Well before the sun rises in Orlando, joggers making their laps around Lake Underhill Park are joined by fishermen outfitting their kayaks on the edge of the boat ramp. Rods and lures safely stowed, the paddling anglers head past the swampy banks and cast their lines through the reeds and lily pads.
It’s a regular steamy summer morning for the locals, but on this day, there will also be strangers above and below the waters of the lake.
SUVs with government tags pull up, hauling a boat emblazoned with US Department of Interior branding. Out of them come scientists, also here to fish, but not for the bream and sunfish that are being caught and released for sport.
Their target is an invasive creature now known to lurk beneath the surface, carrying parasites, damaging waterways and threatening native species: the Asian swamp eel.
The eel, along with other invasive species, are gaining more of a foothold in the southeastern United States, causing concern among resource and wildlife managers, along with state and local agencies tasked with minimizing adverse effects of nonnative plants and animals. And increasing floods linked to climate change are thought to be washing the creatures further and further afield.
The first swamp eel found in this part of Florida was in 2023, and they’ve also been discovered as far north as New Jersey. The scientists from the US Geological Survey and other agencies are here with their own nets to see what the situation is like now, to try to pinpoint new populations and figure out how they got there.
They’re planning an eel version of a “fish slam,” when they catch as many of a single species in a day as possible to survey population growth and geographical spread.
‘Super slimy’ target
The Asian swamp eel isn’t a “true” eel, according to Dr. Daniel Slone, a USGS research ecologist who has a PhD in entomology. “Unlike our native species that may look similar, the Asian swamp eel does not have any dorsal fin and also does not have any reduced legs,” he said.
But more problematic to the scientists today is that the eels are nocturnal, good at hiding and very, very slippery.
The crew and researchers board what at first seems to be a typical johnboat, but once on the water it’s transformed to an “electroshock boat.” After unthreading some lug nuts from attachments secured to the port and starboard sides of the vessel, the crew deploys two metal arms that jut out a half dozen feet from the aft. Hanging from these are chandelier-like devices adorned with five or six electrodes dangling into the water, all powered by a booming gas generator on the boat.
Howard Jelks, a retired USGS fish biologist now captaining the boat, explains the plan.
“We’re applying electricity to the water to stun the fishes so we can collect them,” he said. “As long as you’re in the grounded boat, you’re safe, do not put anything in the water, no body parts, no metallic objects, anything like that when the generator is running. And we consider it ‘hot’ the whole time.”
When the boat goes “hot” and sends electricity into the lake, an ear-splitting trill blasts across the water. With the vessel launched, the crew does a test run to make sure everything is working and to their surprise, get a catch right away.
Biologists Mary Brown and Wesley Daniel spot an eel near the surface and scoop it up in a net. Brown holds it up for the fishermen on the banks to see.
“Wow,” one man’s voice booms across the lake. “Looks like a snake!”
With all systems checked, Brown and Daniel put their ear defenders back on as the boat heads further out. Armed with what look like long and strong pool skimmers, they lean over the bow, staring intensely at the water.
Small panfish and bream float stunned and motionless in the five feet or so of zapped lake around the boat, but the target eels do not seem as susceptible in the marshy edges that are lined with brush.
“Got one!” Daniel yells after jabbing his net into the bank. “Nope, I’m a fibber,” he concedes as his catch turns out to be nothing but plants, mud, and an old fishing lure. The net is emptied net back into the water, minus the errant fishing lure which is tossed into a bucket among other trash collected for disposal.
But it isn’t long before their tactic pays off and the second eel of the day is captured and dumped into a large, white cooler half-filled with water and some plants leaving it slithering from side to side as the hunt continues for more. Close up it does indeed look like a snake, and the thick, shiny layer of mucus that makes them so difficult to hold can also be seen.
Biologist Daniel knows plenty about this species, which can grow up to three feet long. But he can also sum them up simply as “super slimy.”
He and Brown net a third eel. And a fourth. By 11a.m., the heat and humidity are taking a toll on the crew and they take a break under the overpass of State Road Florida 408, which divides Lake Underhill in half.
