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1 l~ FJCECUTIVE SUMMARY rea d b The naffve fish fauna of the Colorado River basin has been g tly affects y human actions that have occurred primarily since the 1930s. Four of the seven large spades in the "big river" assemblage, which is restricted to main channel habitats are endangered and federally listed, and the remaining three are spades at risk Dama, 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 dedine of the endangered fish spades, but changes in the biological environment as a result of fish introductions now may be the most 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. Several nonnative fishes (e.g., channel catfish,. common carp, fathead minnow, and red shiner) have become established in the basin, in part due to °preadaptations" to changed habitat conditions. 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 gradual displacement of natives, espedally where the native fishes occur in low abundance in isolated habitat. to the extreme, displacement can mean local extirpation or complete extinction of one or more native spades. The threat nonnative species pose to native faunas is not restricted to the Colorado River system; it is part of a nationwide problem that is particularly acute where spades .have been introduced intentionally to satisfy recreational or commerdal demands. The possibility of hastening the dedine of native fishes as a direct result of introducing nonnative fishes is real and has been documented in other river systems. Continued decline of rare spades will ultimately lead to extinction and the irrevocable loss of biological diversity. The loss of biodiven;ity has widespread effects, because it encompasses not only the number of native spades present, but also the ecological ~ functions of those spades and the genetic material they contain. "" The problems caused by nonnatives have been investigated at length in the Upper Colorado River Basin (UCRB). There have been numerous surveys documenting the distribution and abundance of nonnatives. There is compelling evidence for the adverse impacts of nonnatives on the native fishes, and the precarious status of the native fish fauna iswell-known.. Researchers have identified control measures that may be applied to most of the common nonnatives. However, a strategic plan is needed for directing control efforts at spedfic nonnatives with selected methods in designated locations. Also needed are the combined efforts of many agendas, governments! and private, and the cooperation of private citizens who may not be viii