American Eel: A Hidden Migration in the Mystic

By: Michael Fager

When the DCR rebuilt the Mystic Lakes Dam they corrected a 146 year old mistake.  They installed a fish ladder.  Almost everyone who was involved with this process knew this fish ladder was being built for the upstream movement of the annual spring herring run.  But the biologists at the Division of Marine Fisheries also built a smaller ladder adjacent to the main fishway for American eels.

The American eel (Anguilla rostrata) is an almost unknown member of the Mystic River’s fauna.  American eels are a migratory fish in which the Mystic River Watershed plays a vital role in its complex life cycle.  But where the herring are anadromous (living in the ocean, but migrating to fresh water to spawn), eels are catadromous, (living and maturing in fresh water, then migrating to the ocean to spawn).  But the life cycle of the American eel is not that simple.

The American eel’s complex life history begins and ends in the Sargasso Sea, that part of the Atlantic Ocean south of Bermuda, east of the West Indies and west of the Azores. American eels, which occur in fresh water rivers from Greenland to Brazil, migrate to the Sargasso Sea to spawn.  It is unknown why eels stop at the Sargasso Sea, but scientists hypothesize that there is some unknown feature of the surface water that cues the American eels to stop migrating and begin spawning.  A unique aspect of American eel reproduction is that American eels can, and do, spawn with American eels that are from Greenland to Brazil.  This creates a single breeding population (scientifically known as a panmictic stock) for all American eels.  This is different from herring and other anadromous species, which only spawn with those herring that migrate up the same stream, thus creating separate breeding populations for each stream.

Spawning female American eels will release up to 30 million eggs.  Following spawning it is assumed that the adult eels die, although humans have never actually observed eels spawning.  The eggs hatch into larva known as leptocephali, the first of five American eel life stages. 

The leptocephali, which are clear, shaped like a willow leaf and can swim in either direction, drift out of the Sargasso Sea on ocean currents such as the Gulf Stream.  As they move north they leave the currents to enter coastal rivers such as the Mystic River.  As the leptocephali cross the continental shelf waters they undergo the first of two metamorphoses that will occur in an eel’s life.  In this first metamorphosis the compressed leaf like shape of the leptocephali changes into a clear, eel-like shape known as glass eels, the second eel life stage. 

When glass eels arrive in coastal waters they began to change color and become the darker elvers that we see in our eel ladder.  The elvers, the third eel life stage, move from the ocean to the river.  Those elvers that move up the fish ladder at the Mystic Lakes Dam end up in a box at the upstream end of the ladder.  There they are counted and placed into a bucket.  From that bucket they are gently transferred into the Upper Mystic Lake to complete their migration and begin the process of maturation.  In the period between April and August this year, MyRWA staff and volunteers counted 2,784 elvers that passed through the fish ladder at the dam.

The elvers transform into fully pigmented yellow eels, their forth life stage, which are sexually immature adults.  Eels do not differentiate sexually until they are about 7.5 to 13.5 inches in length.  Eels remain in the yellow stage until they mature sexually, which can take between three to 24 years.    Somewhere between three and 24 years of age, yellow eels undergo the second metamorphosis, and become sexually mature silver eels, the fifth and last life stage. 

When eels undergo this second metamorphosis they are maturing sexually and preparing physiologically to undergo an ocean migration and spawning.  These changes occur over the summer and fall.  When eels metamorphose to the “silver” stage, not only does their color change, their eyes and nostrils enlarge, their lateral line becomes more visible and their swim bladder changes. As the weather cools eels undergo the final stages of this metamorphosis, they stop feeding and their guts regress.  Then, in winter eels leave the Mystic River and migrate to the Sargasso Sea to spawn, a trip of about 1000 miles.

The eel migrations into the Mystic and then later out to spawn are subtle but significant events in the watershed.  The herring run is a celebration of the life of the river heralding the changing of the seasons in the watershed.  Herring in the fishway, cormorants and gulls circling at the Upper Mystic Lakes dam, great blue herons and black-crowned night-herons lining the shore, are all a part of this migratory movement.  But the elvers move in quietly and unnoticed.  The silver eels leave in the winter under the cover of cold weather.  There is no event heralding their movement.  But the Mystic River watershed plays a vital part in the lifecycle in one of the most complex and little known fish species in the natural world.


Endangered and Threatened Wildlife and Plants; 12-Month Finding on a Petition to list the American Eel as Threatened or Endangered, 72 Fed. Reg. 4967 (February 2, 2007) (Department of Interior, U.S. Fish and Wildlife Service).

U.S. Fish and Wildlife Service, 2005.  American Eel, Anguilla rostrata. Northeast Region, U.S. Fish and Wildlife Service, Hadley, MA

ASMFC, 2012.  Stock Assessment Overview: American Eel. Atlantic States Marine Fisheries Commission, Washington, D.C. 6 p.

This article was written August 31, 2012.