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The Incredible American Eel

 

by David Smithman, Hourly Fisheries Technician
Bureau of Marine Fisheries
December 10, 2015

Slimy, slippery, and snake-like -- the American eel has a life history that may be more complex than any other fish species on the planet. With keen migratory instincts and incredible adaptability, these elusive creatures have survived multiple ice ages as well as a number of substantial historical changes in the ocean environment.

Life Cycle

American eels are a vital part of the marine food web and have a multifaceted life cycle that makes them a hot commodity for the commercial fishing industry. You may have heard of one or more of the following types of eels: glass eels, green eels, yellow eels, or silver eels. These are not completely different species. In fact, all of these are names for the American eel (Anguilla rostrata), depending on which life stage an individual is in.

The life of the American eel starts and ends in an area of warm water in the Atlantic Ocean known as the Sargasso Sea. This area spans more than 1 million square miles and is bounded by a large system of rotating ocean currents known as a gyre. (Did you know that the Sargasso Sea is the only sea on earth with no coastline?) This is where the American eels lay and fertilize eggs that eventually hatch into small leaf-like larvae known as leptocephali. These are free-floating individuals that are swirled around by the Gulf Stream and other surface currents for about a year until they finally reach the Atlantic coast.

By that time, they have matured into glass eels, and are around 2-3 inches long with cylindrical bodies and fins. With no time to waste, they will attempt to move inland into brackish waters while developing a brown or grey pigment. At approximately 4 inches in length, they are identified as elvers (or green eels) and may travel hundreds of miles inland through seemingly impassable obstructions until they reach an ideal habitat.

American Eel life cycle
Click to enlarge

Some eels may move into rivers, lakes, and ponds, while others may remain in marine estuarine waters. (Did you know that eels can absorb oxygen through their skin which enables them to leave the water and travel through wet grass or mud? This is a technique frequently utilized by elvers looking to find a suitable habitat.)

After reaching a suitable location they remain there for up to 20 years! During this time they are identified as yellow eels because of their yellowish to olive green pigment. They spend the majority of their life span in this stage while active nocturnally and feeding on small fish, insects, and some mollusks. It is at this life stage that eels are commonly used as fishing bait that you may be familiar with. (See Baitfish: Profiles of the Top Five for information.)

Depending on water chemistry and other environmental factors, the eels eventually mature sexually to become silver eels and prepare to make the long trip back to the Sargasso Sea. This final metamorphosis is perhaps the most amazing. They undergo a series of physical changes including the growth of long fins, the degeneration of their gut, the doubling of the size of their eyes, and enhancement of their swim bladder. These changes are absolutely necessary in order survive the grueling return migration that can be anywhere from hundreds to thousands of miles long and can take months to complete.

When they finally reach this area, males may be up to three feet long while females can grow close to five feet in length. And it is here, in the Sargasso Sea, where the American eel is able to spawn, thus completing the final stage of its life cycle. (Did you know that the American eel is North America's only facultative catadromous organism? This means that it is born in the ocean, returns to fresh water to mature, and eventually migrates back to the ocean to spawn. Contrary to traditional belief, these individuals are also capable of remaining in saltwater estuaries to mature. This is why they are referred to as "facultative".)
Comparing eels
Eel comparison
Click to enlarge
Net of glass eels
Net of glass eels
Click to enlarge
Measuring glass eel
Measuring glass eel
Click to enlarge
Yellow eel
Yellow eel
Click to enlarge
Unsung Heroes

In recent years, it has been proven that our slimy friends play a much larger role in the food web than once believed. A recent study conducted in the Susquehanna River Basin and Delaware River found that freshwater river mussels use the American eel as a host to transport their larvae upstream where they are able to establish themselves. These mussels naturally filter millions of gallons of water per day and also serve as a food source for many other critters.

The American eel's role does not end there. In freshwater streams, they are a top predator and provide a check on the populations of all other local organisms. Eels in turn become prey themselves to larger fish, aquatic mammals, birds, and turtles. They can also be used as a water quality and pollution indicator because they usually remain in the same habitat and are very long lived.

Something to think about…

The very same eel that anglers place on their hooks in hopes of luring in a trophy striped bass has traveled over 1,000 miles from the middle of the Atlantic Ocean just to get to that point. Other eels reach their demise as a dinner delicacy after fetching hundreds of dollars on the world's sushi market.

Unfortunately, habitat destruction via damns and hydropower facilities in rivers along the east coast has also led to local population declines in recent years. A petition was filed in 2010 that listed the American eel as threatened under the Endangered Species Act (ESA). On October 7, 2015, the U.S. Fish and Wildlife Service announced that the American eel population is stable and does not need protection under the ESA.

To protect the species' long-term stability, it was recommended that states continue efforts to maintain healthy habitats, monitor harvest, and improve river passage for migrating eels. Through several addenda to the Interstate Fishery Management Plan for American eel, many states including New Jersey have taken steps to protect the survival of this species. Ultimately, whether you are a sushi connoisseur or a master angler, these incredible creatures certainly demand a level of respect for their multi-faceted life cycle and undervalued contribution to the ecosystem.

References

American Eel Profile (pdf, US FWS)
Overview of American Eel Stock Status (pdf, ASMFC)
Journey with Nature - The Migratory Journey of the American Eel (The Nature Conservancy)
Eel remains a mystery, except in its importance to ecosystem (Baltimore Sun)

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Last Updated: December 10, 2015