USGS Scientists Find New Invasive Fish Species In Florida

June 25, 1998

The Asian swamp eel, a non-native fish, has been found in canals, ditches, streams and ponds near Tampa and Miami, Fla. The species is spreading and has the capability of invading and harming freshwater ecosystems throughout the Southeast, including the already-besieged Everglades system, according to the U.S. Geological Survey scientists who found the species in Florida.

The exotic creature is a highly adaptable predator, able to breathe air and to live easily in even a few inches of water, especially in warm climates.

"This species exhibits unusual behavior, appearance and adaptations," said Dr. Leo Nico, a biologist with the USGS Florida Caribbean Science Center in Gainesville, Fla. "It has the potential to spread into freshwater ecosystems throughout the Southeast where it could compete with or prey upon native fishes. Imagine a creature with all the attributes necessary to successfully invade and colonize the Everglades and other southeastern wetlands. Well, the swamp eel may be that creature."

The lakes, streams, canals and swamps of Florida and the Southeast are ideal habitats for these eels, said Nico, who discovered the species while conducting scientific samples of fish species in a Tampa Bay drainage. Scientists say they suspect the swamp eel may have escaped from a tropical fish farm or have been a pet released from an aquarium. The species, they believe, is already firmly established in Florida.

Although few non-native fishes invade natural wetlands -- instead being primarily found in disturbed habitats such as canals and drainage ditches -- Nico said the swamp eel's biology makes it well suited for all kinds of habitats. "We expect this foreign fish to rapidly occupy natural wetland habitats," said Nico. "One major concern is for the Everglades ecosystem, not only Everglades National Park and Big Cypress National Preserve, but the surrounding wetlands as well. At this point, the best outlook would be if the eel becomes a favorite food item of native predators such as alligators or water snakes."

Swamp eels -- or rice eels as they are sometimes called -- were first discovered in Florida waters in 1997 in two widely separated sites. In late summer 1997, USGS researchers discovered a population of swamp eels while sampling fishes in ditches, canals and streams flowing into Tampa Bay on Florida's Gulf Coast. At about the same time, students from Florida International University in Miami netted several small swamp eels while collecting aquatic plants from an artificial lake just north of Miami. By now, several dozen eels have been found in the Tampa Bay area and several hundred in Miami waterways.

To determine the size and extent of the swamp eel population in Florida, USGS researchers, working with investigators from Florida International University and biologists from the Florida Game and Freshwater Fish Commission, are actively searching waters surrounding the two sites where the eels were previously found.

In May, USGS biologists found swamp eels in samples they took throughout a major east-west canal near the Dade-Broward County line, including a site near the canal's border with Everglades marsh habitat. Biologists have not found the Asian swamp eel in interior wetlands of the Everglades or other natural wetland systems, but the interconnectedness of the waterways and the eel's biology pose substantial risks of the species becoming established there, Nico said.

In North America, the species is sometimes kept as an aquarium fish, although scientists can only speculate that the species may have escaped or been released into the state's waters. In 1995, swamp eels were found in several ponds at the Chattahoochee Nature Center north of Atlanta, Ga., but USGS biologists are unaware of any possible links between the Georgia and Florida populations. In Georgia, scientists suspect the swamp eel may have spread to other parts of the Chattahoochee River system. To determine if this is true, University of Georgia scientists, in coordination with the National Park Service, are sampling the Chattahoochee system. In Georgia, entire groups of fish have disappeared from one impoundment populated by the eels, making Florida scientists especially aware of the potential effect of this species on the state's native fish communities.

Of particular concern to scientists and resource managers is the ability of swamp eels to thrive in a wide variety of natural habitats and in adverse conditions. In addition to marsh and swamp habitats, Nico said the fish survives quite well in ponds, canals, roadside ditches and rice fields -- "just about any freshwater habitat with a few inches of water."

Another trait that could help these fish successfully colonize southeastern waterways is that swamp eels are air breathers, enabling them to survive long dry spells. In fact, said Nico, their use of air is so efficient that the eels can readily migrate short distances across land from one water body to another.

Swamp eels, which reach lengths of three feet or more, are predators, feeding on animals such as worms, insects, shrimp, crayfish, other fishes and frogs. Yet, said Nico, the eels are also able to survive weeks -- and possibly months -- without food. The eels are highly secretive, with most of their activities occurring at night. In the day, the fish hide in thick aquatic vegetation or in small burrows and crevices along the water's edge. In many populations, all young are hatched as females. Then, after spending part of their life as females, the eels transform into large males.

Swamp eels belong to the family Synbranchidae, a group of fishes found in fresh and brackish waters in Central and South America, Africa, and from India east to Australia. These fish are not true eels, in part because they do not migrate to the ocean to spawn. The species introduced to Florida has been tentatively identified as Monopterus albus, a species native to tropical, subtropical and somewhat temperate climates in Eastern Asia. In Asia, the eel is a popular food fish.
As the nation's largest water, earth and biological science and civilian mapping agency, the USGS works in cooperation with more than 2000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, to contribute to the conservation and the sound economic and physical development of the nation's natural resources, and to enhance the quality of life by monitoring water, biological, energy, and mineral resources.

US Geological Survey

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