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" , ..) 0 C) N .' h ,~ 1 io-.... ~ C) .~,\ ; .) ). .. F .,; INTRODUCTlON The role of non-native fishes is often identified, in association with habitat changes, as a major obstacle to conservation of native fish conununities. Alteration of riverine habitats by dam construction, water diversion, and bank stabilization have contributed to the establishment and spread of non-native fishes in the San Juan River Basin. Non-native fishes established in the San Juan River include species that occur primarily in lentic environments (mainly centrarchids), as well as more widely distributed species such as the small-bodied red shiner Cyprinella /utrensis and large-bodied channel catfish Jc/alurus puncta/us, and conunon carp Cyprinus carpio (Table I). The establishment of non-native fishes in lotic habitats of the Colorado River Basin of the American Southwest was widespread by the end of the nineteenth century, Coldwater sport fish, primarily salmonids. were introduced into high-elevation streams resulting in negative impacts on native trout species (Miller 1950, Minckley 1973, Behnke 1992). Warm water species introduced into lower elevation streams also impacted resident native species, with predation by large piscivores such as channel catfish, flathead catfish Pylodiclis olivaris, and largemouth bass Microp/erus salmoides severely reducing formerly widespread distributions of native fishes (Minckley and Deacon 1968, Marsh and Brooks 1989, Tyus and Nikirk \ 990). Other non-native species introduced primarily as bait and food fish for non-native sport species, such as red shiner and fathead minnow Pimephales promelas, have exerted competitive, as well as predation pressure on native species (McAda and Kaeding 1989, Rupert et aU 993, Douglas et aU 994). Finally, non-native species such as white sucker Ca/ostomus commersoni hybridize with native sucker species (Hubbs et at 1943, Miller and Rees 1999). The result of widespread intentional and accidental stocking of non-native fish species in the western United States, particularly within the Colorado River Basin, is that non-native fish species outnumber native fish species in virtually all artificiallentic habitats. While native species tend to dominate fish communities in lotic habitats that maintain natural flow regimes (Minckley and Meffe 1987, Meffe and Minckley 1987), non-native species can still replace native fishes, as is evidenced in the naturally flowing Salt River in central Arizona (Hendrickson 1993). In the San Juan River, native species numerically dominate the mainstream fish community (Ryden and Pfeifer 1993, 1994, 1995, 1996) while smaller non-native species are more abundant in secondary channels and low-velocity habitats (Buntjer et at 1993, 1994 Propst and Hobbes 1993, 1994, 1995, 1996, Gido and Propst 1994, Gido et aU 997). In the San Juan River sub-basin. at least thirty species of non-native fish have been reported (Platania 1990, Sublette et at 1990, Anderson et at 993, Brooks et at 1994). Of these, four species (red shiner, common carp, fathead minnow, and channel catfish) are comparatively common and regularly collected in the warm water reaches of the San JUaIl River downstream of Farmington, New Mexico to Lake Powell, Utah. Channel catfish is the only widely distributed piscivore (Ryden and Pfeifer 1993, 1994). However, lacustrine non-native predatory species such San Juan River Non-native Species Interactions, Final Repon, I February 2000