Dakota Natural Heritage
The American Eel

By Doug Backlund

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Eel. The word conjures up visions of slimy, vicious, snake-like animals that are to be avoided by people. In reality, eels are unique fish that are both fascinating and of significant economic importance to people throughout the world. Freshwater eels are one of the most heavily utilized yet least understood of fishes. Although similar in appearance, eels are not to be confused with lampreys, which have skeletons composed entirely of cartilage and highly specialized, jawless mouths. Eels may also be confused with burbot, often called eelpout, but the burbot is really a type of codfish. Most eel species live in the oceans of the world. However, the American eel, Anguila rostrata, spends most of its life in freshwater streams and rivers. The life history of this fish and its close relative, the European eel, Anguilla anguilla, is fascinating, complex and even today holds mystery.

For hundreds of years, the life cycle of these eels was completely unknown. Although at times extremely abundant along the coasts and in freshwater rivers and streams, no one knew where these fish spawned. Various hypothesis were put forth by early naturalists such as Aristotle and Pliny. Aristotle thought eels arose spontaneously from the mud of river bottoms. Pliny believed that adult eels rubbed their skin against rocks and the pieces that came off grew into young eels. For years, no one could prove either hypothesis wrong. Later European scientists were especially interested in learning about eels because of their importance as a food fish. Finally, in 1922, a Danish oceanographer, Johannes Schmidt, discovered the European eel’s spawning area in the Sargasso Sea area of the Atlantic Ocean, northeast of Cuba. It was not an easy task. Eighteen years of collecting young eels in the Atlantic Ocean finally revealed the Sargasso Sea location where the smallest eel larvae were found in greatest numbers. Both the European and the American eel are now known migrate to this area to spawn between January and March. Newly hatched eels, called leptocephalus larvae, are transparent and leaf-shaped. They are so different in appearance from the adults that they were considered a separate species of fish for many years. Drifting on the Gulf Stream and other ocean currents, the larval eels are carried back to their respective continents, feeding on plankton as they float along. The Gulf Stream carries the American eel and the European eel past the coast of North America, but only the American eels remain here. The European eels continue to drift for another two years. The young eels undergo metamorphosis during this time to more closely resemble adult eels, and are called "elvers" or "glass eels" by the time they are ready to enter freshwater rivers. The eels are unable to swim well until they have developed to this www. Now, upon reaching their destination they are able to leave the ocean and begin a freshwater existence. The European eel makes this journey in about three years, while the American eel does so in about one year.

It is possible that these species have been spawning in the Sargasso Sea area since the continents of Europe and North America were much closer together, millions of years ago. As the continents drifted apart, the two populations began to separate. One key element that now separates these species is the length of time required for the larvae to undergo the metamorphosis to glass eels. The American eel also has fewer vertebrate and is larger than the European eel. On reaching the coast, male American eels usually remain near the river mouths. Females continue on, penetrating the river systems as far inland as South Dakota, where this species has been taken in the Big Sioux, Vermillion, and Missouri River. American eels undoubtedly were once found throughout the Missouri River system before the construction of dams blocked their migration. Some are still taken below Gavin’s Point Dam, near Yankton. Eels are well known for their ability to maneuver around and over obstacles. They are capable of leaving the water and traveling overland for short distances. In Europe and Canada, eel ladders are provided on some dams to help eels in their upstream migration. How the female eel decides where to end its migration is unknown. Once the destination is reached, the eels become quite secretive and are active only at night. By day, eels burrow into the mud or hide in heavy cover. At night, they prey on small fish, crayfish, and other small animals. In northern latitudes, the eels become inactive in winter months.

Eels may remain in their freshwater habitat for as long as twenty years, but it is thought that most return to spawn in the Sargasso Sea at five to ten years of age. One captive European eel, caught in Sweden in 1863, lived to an estimated 88 years of age. Mature males remain on the small side, but females can attain a maximum length of nearly four feet and a weight of over seven pounds. A mature female can contain over 2.5 million eggs. As the time approaches for the return to the Sargasso Sea, the female eels eat tremendous amounts of food to build fat deposits for sustenance on the long journey ahead. The eels become darker and their eyes nearly double in size. As fall approaches, the eels change in other ways. The stomach and intestines shrink to make room for eggs. The eels eat less and less as they begin their migration. Sometimes the eels gather in great numbers and form balls of eels that float downriver. Once the females reach the coast, the males meet them in vast numbers. Like the females, they have been changing too. The eels return to the sea and now are living entirely off their fat deposits. Their digestive systems are almost gone at this point. Instead of floating on currents as they did in the larval www, the adult eels swim strongly in the deep ocean waters. How they are able to find their way back to the Sargasso Sea where their lives began is a complete mystery. Here they spawn and probably die, but almost nothing is known about this part of the life of the eel. Adult eels have never been seen in the Sargasso Sea, so the spawning activity must take place at considerable depth.

Commercial fisheries for eels have existed in Canada since colonial times, and commercial harvests in Europe have a long history. Today, the market for eel meat is exploding. Demand from Japan, Europe, and Asia can no longer be met and prices are rising. At the same time eel numbers are declining, especially in Europe. Fisheries are ideally managed based on management plans that rely on regular scientific assessments of fish populations. Most eel fisheries have not received the assessments needed to make recommendations for a sustainable harvest. Almost nothing is known about larval survival, adult survival, environmental influences or growth rates in the ocean, coastal, or river environments that support American eels.

An unexpected impact of eel fishing in the Delaware Bay was recently reported. Horseshoe crab are a favored bait for eel. Delaware Bay horseshoe crab are receiving heavy harvest pressure from eel fishermen for use as eel bait. This year, the annual spawning of millions of horseshoe crabs did not occur. As with most significant disturbances of natural systems, the effects are far reaching. The arrival and departure of over one million shorebirds is synchronized to take advantage of the bay’s former enormous population of horseshoe crabs. The shorebirds feast on the eggs of the spawning crabs for several days, gaining fat reserves that allow them to complete their migration. Some shorebird species are nearly dependent on this rich and easy food source to complete their migration. While the heavy harvest of horseshoe crab for eel bait may not be the single cause of this crash, it certainly is contributing.

A new threat to both the European and American eel was discovered in the 1980’s. Demand for eel meat has created a business niche for eel farms. Various species of eel larvae are now shipped around the world to supply these eel farms. The shipment of eels has spread a parasitic nematode that formerly was found only on the Japanese eel (Anguilla japonica). The nematode is rapidly spreading through the European eel and American eel populations. The nematodes are more pathogenic to these eels, since they have little resistance to them. Heavily infected eels experience hemorrhaging and rupture of the swimbladder. It is likely that heavily infected eels would be unable to make the journey to the Sargasso Sea spawning area. What the final impact of this nematode will be remains to be seen.

Although the American eel may not be of much economic or biological importance here in South Dakota, it’s presence here should serve as a reminder of our connection with the rest of the planet’s wildlife. Animals like eels, migratory birds, coastal crabs, and parasitic nematodes all live in habitats that are intertwined in complex ways. State and national boundaries mean nothing to them. Governments and people are beginning to work together to conserve the world’s wildlife, but real, workable solutions to many problems are not easy to find. Those who care about the natural world can only hope that the damage caused by a rapidly increasing human population isn’t irreversible.

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