The National Park Service (NPS) has been concerned with introductions of nonnative (foreign and domestic transplants) species in park areas since 1933 (Dennis 1980). Such introductions were recognized then as potential threats to maintaining areas under NPS jurisdiction as undisturbed as possible. Most activities since then to remove, reduce, or control introduced species in the National Park System have targeted terrestrial species, with only limited focus on aquatic organisms.

Shortly after Yellowstone was established as the first national park in 1872, the U.S. fish commissioner assigned an ichthyologist to assess it for native fishes and advise what nonnative fishes should be introduced for angling purposes (Jordan 1891). For many decades thereafter, NPS policy was to stock nonnative fishes in many national park units for sport fishing. The policy was challenged in the 1940s (Hubbs 1940, Hubbs and Wallis 1948, Hubbs and Lagler 1949) and later (Miller 1963) when sport fishes were recognized as a threat to native fishes in the national parks. What was unimagined then was that nonnative fishes introduced outside park boundaries would invade shared waters as new introduction pathways evolved. For example, visitors to Everglades National Park, Florida, taking time to look into water at Anhinga Trail now see more fishes from Africa, Central and South America, and Asia than native fishes.

In 1989 Courtenay reported at least 20 species of exotic (foreign) fishes known or reported to be established as reproducing populations in waters within or bordering units of the National Park System. That number did not include fishes native to the United States that had been transplanted and became established beyond their native ranges of distribution. Had U.S. transplants been included, the total number of nonnative fishes within or near the national parks would have been vastly higher. The National Park Service maintains a database of nonnative fishes in natural resource parks based on voluntary park input that presently includes 118 species of which 33 are exotics (James T. Tilmant, personal communication, 2003). The data suggest the probability that no national parks are without introduced fishes (fig. 1).

Fuller et al. (1999) reported nonnative fishes as having been introduced to all 50 states, with 536 taxa found beyond their native ranges (fig. 2). Although many failed to become established, those fishes came from all continents, including North America, except Antarctica.