Biologists warn that the Asian swamp eel, a non-native species, may turn out to be the animal equivalent of the kudzu vine. Here is a fish that seems very capable of dominating the waters of the Everglades. It’s poised to do just that, too. Unfortunately, it may already be too late to stop it.
The Asian swamp eel (genus Monopterus), also known as the rice eel, rice paddy eel, or swamp eel, is a three-foot long, one-pound fish that is native to Central and South America, Africa, the Indian subcontinent, Southeast Asia, and East Asia (including Japan, Korea, and China). There are swamp eels in Australia, but it's not certain that they're native there.
Swamp eels are a common food in many parts of Asia, where they are typically sold live in markets.
The swamp eel's preferred habitat includes swamps, rice fields, muddy ponds, and canals. This fish doesn't migrate to the ocean to spawn, and it isn't closely related to American eels or snake-shaped fish native to American water bodies.
Swamp eels were introduced in Hawaii about 1900, reportedly by Chinese immigrants. Later, they were apparently imported to the mainland U.S. as an exotic food source or as aquarium pet. It is assumed that somebody with a home aquarium dumped some of the eels into an area canal, or perhaps they escaped from one of the region’s numerous fish farms or were illegally released on purpose. Whatever the cause, swamp eels were already loose in several Florida locales by 1993. Four years later they were discovered in canals near Homestead less than a mile from the eastern border of Everglades National Park. To the dismay of concerned managers, they are apparently in the Everglades to stay.
That is bad news, indeed, for the Everglades ecosystem. The swamp eel is a voracious feeder with a generalized diet that includes food critical to the survival of wading birds, fish, and many other native species of the Everglades. The results could very well be catastrophic if swamp eels were to become abundant enough.
Don’t doubt the swamp eel’s ability to invade new territory and reach “extraordinary levels of abundance” (as some scientists have put it). The more you learn about the swamp eel, the more you appreciate that it is not an ordinary fish – not by any stretch of the imagination. It is almost breathtakingly prolific, versatile, and hardy. Consider these facts about the swamp eel:
• It eats aquatic insects, crawfish, shrimp, tadpoles, frogs, small fish, worms, and other living things small enough for it to catch and ingest.
• It can live in fresh or salty water.
• It can survive cold temperatures, which means it can spread to many water bodies throughout the southern half of the U.S.
• It is an air breather, which means that it can live in areas that are seasonally dry (it builds mud burrows) and can disperse over dry land. A swamp eel reportedly lived in a wet towel for seven months without food or water.
• It breeds year round and can lay 1,000 eggs.
• It is a protogynous hermaphrodite (translation: it can change sex). Some females become males in later life. All of the largest eels are males.
• It has no natural enemies in the United States that we know of.
• Because they are air breathers, swamp eels aren't affected by ichthyocides (fish poisons that deny oxygen to the gills). Rotenone works to some degree, but it takes a disturbingly high dose to kill an adult.
Add in the fact that it may be difficult to detect its presence, at least initially. Swamp eels feed primarily at night and then hide during the day in burrows, thick aquatic vegetation, rock crevices, or other hidey-holes.
Now, if you think all of that that sounds bad, take a look at the eel in action:
Park lovers should also consider this: Levee removal and other features of the Comprehensive Everglades Restoration Plan (CERP) designed to restore natural flow patterns to Everglades National Park could speed up the eel’s spread to some areas of Everglades National Park.
Scientists who’ve studied the swamp eel admit that it might not be possible to stop this eel in the long run. It may, indeed, be the “animal equivalent of the kudzu vine.”
That is not to say that we shouldn’t try. Electroshocking, rotenone poisoning, and other measures might slow the invasion somewhat – hopefully until some sort of biological control strategy can be worked out
Scientists are conducting research that should prove helpful. To get a better handle on the status of current research and control methods I spoke with Dr. Leo G. Nico at the U.S. Geological Survey’s Florida Integrated Science Center. Dr. Nicxo, a Research Fishery Biologist/Ichthyologist with the Center’s Nonindigenous Fishes Research Program, has been conducting research on the Asian swamp eel for about ten years, found some of the first populations in the wild, and has published fairly extensively on the subject. He’s even kept swamp eels at his home to study and photograph them (as in the act of consuming fingerling bass, for example).
Like any responsible scientist (especially one working for a federal agency), Dr. Nico is careful not to extrapolate preliminary research results too far. This is an area of scientific inquiry in which questions abound and answers remain elusive. How is the swamp eel population distributed in South Florida? Was there just one initial release/escape into the wild from a single site, or have there been multiple introductions at various places? How fast is the swamp eel spreading into new areas, and with what impacts on native species? How many different species are there? What are the species’ tolerances for salinity, cold, and other parameters? The list of significant unknowns is very long, indeed.
As Dr. Nico explained it to me, there are at least three populations of swamp eels already established in the wild in South Florida. There is one in the North Miami area, and genetic evidence show that it originally came from China. There is another in the Tampa vicinity, and that one’s genetic inheritance can also be traced to China. Still another is in canals along the park border in south Miami-Dade County. That population carries DNA from eels in Southeast Asia. These are not necessarily the only three populations in South Florida – just the ones that have been confirmed to date. It’s known that there are at least three distinct species of swamp eels in the wild, and scientists suspect there may be four or more out there.
There are unconfirmed reports that swamp eels have already been detected within Everglades National Park. Though they may be present, they have not yet caused substantial ecosystem disruption.
For additional information, have a look at the Final report of CSEI Assessment Project 98-6. Stay tuned. We’ve certainly not heard the last of the Asian swamp eel story.
Postscript: South Florida is not the only place in America where the Asian swamp eel is established in the wild. Hawaii has some, and a small population of swamp eels has survived for at least ten years in an area north of Atlanta. (This latter population was from Japanese or Korean stock, which apparently accounts for their enhanced ability to survive colder winter temperatures.) There has apparently been at least one report of a swamp eel population in the Northeast, too.