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(;~~' '1 uC'tJ 002.yCo\ EXECUTIVE SUMMARY The native fish fauna of the Colorado River basin has been greatly affected by human actions that have occurred primarily since the 1930s. Four of the seven large species in the "big river" assemblage are endangered and federally listed, and the remaining three are species at risk. Dams, diversions, and extensive flow regulation have produced significant changes in the physical environment including habitat loss and fragmentation. These changes were undoubtedly a major factor in the decline of the endangered fish species, but changes in the biological environment now may be an equally significant threat. Many nonnative fishes have been introduced into the Colorado River basin in this century. Nonnative fishes were introduced by various federal and state agencies, as well as by private individuals. Successful establishment of several nonnative fishes (e.g., channel catfish, common carp, fathead minnow, and red shiner) has presumably been facilitated by their "preadaptations' to the changed conditions now found in the Colorado River system. However well-intentioned the introductions may have been, the effects have been almost exclusively detrimental to the native fauna. A successful introduction results not only in the addition of nonnative individuals to the receiving waters, but almost always in displacement of natives, especially where the native fishes occur in low abundance in isolated habitat. In the extreme, displacement can mean local extirpation or complete extinction of one or more native species. The threat to natural systems posed by non natives is by no means restricted to the Colorado River system; it is part of a nationwide problem that is particularly acute where species have been introduced intentionally to satisfy recreational or commercial demands. The possibility of hastening the decline of native fishes as a direct result of these fish introductions is real, and displacement has been documented in other river systems, Continued decline of rare species will ultimately lead to extinction and the irrevocable loss of biological diversity. This loss has widespread effects, because it encompasses not only the number of native species present, but also the ecological functions of those species and the genetic material they contain. The scope of problems with nonnatives has been investigated at length in the Upper Colorado River Basin (UCRB). There have been numerous surveys documenting the distribution and abundance of nonnatives. Direct and indirect evidence for the adverse impacts of non natives on the native fishes and the precarious status of the native fauna is well-known. Researchers have identified control measures that may be applied to most of the common nonnatives. However, a strategic plan is needed to target specific nonnatives for control by selected methods in designated locations. Also needed are combined efforts of many agencies, governmental and private, and the cooperation of private citizens who may not be aware that nonnative iv