Creature Feature: Meet the Asian Swamp Eel, "the Animal Equivalent of the Kudzu Vine?"

The Asian swamp eel is a pretty ugly fish, and its impact on native aquatic species in the Everglades may be just as unpleasant. USGS photo.

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.


Not good news for the Everglades and other parts of the country, and further proof that introducing an exotic species can lead to big problems.

You're correct - as far as fish go, this one is downright ugly!

Thanks for bringing this to our attention.

I want to give a shout out to Tim Collins at FIU, who did the molecular genetic work that demonstrated the multiple introductions (of several species from different SE Asian countries), and to Bill Loftus, who recently retired from USGS but performed the long-term fish monitoring in ENP and S. Florida.

Collins T.M., Trexler J., Nico L., and T. A. Rawlings. 2002. Genetic diversity in a morphologically conservative invasive taxon: Multiple introductions of swamp eels to the southeastern United States. Conservation Biology 16: 1024-1035.

Also, we'll know a whole lot more about swamp eels if and when Duane Choquette finishes his dissertation in Collins' lab.

My point is that much of what's known about swamp eels has been nickel & dime projects, and beyond the minimum job duties for the USGS folks.

ps: Tim's also done the molecular genetics work on the burmese pythons, showing not just reproduction in the Everglades (as opposed to mere survival of released pets) but parthenogenic reproduction (no mate needed). Not bad for someone who's real research interests are snails.

Thanks for the Usefulle Info, tomp. You obviously know a lot more about swamp eels -- and the people who are studying them -- than I ever will. One of the truly frustrating aspects of writing for Traveler is that I don't have the time, energy, and expertise to dig as deeply as I should into complex subject matter like this. Sigh......

It is truly sad to see this tragedy unfold. The potential destruction this fish can cause is almost unimaginable. Here in Hawaii exotic plant and animal species have decimated the native habitat and caused scores of indigenous species to become extinct. National parks are certainly not exempt from the ravages of alien species. As this fish expands its range not only will it consume resident wildlife, it will set off secondary impacts throughout affected ecosystems that can have unforeseen consequences. Climate warming will likely help to carry this eel even further north.

would you happen to know what the life span of these animals could be?

I'm not sure about the life span of the Asian swamp eel. Several of the sources I consulted say 8 to 12 years, but that would certainly vary with local conditions.

A few years ago while catching crayfish in a creek we also caught what we thought was a salamander. We put it in our fish tank alone with some other small fish and the crayfish, it was approx 6 inches long at that time. Now it has grown to approx 2 1/2 feet long and eats everthing else in the tank. Now that it is bigger and after doing research we have discovered that it is the swamp eel. It looks identical to what you have in this video. A couple of times he got out of the tank and we found him under the couch in the living room. We live in Florida southwest of Jacksonville. Can you give me an idea of what to do with him (not really sure if it a male or female?). I'm also wondering how he got in the creek this far north of the everglades?
We don't want to put him back in the creek so any ideas would be helpful.

Well, Wanda, if this thing is indeed a swamp eel as you believe, you sure don't want to release it back into the wild. The swamp eel is a very unwelcome non-native species, and if we could wave a magic wand and get rid of it, we'd certainly waste no time doing it. (The thought that the swamp eel might eventually become established throughout the temperate U.S. should send chills up the spine of anybody who cares about the indigenous aquatic species that inhabit the invaded waters.) If you continue to provide a home for this particular specimen, you can expect it to live 8 to 12 years. BTW, "sexing" a swamp eel is tricky business. All swamp eels are born male, but since the species is a protandrous hermaphrodite, males can change into females. The older and bigger a swamp eel is, the more likely that it is a female.

[My thanks to octopd for educating me on the fact that a swamp eel is a protandrous hermaphrodite.]

That it (swamp eel) got out and hid under your couch is disturbing, these predators can live out of water for long periods of time (7 months in a wet cloth), the thing to do is kill it or find some asians that would gladly eat it, they are considered a food source in southeast asia. Trust me you won't be harming the population by killing it, but by no means should you release it in the wild that would be wrong. Another consideration would be donating it to an aquarium or university studying them?

Good luck