The problem of introduced nonnative species, including fishes, in North America is not new. It began with the arrival of the first European settlers more than 500 years ago and has continued at an increasing rate ever since. Many of the early introductions of plants and animals were intentional and generally viewed as welcome additions to the North American flora and fauna. Some of the first introductions were domesticated animals and plants, which offered considerable economic and recreational benefits. As the number of nonnative species increased, however, their economic and ecological costs became apparent. Today the non-indigenous species problem begs our attention to prevent further loss of our native fishes.
The definition of nonnative species has often been inconsistent, and such species have been called exotic, alien, transplanted, introduced, nonindigenous, and invasive. Political boundaries have sometimes been used to determine the status of an introduction. For example, some natural resource managers would not consider it an introduction to move a fish beyond its native range inside a state or national boundary (such as from northern to southern California, or from the East to West Coast). However, many biologists now define introduction as "the movement of a species beyond its natural or native range by humans."
The ability of nonnative fishes to alter native fish populations, aquatic community structure, and ecosystem structure and function is well known. Ecosystem level changes that alter nutrients, energy cycles, and productivity directly impact human society. One of the major effects of invasive nonindigenous species on biodiversity is outright loss of native species. Invasive nonindigenous species should be treated as biological pollutants that can push native biota to or past the brink of extinction, especially when they occur in the presence of other forms of pollution, such as physical habitat alteration or chemical pollution.
There is no question that the introduction of nonnative species is one of the most important issues in the conservation of aquatic natural resources today. There are now more than 80 foreign fishes established in the waters of North America. An additional 210 species of North American fishes have been moved and are established beyond their native range. The U.S. Fish and Wildlife Service currently lists more than 100 native fishes as endangered or threatened, and nonindigenous species have contributed to the decline of about half of these. Of the 30 extinct fishes in the United States, nonindigenous species were a factor in the extinction process of 24. The mechanism of the extinction varied from predation and competition for food and space resources to genetic contamination through hybridization.
The effects of nonindigenous fishes on endangered species and aquatic biodiversity will probably significantly increase during the next 25 years. The basis for this prediction is the drastic increase in introduced fishes during the past 45 years. Analysis of records of introduced fishes revealed that between 1831, the date of the first known release of nonindigenous fishes, and 1950, a period of 120 years, fewer than 117 known fishes were introduced. Between 1950 and 1995, only 45 years, more than 458 fish species were introduced, but not all became established. Introductions of nonindigenous us fishes were made in all states, but more were made in California, Florida, Hawaii, Nevada, and Texas than in the other states.
Nonindigenous fishes have originated from a variety of locations. Intentional introductions have been made by management agencies as part of game or forage fish stocking. However, many game fishes were illegally stocked by well intentioned but misguided anglers. Several species, such as Grass Carp (Ctenopharyngodon idella), were widely introduced for biological control of aquatic plants, many of which were also introduced. Aquarium fishes represent another group of nonindigenous fishes that now occur in a variety of habitats. The tropical origin of most aquarium fishes has limited their distribution to the extreme southern portion of the United States or to thermal springs in colder areas.
Native fishes throughout the desert Southwest are in serious jeopardy from nonindigenous fishes. This region, characterized by low native fish diversity and high endemism, received the most fish introductions and suffered the greatest loss of native fishes. Species such as Bony tail (Gila elegans) and Razorback Sucker (Xyratichen texanus), which inhabit large rivers in the Colorado basin, the Sonoran Topminnow (Poeciliopsis occidentalis), and several pupfishes and springfishes, inhabitants of small desert streams and springs, are directly threatened by the presence of numerous predaceous nonindigenous fishes.
