Public Release: 

Invasive fishes pose increasing threat to U.S. waters and native fishes, says USGS

US Geological Survey

Skyrocketing numbers of invasive non-native fishes in the nation's waters are increasingly threatening aquatic systems, according to three USGS biologists writing in a book recently published by the American Fisheries Society.

The book, Nonindigenous Fishes Introduced into Inland Waters of the United States, provides a wealth of current information on the status and impacts of non-native fish in America's waterways.

For example, the authors write, non-native fish species are causing declines in the abundance and genetic integrity of native fish species -- including highly valued game and fisheries species -- and are probably an important factor in the extinctions of many of our native fish species.

The spread of non-native plants and animals has become a serious problem both in the United States and worldwide. A number of resources are being mobilized to address this issue, particularly following the recent signing of a presidential executive order on invasive species.

Non-native, or nonindigenous, species, are those which have established populations outside their native range. Many non-native fish in the United States were introduced by deliberate stocking, others have spread through releases of live bait or unwanted aquarium pets, or arrived in ballast water discharged from ships.

The new book provides detailed information on more than 500 non-native fish species, including methods of introduction, ecological and economic impacts, range maps and identification aids. "It represents the state of our current knowledge of nonindigenous fishes, and fills a large void by consolidating previously scattered information," says co-author Dr. Leo Nico, a biologist with the USGS Florida Caribbean Science Center in Gainesville, Fla.

Over the past 50 years the number of introductions of non-native fishes has increased dramatically as a result of the rapid expansion of travel and international shipping, as well as the increased interest in the aquarium fishes. About 40 percent of U.S. non-native fish species come from foreign countries; the rest are species that have spread into new environments outside of their native U.S. geographic range. "Many of these transplants are just as bad or worse than those from foreign countries," says Pam Fuller, another co-author of the book and a USGS scientist in Florida.

Scientists believe that non-native species were a factor in 24 of the 30 known cases of a native fish species becoming extinct. USGS Chief Biologist Dr. Denny Fenn says that the effects of invasive fishes on endangered species and aquatic biodiversity will likely increase in the next 25 years. "They can and do impact entire ecosystems," Fenn says.

Examples of systems affected by exotic fish species, including:

  • The Great Lakes, where populations of the introduced ruffe and round goby are exploding and where introduced sea lampreys, which parasitize other fishes, have been implicated in the collapse of the lake trout fishery. Ruffe and round goby were introduced to the Great Lakes via ship ballast water, and the sea lamprey entered through the Welland Canal.

  • The Desert Southwest, where a number of small endangered fish species such as sunfishes, catfishes, and bullheads, are being wiped out by stocked largemouth bass and other species.

  • South Florida, where populations of Asian swamp eel were discovered in 1998 in several locations, including just outside the Everglades. These fish, which are sold in the aquarium trade and which may have escaped or been released into the state's waters by aquarists, are voracious predators that could threaten a number of native species.

While many state agencies still stock nonindigenous species for sport fishing, Fuller says there is a price to pay for introducing any fish into waters where they did not evolve. Non-native fish often prey on native species, or compete with them for food and spawning sites.

Another danger invasive species pose is the loss of biological diversity through cross-breeding, or hybridization, between exotic and native species. This has become a particular problem for some western trout species in areas where rainbow trout have been introduced. "We are losing the genetic integrity of many of the western cutthroat subspecies," says co-author Dr. Jim Williams, also a biologist with the USGS Florida Caribbean Science Center. "Many populations are being wiped out, and once lost, they can't be replaced."

Fuller says she hopes the publication will help educate people about the magnitude of the problem. "I think a lot of people will be astonished at the extent to which fish have been moved around," she says. Fuller notes that many familiar species that people often assume are native -- such as the brown trout -- were actually introduced to this country in the 19th century.

The authors also hope to educate anglers and aquarium hobbyists about the consequences of releasing live fish into natural waterways. "These fish sometimes carry diseases and parasites that can affect the native species, " says Fuller. "I've had fishermen tell me they never gave a thought to dumping their unused bait minnows at the end of the day. They didn't realize it could be a problem."

The book is published by the American Fisheries Society, a professional organization composed of fisheries scientists and managers dedicated to the conservation and sustainability of fisheries and aquatic ecosystems. The Society's Acting Executive Director, Dr. Robert Kendell, says the organization seeks to publish the latest research in fisheries and aquatic sciences. The book is available from AFS by calling 412-741-5700.

More information on problems associated with non-native fish can be obtained from the USGS Nonindigenous Aquatic Species web site: Users can perform queries to see which species have been introduced into their state and to get species-specific information.


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.

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