The eastern and western mosquitofishes are small, harmless-looking, guppylike fishes native to rivers, ponds, lakes, and springs of the United States from New Jersey south through the Atlantic Coastal Plain, up the Mississippi River to Illinois, and west to Texas. These fishes were purposely introduced throughout the world in the mistaken belief that they control mosquitoes better than native, larvae-eating fishes (Courtenay and Meffe 1989).

Almost everywhere, introduced mosquitofishes have harmed aquatic ecosystems and faunas because of their highly predaceous habits, and they have reduced or eliminated populations of at least 20 other species of fish, including largemouth bass (Schoenherr 1981) and numerous invertebrate species. They have been especially devastating in the American Southwest, where they interact with a wide range of endangered and threatened native fishes, most notably the Gila topminnow. The topminnow was formerly the most abundant fish in the southern half of Arizona but is now endangered, with only 13 remaining natural populations (Meffe et al. 1983). One of the major reasons for the decline of the Gila topminnow is predation by nonindigenous mosquitofishes, which prey on juvenile topminnows, harass adults (Meffe 1985), and swarm through whatever stabilized habitat they colonize. Mosquitofishes also reduce or eliminate endangered pupfishes and springfishes in Arizona, California, and Nevada (Minckley et al. 1991). Mosquitofishes are difficult to eliminate once established (Meffe 1983; Marsh and Minckley 1990), and the best way to reduce their effects is to control their further spread. One of the main avenues of spread is continued, intentional release by mosquito-control agencies.