American Eel
Island Science

As American as … the American Eel

American Eel As we finish celebrating our country’s birth as a nation, everyone is in a patriotic mood and what could be more patriotic than, the American Eel. Okay, I am sure you can think of something more patriotic, but this is a nature column. We have an adorable baby eel swimming around in one of the field station aquariums right now and I am betting the Maria Mitchell aquarium has a couple of them also. Eels are extremely interesting creatures that have fed Nantucketers (native Wampanoag to islanders today) for thousands of years. Eels live most of their life in fresh water, swimming to salt water to spawn in the opposite direction (salt to fresh) of the better known salmon migration.  This two habitat lifestyle is called being catadromous from the greek “kata” for “down or away or off” and “dromous” from the Greek for the “act of running”. So catadromous fish run away from the coast and their freshwater homes to breed or away from their birthplace to live depending on one’s take on the matter. For comparison, river herring follow an anadromous life cycle, maturing at sea, then migrating up freshwater streams to spawn in freshwater ponds.

The American Eel Anguilla rostrata was first described in 1817 by Charles Alexandre’ Lesueur. Anguilla is Latin for eel, and rostrata is a Latin word that can mean either “breaked or curved” or “long nose”. The European eel (Anguilla Anguilla or “eel eel” – how original) was the eel that was studied most frequently by 18th and 19th century scientists. Where eels came from was a mystery for hundreds of years because fishermen never caught anything that looked like baby eels like the little guy swimming in my tank. The first person to study eels was Aristotle. “He stated that they are born of “earth worms”, which emerged from the mud with no fertilization needed — they grew from the “guts of wet soil”. For a long time, nobody could prove Aristotle wrong. In 1777, the Italian Carlo Mondini found the creature’s gonads (negating that whole virgin birth idea) and proved that eels are fish. In 1876, the young Austrian student Sigmund Freud dissected hundreds of eels in search of the male sex organs (that explains a lot). He had to concede failure in his first published research paper, and turned to other issues in frustration” (parts excerpted from

American eels (Anguilla rostrata), hatch from eggs in the Sargasso Sea in the North Atlantic and migrate to freshwater systems. First it was believed European and American eels were the same species due to their similar appearance and behavior, but research has shown that they differ in chromosome count and various molecular genetic markers, and in the number of vertebrae, Anguilla anguilla have 110 to 119 and Anguilla rostrata 103 to 110 (sounds like fun huh, counting vertebrae?). The spawning grounds for the two species are in an overlapping area of the southern Sargasso Sea, with A. rostrata spawning a bit westward of A. Anguilla. After spawning in the Sargasso Sea and moving to the west, the leptocephali (eel stage two, see more below) of the American eel exit the Gulf Stream earlier than the European eel and begin migrating into the estuaries along the east coast of North America between February and late April at an age of about one year and a length of about 60 mm. Their European cousins keep riding the Gulf Stream across to Europe where they proceed to travel up creeks and streams to live out their adult lives.

American (and European) eels go through a total of six stages in their lives.

In October, sexually mature eels swim away from freshwater and brackish harbor, marshes, and bays up and down the east coast down to the Sargasso Sea, an area of the Atlantic Ocean west of the Bahamas. In some cases, the trip is thousands of miles as they eels instinctively head for their spawning grounds.  In January, the eels spawn there, and then die. The next generation of eels begins their journey as eggs released by the females in the Sargasso Sea, floating around in buoyant bubbles with up to 4 million of their brethren.

The second stage is so strange that for over a hundred years of years (until about 1893) scientists thought it was a separate species called Leptocephalus brevirostris. These transparent, leaflike two-inch (five cm) creatures of the open ocean were named from the Greek leptocephalus meaning “thin- or flat-head”. In 1886, French zoologist Yves Delage kept leptocephali alive in a laboratory tank in Roscoff until they matured into eels, and in 1896 Italian zoologist Giovanni Battista Grassi observed the transformation of a Leptocephalus into a round glass eel in the Mediterranean Sea, and recognized the importance of salt water to the process. Despite this discovery, the name leptocephalus is still used for larval eel ( So this second stage is a flattened, very larval looking see through “leaf” with eyes and a small gut floating around the Atlantic Ocean.

