must cope with a deep ocean. These extra blood vessels around her swim bladder will help her stay buoyant and help her swim. Her color changes too. She transforms from a yellow-brown to dark grey on top and pale underneath. She has now entered the Silver Eel stage (Figure 6). She makes the harrowing journey back to the Sargasso sea where she was born, where she herself lays her millions of eggs and completes her life cycle. In a few weeks, her legacy will go on in the many little Leptocephali that will be born and begin the journey back to freshwater1,3.

There are many mysteries that we have not solved about the American Eel and other freshwater eels. No one has ever witnessed spawning. We do not know how they migrate through the ocean or what triggers them to spawn. There is much more to discover about these fish. Overfishing and the building of dams are well known to have negative impacts on their populations by harvesting the glass eels and preventing their migrations up into rivers. Fishing regulations, dam removal, and the use of eel ladders at existing dams have all made a difference. Continued research and monitoring are key to learning more about this incredible species and preserving it for the future. I hope you have enjoyed the journey of the American Eel. Be sure to check in on February 5st to learn about some of the top invasive plants.

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