Distribution and Abundance of Nonnative Fishes in Streams of the Western United States

—This report presents data from one of the largest standardized stream surveys conducted in the western United States, which shows that one of every four individual fish in streams of 12 western states are nonnative. The states surveyed included Arizona, California, Colorado, Idaho, Montana, Nevada, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming. The most widely distributed and abundant nonnative fishes in the western USA were brook trout Salvelinus fontinalis, brown trout Salmo trutta, rainbow trout Oncorhynchus mykiss, common carp Cyprinus carpio, smallmouth bass Micropterus dolomieu, largemouth bass M. salmoides, green sunfish Lepomis cyanellus, fathead minnow Pimephales promelas, yellow perch Perca flavescens, yellow bullhead Ameiurus natalis, cutthroat trout O. clarkii, western mosquitofish Gambusia affinis, golden shiner Notemigonus crysoleucas, channel catfish Ictalurus punctatus, and red shiner Cyprinella lutrensis. The greatest abundance and distribution of nonnative fishes was in interior states, and the most common nonnatives were introduced for angling. Nonnative fishes were widespread in pristine to highly disturbed streams influenced by all types of land use practices. We present ranges in water temperature, flow, stream order, riparian cover, human disturbance, and other environmental conditions where the 10 most common introduced species were found. Of the total western U.S. stream length bearing fish, 50.1% contained nonnative fishes while 17.9% contained physical environment that was ranked highly or moderately disturbed by humans. Introduced fishes can adversely affect stream communities, and they are much more widespread in western U.S. streams than habitat destruction. The widespread distribution and high relative abundance of nonnative fishes and their documented negative effects suggest their management and control should elicit at least as much attention as habitat preservation in the protection of native western U.S. stream biota. Through predation, competition, and hybridization, nonnative fishes have contributed to the decline of biological diversity of U.S. stream fauna. A 1999 compilation of over 17,000 records of nonnative fishes occurrence reported 536 nonnative species in the inland waters of the USA (Fuller et al. 1999; Nico and Fuller 1999). A study of extinctions of North American fish during a 100-year period (1889–1989) indicates that 40 taxa (including 27 species, 13 subspecies, and three genera) were lost (Miller et al. 1989). Nonnative fish species were implicated in 49% of endangered species listings, second only to habitat loss (Magnusson et al. 1998; Wilcove et al. 1998). Since the mid-19th century, western U.S. streams have been the focus of fish introduction efforts. Believing western waters to be species depauperate, the U.S. Fish Commission used the transcontinental railway system to bring eastern * Corresponding author: [email protected] Received March 14, 2005; accepted May 23, 2005 Published online October 20, 2005 species west. The rate of introductions and the number of species introduced increased since the 1950s with advances in propagation, transportation of species, and the introduction of species from other continents (Fuller et al. 1999). Because eradication of nonnative species often is ineffective or impractical, most nonnative species will remain a part of ecosystems, necessitating further study of these organisms to best predict and manage their impact (Mooney and Drake 1989; Carey 1996). Few studies have examined the concepts of species introductions in a landscape larger than a watershed or pooled data from many sources to patch together a larger region for analysis (Moyle and Light 1996; Gido and Brown 1999). The Environmental Monitoring and Assessment Program (EMAP; Peck et al. 2001), a standardized survey by the U.S. Environmental Protection Agency (USEPA) of the streams of 12 western states, provided a rare opportunity to investigate the distribution and abundance of nonnative fishes and environmental factors related to their distribution across the western USA. Access to standardized 1387 NONNATIVE FISHES IN STREAMS OF THE WESTERN USA data from across the western USA allowed us to document the distribution and abundance of nonnative fish species at a regional scale to seek patterns that may not be evident at smaller scales. Specific objectives of our study were to (1) quantify the distribution of nonnative fishes across the western USA and at a statewide level; (2) identify the most common nonnative fishes of 12 western states; (3) report the range of conditions that nonnative species inhabit throughout the western USA; (4) evaluate the relationships between nonnative fish abundance and native fish abundance; and (5) examine the relationship between nonnative fish presence and relative abundance and several measures of human disturbance and land use practice. This information will help managers understand the scope of the distribution of nonnative fishes in the western USA. Such information may also allow scientists to develop workable hypotheses in studying the processes or mechanisms involved with invasions of nonnative fishes on a large scale.