The third stage is when the eel, becomes more “eel -shaped” although it is still transparent and thin and ribbon-like, but looking like an honest to god eel and called a “glass eel” at this point.  It takes almost a year before these tiny eel larvae reach the coast of the United States as they are carried by ocean currents. Many glass eels are eaten by predators before they ever reach fresh water. When glass eels reach the coast, they metamorphosize into a new body shape and they become pigmented. These rounder, darker eels are now called “elvers” and they are on average about 2.4 inches long.

As eels mature they go though at least 2 more stages of coloration. After a few months as they begin to enter coastal bays and harbors and estuaries, the elvers transform into the first adult “yellow eel” stage. For their final stage as full grown adults they will have a greenish, yellowish-brown or black/gray body with a white belly and a rounded tail.  This final mature stage is known as a “silver eel”. Adults remain in freshwater rivers and streams for the majority of their lives. Once they reach sexual maturity, they return to the Sargasso Sea to spawn and die. Eels usually live for at least five years. Some eels can reach 15-20 years old and one lived in an aquarium for 85 years (!) ( ).

American eels have a continuous fin stretching around the tail from the back to the belly. Like many animal species, the females are the larger specimen; on average the males grow to 2 feet long and females grow 3-5 feet long. Eels are nocturnal, or active at night and not active in the day. I see the most eels around the Field Station marsh while doing the midnight horseshoe crab surveys in which they’ll slither around my waders in the grass or near the shore. In the daytime they usually bury themselves in sediments or hide in rocks or within vegetation. Eel predators include larger fish and fish-eating birds such as gulls, eagles and ospreys. I have seen several ospreys take eels from the area over by the Life Saving Museum.

American eels are opportunistic eaters consuming everything from carrion (dead animals) to fish, crustaceans, bivalves, polychaete worms, aquatic insects, and things that fall In the water (but not voraciously  like piranha so don’t get images of scary B grade horror movies stuck in your head).

The color of eels depends on not only their age but sometimes their habitat. Older eels are usually dark brown or greenish, with yellowish-white bellies. Their color can change from light to dark within a few hours. This helps them blend in with their surroundings (more gray for rocky areas and greenish for heavily vegetated bottoms). The scales of these fish are embedded in their skin in an interesting somewhat haphazard opposite direction parquet floor effect, and then covered with a thick mucus coating (“slippery as an eel”). American Eels have large heads. American eel are found in fresh, brackish, and coastal waters from the southern tip of Greenland to northeastern South America. The American eel can survive even on moist ground for a few hours and has been found in landlocked lakes, traveling either over short distance in moist sloughs or underground in groundwater plumes (or often transported there by helpful humans).

This site has some amazing pictures of eels in all stages and quite a bit of great information about eels:

“As American Eels move upstream, they often have to navigate around obstacles. Eels can climb over rocks, dams, and even waterfalls. They have the ability to absorb oxygen through their skins to breathe. This allows them to survive out of water for several hours. If an eel is found doing this, it is most often on a damp, rainy night. Eels can also travel by underground waterways. This explains how eels are found in ponds that don’t have a stream leading to it.”

I was fascinated to learn of new citizen science projects called “eel ramps” which aim to restore the connection between fresh water and salt water so that eels can move freely between their homes and their spawning sites. More about a constructed eel ramp in Massachusetts can be found at

American eels are economically important in various areas along the East Coast as bait for fishing for sport fishes such as the striped bass, or as a food fish in some areas. Their recruitment stage, the glass eel, are also caught and sold for use in aquaculture in a few areas, although this is now restricted in most areas. Eels were once an abundant species in rivers, and were an important fishery for aboriginal people. The construction of hydroelectric dams, however, has blocked their migrations and locally exterminated eels in many watersheds. For example, in Canada, the vast numbers of eels in the St. Lawrence and Ottawa Rivers have dwindled. The number of juvenile eels in the Lake Ontario area decreased from 935,000 in 1985 to about 8000 in 1993 and was approaching zero levels in 2001. Rapid declines were also recorded in Virginia, as well as in New Brunswick and Prince Edward Island in Canada” (from

The following information is from Clint Andrew’s excellent and highly recommended article about historic methods of fishing for both Wampanoag and early settlers republished in the Historic Nantucket Vol. 43, Number 3. (Fall 1994) p. 70-73 ( and originally published in the Bulletin of the Massachusetts Archeological Society in 1986.

