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The American Eel

Introduction to the American eel

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Photo: USFW

The elver

Photo: USFW

Found from Greenland to Brazil, the American eel enguilla rostrata is firstly a creature of the ocean, using ocean currents to move from its natal waters. The American eel seems to be a flexible species well-equipped to withstand the short- and long-term cycles and fluctuations inherent in ocean dynamics.

Some American eels swim up freshwater streams to mature, others remain and mature in both estuarine and marine waters, and still others move between habitats. American eels had long been considered the only catadromous fish in North America but the discovery of these marine and estuarine population segments leads biologists to revise that description to facultative catadromy, meaning taking place under some conditions but not under others, or optional.

American eels begin their lives as eggs hatching in the Sargasso Sea, a 2-million-square-mile warm-water lens in the North Atlantic between the West Indies and the Azores. After hatching, the buoyant eel eggs float to the ocean surface and hatch into small, transparent larvae shaped like willow leaves. These larvae drift with the Gulf Stream and other currents, taking about a year to reach the Atlantic coast. By this time, the larval eels have developed fins and the shape of adult eels. In this first phase, the juveniles – called glass eels – are without pigment and still transparent. In the second phase, juvenile eels develop gray to greenish-brown pigmentation and are called elvers. Juveniles slowly develop into yellow eels, the sexually immature adults that are actually yellow-greenish to olive-brown.

American eels in fresh water find homes in a variety of stream habitats, particularly where they can hide under logs and rocks. Such female American eels can grow to 5 feet in length, and males usually reach about 3 feet. People have fished and farmed eels for thousands of years.

In their yellow phase, American eels are nocturnal, swimming and feeding at night. Carnivores that feed on insects, fish, fish eggs, crabs, worms, clams and frogs, eels also will eat dead animal matter. Eels can move equally well and forcefully forward and backward, giving them the ability to pull, twist and spin to tear apart large prey.

American eels can absorb oxygen through their skin as well as their gills, making it possible for them to travel over land, particularly in wet grass or mud, which may help them move around barriers in streams. Eels also can cover their entire bodies with a mucous layer.

Eels undergo amazing physical changes enabling them to return to ocean water. This metamorphosis is a gradual process, transforming the fish from a shallow water bottom dweller to ocean travelers. Called silver eels, they are dark, with bronze-black backs and silver undersides. Fat reserves increase to fuel the long ocean swim, for eels do not feed during their migration in the open ocean; in fact, their gut begins to degenerate. Eyes double in size and change in sensitivity toward blue, enhancing eels’ vision in deep water. The blood vessels feeding their swim bladders increase in number, allowing increased gas deposition and reduced loss of gas, both critical for buoyancy.

Silver eels migrate to the ocean and return to the Sargasso Sea, where females release between 20 and 30 million eggs and the males fertilize them. Eels have been a part of the human diet, especially in Europe and Asia. Like many other species, American eels no longer have access to much of their historical habitat because of dams and other obstructions in rivers.

An Asian parasite likely introduced in aquaculture has spread rapidly in American eels in the past few years. This worm infests the eel’s swim bladder. While it may not be a problem in shallow water, once the eels mature and begin their long return swim to the Sargasso, a non-functioning or even somewhat impaired swim bladder could doom eels to dying in the open ocean. Biologists simply do not know the extent of this parasite’s damage to the American eel population.

The U.S. Fish and Wildlife Service and the National Marine Fisheries Service reviewed the status of the American eel in 2007 and found at that time that Endangered Species Act protection for the American eel was not warranted. This review followed a series of workshops and actions initiated by a 2004 petition seeking to list the American eel.

After examining all available information about the eel population from Greenland south along the North American coast to Brazil in South America and as far inland as the Great Lakes and the Mississippi River drainage, the Service found in 2007 that declines of eel populations in some areas had not put the overall population in danger of extinction.

The Service received another petition in 2010 seeking to extend federal protection to the American eel. The Service found that this petition, from the Council for Endangered Species Act Reliability, presents substantial information that warrants the initiation of a more extensive status review of the species.

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