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The Migratory Journey of the American Eel


The Migration of the American Eel

View the migration route of the American Eel.

The American Eel

Scientific name: Anguilla rostrata
Length: averages 24 - 40 inches
Mass: up to 30 pounds
Coloring: varies during life cycle; transparent to olive green to yellowish brown to a silvery gray
Feeding Habits: differentiates while maturing; plankton, dead fish while young & small crustaceans, frogs and fish when in freshwaters
Predators: large aquatic mammals, fish-eating birds and large fish like bass
Defenses: body is covered in a slimy substance in hopes to deter predators, and fishermen, from holding on
Habitat: catadromous species; lives life in freshwater, but spawns in salt water
Importance to Society: elvers & adults are considered delicacies in several countries; over-harvesting is a major concern in this market
Conservation status: unprotected but carefully monitored due to decreases in populations worldwide.

The American eel is a species that has intrigued even the greatest thinkers of our time. While adult eels were abundant in numbers along coasts and in freshwater waterways, no one could find the fish in juvenile form nor did they know where the eels spawned. In 4th century BC, Aristotle recorded the first observation on the life of an eel. He believed that eels were created spontaneously from the mud of river bottoms. Pliny the Elder – an ancient natural philosopher – thought that young eels grew from skin that adult eels had rubbed off against rocks. While both theories seem entirely unbelievable, no one could prove them false until Johannes Schmidt found the eel’s spawning grounds in the Sargasso Sea in 1922.

Since Schmidt’s discovery more information has been discovered about the American eel, but the snakelike fish still eludes scientists in many ways. Although where the eels spawn is well-established, no adult eel has even been found in that area. Spawning behavior and the fate of the adult eels are also a mystery. However, what we do know is that the American eel is an amazing species with keen migratory instincts and physical transformations that help it along its way.  

Transformations with each Destination: The Migration of the American Eel  

The American eel’s migratory instincts are beyond incredible. Beginning its life in the Atlantic Ocean’s Sargasso Sea, the catadromous fish must find its way to freshwater rivers, lakes and estuaries. The range of habitat is expansive; from the southern tip of Greenland, along the Atlantic coast of North America, throughout the Gulf of Mexico and the Caribbean and inland for the St. Lawrence Seaway in New York to the Great Lakes of the Midwest, the eel makes its way there and all the way back to its salty spawning grounds. 

With each geographical change the American eel makes comes a physical transformation. The larvae, called leptocephalus, are transparent, leaf-shaped and appear nothing like an adult eel. For about a year the leptocephalus drifts along with the Gulf Stream before entering coastal waters. Prior to entering the coast, the eel will make its first transformation into a more eel-like shape, though still transparent, and are called glass eels. As it makes its way into estuaries, the glass eels become longer and pigmented - taking on a brown or gray color - and become elvers. 

Elvers will overcome seemingly impassable obstacles to get where they want to go, even if it means leaving the water to travel on land for a short distance, swimming through rapids or up dams. It can take elvers years to reach its destination, traveling for hundreds of miles to do so. When it completes its journey, the elver will make another transformation into a longer, more yellowish creature known as the yellow eel. The yellow eel is evasive and nocturnal, preying on aquatic invertebrates, amphibians and fish. 

During the yellow eel stage, the animal will recognizably become a male or female. Usually females are found in freshwater habitats while males are found along the coastal areas. Females will return towards the males once they have sexually matured – which can take as long as twenty years. This takes place during the final metamorphosis when the yellow eels become silver eels. Silver eels are longer, thicker and take on a metallic black-bronze sheen with a pure white belly. Just prior to their return migration, the eel stops feeding and its eyes and pectoral fins enlarge. Silver eels swim towards their salty grounds during their winter months in order for a January spawning in the Sargasso Sea’s warm waters. It has been determined that American eels die after reproducing, but like the many aspects of the eel’s life, this is not for certain.

On the Decline

Despite the fact that the total population of American eel’s exceeds over a million worldwide, their numbers appear to be on the decline. In December 2004, the U.S. Fish and Wildlife Services received a petition to consider protecting the American eel under the Endangered Species Act. Many believed that overall population has waned due to barriers to migration routes, habitat loss, hydropower plant mortalities, pollution and overfishing. After a few years of research and debate, the FWS concluded that there was not enough proof indicating that American eels where threatened or endangered of becoming extinct in the near future. According to fishery biologist Heather Bell, fluctuations in American eel populations are common, and the recent decline in numbers don’t indicate an irreversible trend. In January of 2007, the U.S. Fish and Wildlife Services determined that placing the American eel under the protection of the Endangered Species act was not warranted at this time.

Although federal protection has been deemed unnecessary, many states work to protect the American eel in their waters. Harvest limitations have been implemented as well as projects to remove man-made dams interrupting historic eel migrations routes and building eel ladders at hydropower plants.  While the American eel is biologically and economically important to the states off the Atlantic coast, it’s presence in Indiana is a reminder of how amazing the biodiversity of our planet truly is and how each creature should have the opportunity to thrive. Historically, eels were found in Lake Michigan, Lake Erie, Kankakee River and the Ohio River. 

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