“The Beginnings: Indian Fish and Fishing Off Coastal Massachusetts” By J. Clinton Andrews

“People using primitive equipment depend on great concentrations of fish to be successful. When the surf breaks over the barrier beaches of the fresh or brackish water ponds, eels, white perch, and winter flounders swarm at the very edge of the sand, particularly at night. Eels may be picked up by hand when they are left stranded between surges of the ocean waves crossing the beach. In fall, the inner beach is where the mature eels in breeding condition, with large eyes and bronze and silver coloring, cruise back and forth waiting for the storm swells to wash over the beach. These eels were called “eeshaws” by the Indians, and are still known by that name on the islands. This is one of the principal reasons leading us to believe that many of the fishing methods and much of the fishing gear originated with the Indians rather than with the European colonists.

Eels were a staple food of the early settlers and, apparently, for the Indians as well. In winter eels hibernate in muddy areas where they can be speared through holes cut in the ice. The prongs of the island’s traditional eel spear terminate in hooks that grasp the eel when the spear is pulled up. This design differs from the European trident, which ends in sharp barbed points.

There is a spring run of eels as well. Beginning with the first warm rain after the ice has melted, eels that have wintered in the muddy bottoms of ponds and estuaries work their way to the salt water. They progress slowly and in the daytime, or when the temperature drops, they shelter in aquatic vegetation. Finding and catching eels during this migration is also very easy.”

Nantucket would not be Nantucket without Eel Point, named for the eels often caught in the shallows there. Local historian and author Fran Karttunen has written about early eeling and Madaket ditch in Yesterday’s Island Using eels as bait for stripers (bass) is a long time practice and a variety of books and web sites describe how to best hook a large bass with live eel bait.

The population of American eels has been declining in an alarming manner since the 1970s, and has been in decline since Europeans arrived centuries ago. Their multi-stage life history has made American eels particularly susceptible to both over-fishing and habitat destruction or obstruction. In the same manner as herring and salmon, eels can be easily obstructed from their travels by the construction of dams ( In situ (in the water) measurements of glass eel migration are now being observed by data stations and observatories (Long term ecological research or LEO by Rutgers) in the Atlantic Ocean near New Jersey (

Many problems face eels today including the destruction of their freshwater habitats and the continued pollution of the oceans. In many parts of the world, a swim-bladder nematode called Anguillicola crassus has parasitized eels, causing them difficulty in migration. The harvest of Sargassum from the Sargasso Sea has also been implicated in eel population degradation. Sargassum is a combination of several species of large-bodied pelagic macroalgae which arise in the Sargasso Sea and drift into other portions of the Atlantic. Floating masses of sargassum are known to provide critical habitat for a wide variety of sea life including dolphin fish (Coryphaena hippurus) (also called mahi mahi), juvenile sea turtles, and seabirds and may represent the single most important fish habitat in the blue waters off the Atlantic Coast. Last but not least, eels have very sensitive noses required for their migration and they are susceptible to low oxygen levels and confusing man-made water column chemical olfactory residual artifacts, both of which are exacerbated by dams, locks, and coastal pollution.

American eels are currently managed under the Interstate Fishery Management Plan (FMP) for American Eel, approved in 1999. Long-term FMP objectives include: encourage protection of eel spawning, nursery and growth habitats; and protect and enhance inland and coastal water quality to protect the health of the eel population and to reduce bioaccumulation of toxic substances. Additional information is contained in the ASMFC’s Source Document for Diadromous Fish. Both documents can be found on the ASMFC website at or by contacting the ASMFC Habitat Specialist at (202) 289-6400.

More info on eels can be found at

Articles by Date from 2012