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ENDANGERED SPECIES Figure 1. Northern pike, introduced into the Yampa River of Col- orado, rapidly spread downstream into critical habitat of endangered fishes, . . the decline of four endangered Colorado River fishes of the native big-river fish community (Minckley et at 1986), Colorado pike minnow (Ptychocheilus 11lCius), humpback chub (Gila cypha), bony tail (G. elegans), and razorback sucker (Xyrallchen texaiz.us},..Fish control needs and options were considered, in part, by using information obtained from a fish control workshop sponsored by the Upper Col- orado River Endangered Fish Recovery Program, Finally, we present strategies for controlling the most troublesome nonnative species in the upper Colorado River basin, and discuss some of the lessons learned from our evaluation of recovery efforts. Introduction and effects of nonnative fishes Native Colorado River fishes have been swamped by introduction of at least 100 nonnative fishes during the last century (Fuller et at 1999), Nonnative fishes adapt well to altered habitats (e,g" reservoirs and their tailwaters) in the lower Colorado River basin (i.e" below Lee Ferry Arizona; Minckley 1982), Riverine habitat in the upper Colorado River basin ha,s been less affected by habitat alteration, and thus supports most of the remaining big-river fish popula- tions (Tyus et at 1982; Carlson and Muth 1989), Main chan- nels in the upper basin that supported only 13 native fishes (Tyus et at 1982) have been invaded by over 60 fishes (Fuller et at 1999), many of which are now self-sustaining, Scientists have expressed concern for more than 50 years about potential problems of nonnative fish introductions on native Colorado River fishes, In about 1930, Dill (1944) first noted declining native fish populations coincident with increases in channel catfish (Ictalurus punctatus), largemouth bass (Micropterus salmoides) and other nonna- tive fishes, and suggested that introduced fishes were the cause, This pattern of declining native fish populations with increasing nonnative populations has become more pronounced and widespread (Miller 1961; Minckley 1982; Carlson and Muth 1989), 18 Fisheries Federal, state, and local agencies have purposefully stocked vast regions of the United States with nonnative fishes, including waters of the upper Colorado River basin, The largest drainage areas in the upper basin in- clude lands in the states of Colorado and Utah, Nonnative sportfish stocking in those states has been intense, and supports 99% and 96% of their recreational fisheries, respectively (Horak 1995), The state of Colorado also has stocked the greater diversity of fishes, and it ranks third in the number of nonnative fish introductions in the United States (106 species; Fuller et at 1999), Recently however, after years of negotiation, all agencies have agreed to cease stocking nonnative fish, with the exception of trout, into waters of the upper Colorado River basin. Although major stocking efforts in the waters of the upper Colorado River basin have been suspended, previous stocking efforts left a potent legacy, Nonnative sport fishes continue to proliferate by reproducing in river channels and invading from off-channel habitat For example, smallmouth bass (Micropterus dolomieu) and northern pike (Esox lucius; Figure 1) reproduce annually in main river channels, result- ing in a steady supply of young predators, Impoundments and other water bodies with seasonal or permanent con- nections to river channels provide crops of sunfishes, pike, catfish, and carp that subsequently invade riverine habitat In addition to intentionally introduced game fishes, one of the most troublesome groups of nonnative fishes are small, nongame fishes [e,g" red shiner (Cyprinella lutrensis) and fathead minnow (Pimephales promelas)] that were unin- tentionally introduced into the Colorado system via bait- buckets and as "hitch-hikers" accompanying stockings of other species, These small fishes are aggressive, abundant, and widely distributed, constituting over 90% of the standing crop of fishes in backwater habitat used as nurs- ery areas by the listed fishes (McAda et at 1994), As a result of all of the preceding introductions, the fauna of the mainstream river ecosystem is continually changing with ongoing "biological pollution" (McKnight 1993), Not only has the decline of the big-river fishes been associated with fish introductions, but there is a large and growing body of evidence that reveals the mechanisms (Hawkins and Nesler 1991; Minckley et at 1991; Lentsch et at 1996), [Note: Much of the information is available only in report form, Unpublished reports cited here can be obtained from the U5. Fish and Wildlife Service, Upper Colorado River Endangered Fish Recovery Program, Box 25486, Denver Federal Center, Denver, CO 80225], Preda- tion and competition from nonnative fishes are the most frequent mechanism cited, but they are not the only nega- tive interactions, There are several accounts of the piscivo- rous Colorado pikeminnow "choking" on spines of chan- nel catfish (McAda 1983; Pimental et at 1985), and introgressive hybridization with nonnatives can pose a serious threat (reviewed by Ross 1997), Competition between native and nonnative fishes is certain to occur to some degree (Taylor et at 1984), but there is little quantitative information. Red shiner and fat- head minnow are of great concern because of their Vol. 25, No, 9