“They are diving in the rocks,” Daniel said of their targets. “The water is down quite a bit, so the eels are in the small water pockets between the rocks, so even though we can see them initially, when we are hitting them with electricity, they’re going straight back down into the rocks making it very difficult to pull them out with the nets,” he explained.
Invasive frogs, eels and alligatorweed
By day’s end, the team has caught seven Asian swamp eels and documented the presence of many more that slipped through their nets. The captured eels, ranging in color and size, will go with Brown who study their diet, along with the US Army Corps of Engineers and the Florida Fish and Wildlife Conservation Commission. Other studies will be done to learn more about the environmental tolerances of the eels to determine in what conditions they can survive and thrive.
There are a lot of unknowns when it comes to non-native species, such as how they get into the country’s waterways. “Sometimes we have evidence of things like something was distributed by a boat or someone dumped it out of an aquarium,” said Ian Pfingsten, a botanist with the USGS Nonindigenous Aquatic Species Database who focuses on aquatic plants.
The arrival of swamp eels was likely the result of human release. Daniel said: “You can find them in many Asian food markets and markets that serve live animals to people (and) they are also in the bait trade, they are used for offshore fishing for marlins or swordfish.”
Along with the swamp eels, scientists in the USGS Wetland and Aquatic Research Center have been most concerned about the spread of Cuban tree frogs, which secrete noxious chemicals that are harmful to humans, and alligatorweed, which clogs waterways that can impact infrastructure.
They use publicly available databases to catalogue and track the spread of more than 1,400 non-native species across the US, including their Flood and Storm Tracker (FaST) maps, which were created to see the impact of floods on non-native populations of plants and animals.
The devastating one-two punch of Hurricanes Helene and Milton in 2024 caused opportunities for new areas to be hit, primarily along the Florida and Georgia coasts.
Well-known invasive species such as the Burmese python may have been washed to locations where they had not been present before. Pfingsten said there was also concern about the Asian swamp eel impacting native species, and the giant applesnail, which is an agricultural pest. And both are known to carry parasites that can hurt humans.
Different species pose different risks depending on the region. And it’s not just invasive animals — the team tracks invasive plant life as well.
“We have some plants that are of interest in Florida, like giant salvinia, which is very common in parts of Louisiana and Texas, and less so in Florida because there’s been such a big management effort to eradicate any known populations,” Pfingsten said. The worry, he added, was that the dense, matting weed that can reduce oxygen levels in water could be spread in the St. Johns River, a major commercial and recreational waterway.
Initial research shows flooding linked to Helene may have helped 222 non-native species spread, 90 of them considered invasive. For Milton, up to 114 non-native species could have relocated amid the floods, 56 of which are invasive, Pfingsten said. Without actually witnessing a swamp eel be swept from one lake to another, it’s impossible to say for certain that flooding allowed the spread. But this specialized work that began after Hurricane Harvey flooded Houston and surrounding areas in 2017 shows how it’s likely to happen. In the town of Wharton, Texas, after Harvey, for instance, floodwaters were so high they merged the Colorado and San Bernard rivers, effectively creating a new corridor for aquatic species to move.
“So, we’re just providing that information to pinpoint areas on the landscape where there could potentially be a new introduction,” Pfingsten said, adding different areas of the country will have different species to look at.
“Right now, the swamp eel is a high priority, one that states really care about, the Southeast really cares about,” he said, adding that other invaders — like tilapia — might be more abundant but less troublesome.
Information from the “eel slam” will be catalogued and distributed. It’s still anecdotal evidence, but the scientists are getting to paint a larger picture and test their theories.
Whether the lone swamp eel found in Orlando in 2023 was part of a population that spread through hurricane flooding may never be known. But the team has now recorded more than a dozen eels in just a few hours at Lake Underhill, which has no usual water connection to the initial lake.
“I mean, that’s the tricky part, right?” Pfingsten said. “If we know that there was a swamp eel population in a neighboring drainage and then after the storm we went back and surveyed and it’s like — oh yeah, they are there now. Now, did we see that actually happen? No, it’s still circumstantial.”