Non-native fish species are causing declines in the abundance and genetic integrity of native fish species, including highly valued game and fisheries species. Non-indigenous species often prey on native species, or compete with them for food and spawning sites. They also cause a loss of biological diversity through cross-breeding, or hybridization, between exotic and native species. Invasive fish species may be an important factor in the extinctions of many native fish species.
Alien fish are introduced deliberately or accidentally. Accidental introduction into the marine environment occurs frequently through trasfer on ships hulls or in ballast water.
Accidental and deliberate introductions of fish species have occurred in several regions. The results are sometimes unfavourable, as with the rapid spread in Australia of the European carp, Cyprinus carpio. This now threatens the environment of native and introduced sport species, mainly because it causes water turbidity which affects the productivity of aquatic plants.
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. Some of the most notorious species, such as the zebra mussel, ruffe and round goby, arrived in US waters as stowaways in the ballast tanks of ships entering US ports from overseas. Over the past 50 years the number of introductions of non-native fishes has increased dramatically as a result of the rapid expansion in travel and international shipping, as well as increased interest in 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.
US examples of systems affected by exotic fish species, include: 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 may have escaped or been released into the state's waters by aquarists, are voracious predators that could threaten a number of native species.
Some of the most drastic impacts of invasive animal species have been recorded in South African rivers, where alien fish, and to a lesser extent invertebrate and reptile species, have altered habitats and successfully outcompeted native fauna. Up to 60% of the threatened endemic freshwater fish of South Africa may be threatened by introduced fish species such as trout, carp and bass. Examples are the introduction and consequent spread of of the Nile Perch, a large fish predator, in Lake Victoria, and species of small clupeid fish deliberately introduced from Lake Tanganyika into Lake Kariba to increase the fishing potential there.