Dealing With an Eel Invader in Florida

Sept. 30, 2000 -- When John Curnutt first netted a catch of Asian swamp eels and dumped the slithery, brown, tubelike creatures into a bucket, he knew Florida was facing an unusually fierce invader.

“Usually the fish we capture calm down after sitting in the bucket in warm weather,” says Curnutt, a research ecologist at the Florida biological division of the U.S. Geological Survey. “These eels just kept flopping. We finally had to pour a chemical in the bucket to kill them. They’re solid muscle.”

Savored and Feared

In China and Japan, the Asian swamp eel, also known as the rice eel, is a delicacy. Sometimes called unagi when it’s on the table, the eel is eaten pickled, with sweet sauces, grilled on a stick and broiled over rice, and other variations. Not surprisingly, the muscular fish is prized in China for its stamina-lending properties.

But in Florida, appetites for the 3-foot-long eel are rare. Indeed, the creature, introduced as an exotic species sometime in the early to mid-1990s, is doing very well near the top of the food chain.

That’s worrisome to ecologists, who fear the species could suck up food supplies of native fish and wading birds in Florida’s 1.4 million-acre Everglades National Park. No one is sure how the eel first arrived in the state, but Curnutt suggests it may have been released by someone who had tried to raise the species on a fish farm.

Although the eels have not reached the interior of the Everglades park, their numbers are dense — possibly in the tens of thousands — just around its borders, in nearly 55 miles of outlying canal systems. Populations of the eel have also been found in Hawaii, north of Miami and in southern Georgia.

Some fear the extremely mobile and hardy eel could spread even further. Leo Nico, the first USGS biologist to discover the eel in Florida, believes it could eventually penetrate large portions of the United States if left unchecked.

The problem is finding a way to stop it.

Indestructable Invader?

The eel doesn’t appear to be affected by most fish poisons. Biologists are prevented from using stronger ones, such as cyanide, since the canals around the park are linked up with a Florida aquifer.

It’s nearly impossible to hunt down the eels by hand and net since they’re mostly nocturnal and can take quick shelter in crevices. Exploding dynamite in the water, a technique that kills most fish by popping blood vessels in their air sacs, doesn’t work with eels because they have no such organ. Draining ditches is no solution, either, since the eels can survive in water and on land.

Even encouraging people to eat the animal — like authorities in Louisiana have done to reduce numbers of the swamp rat, or nutria — poses risks. The Asian swamp eel bears some resemblance to the darker American eel, whose numbers are dangerously low.

“Each creature presents its own kind of soap opera when it’s introduced, depending on its behavior and the timing of its introduction.” says Carole Goodyear, a biologist at the National Marine Fisheries Service who recently authored a survey of unwanted species in the United States.

The Asian eel’s soap opera, so to speak, is its adaptability. The Asian import is equipped with both gills and lung-like organs to breathe. It can survive in marshes and swamps, as well as in ponds, canals, roadside ditches and rice fields. If conditions become dry, the eel simply slithers into mud or grass and can live there for as long as seven months with no food.

USGS biologists have settled on a rather extreme approach for controlling the thriving fish — electric shocks.

Shocked Out of Water

In a little over a month, teams of biologists from the USGS, the Florida Game and Freshwater Fish Commission and Florida International University will set off in boats equipped with some lethal machinery. Each boat will carry an 800-volt generator and two metal arms that can be lowered into the water. When a current is run between the two prongs in the water, everything within about two feet of its reach will receive a big shock.

While most fish quiver, or swim to the bottom at such a shock, eels react more dramatically.

“It makes them crazy,” says Curnutt. “They shoot up out of the water and then we catch them with nets.”

By shocking and capturing eels from a range of locations throughout southern Florida, the team hopes to compile a thorough survey of the eel’s numbers and locations. Armed with that information, the plan is to go ahead with a targeted attack on the eels, possibly using poisons.

Why go to so much trouble over a single breed of fish?

One reason is it’s a tenacious predator. To eat its prey, which Curnutt has observed includes “almost anything” from frogs to small fish to shrimp to turtle eggs, the eel uses its mouth as a vacuum cleaner and sucks in smaller creatures. Those that it can’t suck in, it grabs with its teeth and spins quickly until they are torn in half and can be ingested in smaller chunks.

“The concern is the space they take up as predators means they’re replacing another native creature in the food chain,” says Paul Shafford, head of non-native fish research at the Florida Fish and Wildlife Conservation Commission. “They change the energy flow of the ecosystem.”

Uncertain Effect

Still, Shafford is quick to point out that adding an animal to the food chain doesn’t always have a dire impact on an ecosystem.

“We don’t know yet is if the eel’s entry into the aquatic ecosystem is detrimental, innocuous or even valuable,” he says.

Shafford notes that at least one exotic fish species in Florida’s waters has proven helpful. The imported butterfly peacock bass has preyed upon and reduced populations of another unwanted exotic fish by 25 percent. The exotic bass has also proven a favorite of anglers who provide Florida with a multimillion-dollar fishing industry.

But Curnott argues the Asian eel may be an exceptional threat.

Florida’s warm, wet climate is known for accommodating outside species — 28 fish from Central America, Africa and Asia now thrive in its waters. Curnutt notes almost none has managed to reach the deep interior of the Everglades park. Most fish, he explains, get stuck in marsh land and expire before reaching the interior.

That’s where the Asian eel has a clear advantage.

“This eel can burrow in the mud and wait for months and then get going again,” he says. “As an adaptable predator, it has the potential to colonize the whole system.”

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