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,. aggressive nature. Both fishes compete for space with, and are more aggressive than young of native cyprinids, in- cluding Colorado pikeminnow (Karp and Tyus 1990). Diets of the red shiner and the fathead minnow overlap with young Colorado pikeminnow (Muth and Snyder 1995). Evidence for nonnative fish predation on native Col- orado River fishes includes visual observations and stom- ach content analyses, We have compiled a list of such pre- dation (Table 1), which should leave no doubt that nonnative predation is a potentially powerful force, Especially affect- ed is the razorback sucker, in which basinwide recruitment failure has been linked to predation by nonnative fishes (reviewed by Minckley et aL 1991; USFWS 1998b). Predation on larval Colorado River fishes by some of the smaller introduced cyprinids has been suspected for some time, and several species have been recently impli- cated, The small, but highly territorial fathead minnow, has been shown to tear apart and consume sucker larvae even in the presence of other suitab~e pr~y (Dunsmoor 1996). Red shiner, an opportunistic feeder ef larval fishes, also is suspected of causing declines of several native western fishes (Rinne 1991; Ruppert et aL 1993; USFWS 1995). Less is known about other small nonnative fishes, including redside shiner (BaIteatlls richardsonius) and sand shiner (Notropis stram/Ilells), but both pose threats to Col- orado River fishe~ ~hrough predation and competition (reviewed by Tyus'andSaunders 1996), Evidence for deleterious effects of nonnatives on Col- orado River fishes is so compelling that 81 % of regional fisheries experts are convinced that nonnative fishes have caused significant problems in the upper basin (Hawkins and Nesler 1991). The nonnative fish issue has been stud- ied thoroughly and the conclusions are inescapable: intro- duced species have played, and continue to play, a signifi- cant role in the decline of the native big river fishes, Developing solutions No effective program exists for the control of nonnative fish populations in the Colorado River, However, recovery of the listed big-river fishes will require developing long- term programs for removing or reducing at least some nonnative fish populations, The Upper Colorado River Endangered Fish Recovery Program has been concerned with the nonnative fish problem since its 1987 inception (Wydoski and Hamill 1991), This program, which present- ly coordinates recovery efforts for the four endangered big-river fishes in the upper Colorado River above Lake Powell (Figure 2), has funded efforts to define the problem and to develop solutions (e,g" Lentsch et aL 1996), In November 1995, the program contracted us to convene a workshop of scientists and managers familiar with prob- lems in the upper Colorado River basin, and to prepare a report and strategic plan for nonnative fish control (i.e" Tyus and Saunders 1996), We charged workshop partici- pants with providing the following information: (1) which nonnatives pose serious threats, (2) which endangered spe- cies (and life stage) are the most affected, (3) what are the geographic areas where nonnatives cause problems for September 2000 ENDANGERED SPECIES endangered species, and (4) what are the most promising techniques for controlling nonnative populations, Workshop consensus was that predation is the most important mechanism whereby nonnatives affect endan- gered fishes, and that several species posed significant threats (Table 2), Razorback sucker and Colorado pikemin- now larvae were considered the most vulnerable species WYOMING r o 0 Figure 2, Geographic area covered by the upper Colorado River Endangered Fish Recovery Program (numbers are kilometers above and below the confluence of the Colorado and Green rivers). Fisheries 19