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INTRODUCTION The Colorado pikeminnow (Ptychocheilus lucius), formerly known as the Colorado pikeminnow, is presently restricted to the upper Colorado River basin and inhabits warm- water reaches of the Colorado, Green, and San Juan rivers and associated tributaries. The Green River and its two large tributaries (White and Yampa rivers) support the largest (Tyus 1991) and perhaps most viable (Gilpin 1993) population(s), while the San Juan River contains the smallest and most tenuous population (Platania et al. 1991). A population also persists in the upper Colorado River. There, reproduction and recruitment occur annually but strong cohorts are infrequent (Osmundson and Burnham 1998), and the extent of suitable adult habitat is limited. Goals for recovering the endangered Colorado pikeminnow include building self-sustaining populations to levels that assure long-term species viability. To achieve this goal, reproduc- tion and recruitment to the adult phase may need to be enhanced by focused management activities. In addition, physical habitat and food resources must be in adequate supply to support an expanding adult population. To assure viability over the next 100 years, Lentsch et al. (1998) estimated that each population needs a minimum of approximately 5,500 adults. In the Colorado River mainstem upstream of the Green River confluence, the population size during 1991-1994 was estimated at 500-600 adults (Osmundson and Burnham 1998). To date, no effort has been made to estimate carrying capacity (the number of adult fish the river can support in good condition) for Colorado pikeminnow in the upper Colorado River. It is therefore unknown whether the system is capable of supporting an additional 5,000 adults. A data-based evaluation of the carrying capacity of the upper Colorado River is essential for the development of realistic recovery goals. Longitudinal variation in native fish abundance and Colorado pikeminnow body condition led Osmundson et al. (1998) to suggest that limited food availability in lower reaches of the upper Colorado River was responsible for motivating young adults to disperse upstream. Because appropriate-sized prey were significantly more abundant in reaches upstream of Westwater Canyon, the authors concluded that upstream reaches had a much higher carrying capacity for adult Colorado pikeminnow than downstream reaches. Also, body condition of adult Colorado pikeminnow in downstream reaches was low, suggesting that carrying capacity had been exceeded. In contrast, body condition in upstream reaches was significantly higher, suggesting that numbers there remained below carrying capacity. Historical accounts suggest that the Colorado pikeminnow was formerly much more abundant in the upper Colorado River than it is today (Jordan and Evermann 1896, Quarterone 1993). This decrease in population size may be the result of lowered rates of reproduction and recruitment (Osmundson and Burnham in 1998). In addition, carrying capacity of the system has probably been substantially reduced from historic levels. Mechanisms likely responsible for reducing carrying capacity include: (1) a reduction in adult range, (2) a reduction in the suitability of prey, (3) a reduction in the quantity of prey, and (4) a reduction of optimal or preferred physical habitats.