Monday, December 10, 2012

Exotic, exciting eels!

This week’s endangered species isn’t listed under the ESA yet, but many experts believe the U.S. Fish and Wildlife Service should consider listing the American Eel (Anguilla rostrata) (photo copyright Garold W. Sneegas)  under the ESA.  Found from Greenland to Brazil, these fish have a fascinating life cycle.  They’re catadromous, which means they do things opposite of the anadromous salmon.  They spawn in the ocean and live most of their lives in estuarine or freshwater.  Here’s how it works (USFWS 2009):

  • American eels are hatched from floating eggs in the Sargasso Sea in the Atlantic Ocean.
  • The small, transparent larvae that are shaped like willow leaves float toward the Atlantic coast for about 1 year (photo from Northeast Eel and Elver Company,
  • By the time they reach the coast, the young fish have developed fins and look like adult eels, except that they’re still transparent.  This stage is called “glass eels”.
  • During the second phase of life, the fish are gray to grayish brown and are called “elvers”.
  • Juveniles mature to the “yellow” phase, during which the sexually immature fish are yellow-green to olive brown in color.
  • Eels can take 3 to 40 years to mature (it’s a faster process in estuarine and marine waters), before they head back out to sea to breed.
  • Silver eels, the sexually mature migrants, change from bottom dwelling fish to open ocean swimmers.  They build up fat reserves for the long migration, because they don’t eat on their way to the Sargasso Sea.  Their eyes double in size in order to be able to sea in the ocean!
  • Once in the Sargasso Sea, the females lay 20 to 30 million eggs to be fertilized by the males! 


While the eels are living upstream, they hide under rocks and feed at night eating insects, fish, fish eggs, crabs, worms, clams and frogs, as well as dead animal matter.  And there are even more fascinating facts about eels.  The USFWS (2009) reports, “with their relatively weak jaws and many small teeth, eels have developed an unusual feeding process with food that cannot be consumed whole or readily broken into pieces by jerking or pulling.  Holding on with their mouths, adult eels spin their bodies to break apart food, and have been recorded at six to fourteen spins per second.  In comparison, Olympic ice skaters can spin five times per second.”  Additionally, eels can breath through their skin as well as their gills, making short transits on land possible!

So what’s threatening the eel population? 

  • Overfishing.  In some European countries (where eels are eaten regularly), populations are down by 99% of their historical size.
  • Dams and other blockages.  Eels have trouble getting to their upstream habitat because of man-made structures.  They are also often killed in the turbines of hydropower plants.
  • Parasites.  An Asian parasite (introduced via aquaculture) that affects the eel swim bladder has infected the population.

Here’s one project designed to help with eel conservation:


Want to learn more?  Be sure to check out this in-depth and informative report from the Gulf of Maine Council on the Marine Environment.



Blankenship, K.  2012.  “Large number of eels caught at Conowingo give biologists hope.”  Chesapeake Bay Journal, December 2012.  Accessed online 12/9/12.

U.S. Fish and Wildlife Service Newsroom.  2009.  “The American Eel.” October 20, 2009.   Accessed online 12/9/12.

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