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Limited population estimates are available for humpback chub. Valdez and Ryel (1995) estimated the mainstem Colorado River in Grand Canyon population to be from 2682 to 4281 individuals ' and, using several different estimators, the Little Colorado River population was estimated from 4111 to 4870 individuals. Douglas and Marsh (1996) estimated the Little Colorado River population using five different models. Their estimates ranged from 4,508 to 10,444 with the former having the highest rating. For the upper Colorado River basin, Karp and Tyus (1990) reported a population of 600 in the Yampa River. Approximately 1000 and 7000 mature individuals were estimated through mark/recapture to occupy Black Rock's and Westwater canyons on the Colorado River, respectively (T. Chart, UDWR; unpublished data). Bow Although several historical accounts describe bonytail throughout the Colorado River system more than 100 years ago (e.g., USFWS 1990; Valdez and Clemmer 1982), the basic biology of G. elegans was not studied in detail until the late 1960's (Vanicek 1967; Vanicek and Kramer 1969; Holden and Stalnaker 1970; Vanicek et al. 1970). Even then, early studies focused on the abundance, He history, and distribution, and little was determined about its ecology. During this period, bonytail numbers were greatly reduced. Thus, the ecological requirements of the bonytail remain poorly understood. The last known concentration of bonytail were captured in Split Mountain Canyon of the Green River through Dinosaur National Monument, Utah (Vanicek 1967). Valdez (1985) identified 14 possible bonytail that included juvenile and young-of- year fish in Cataract Canyon, Utah. Kaeding et al. (1986) captured one bonytail from the Black ' Rocks section of the Colorado River, Utah. Tyus et al. (1987) captured one bonytail in Gray Canyon. In 1993, a suspected adult bonytail was captured in the Colorado River about four miles upstream from its confluence with the Green River (D. Osmundson, personal communication). Utah Division of Wildlife captured two potential bonytail in 1996 in the Colorado River in Cataract Canyon and in Desolation Canyon (T. Chart, personal communication). I Demographic Life-Stage Model Crowl and Bouwes (1998) developed a generic life-stage model for Colorado River endangered fish as a tool to aid in the development of management objectives. The purpose of the model was to simulate demographic characteristics of all the populations. It was based on the best available life-history information for each species and geomorphic reach characteristics as identified by species experts. For purposes of the model, the Upper Basin was divided into three river segments: (1) Yampa/Green Rivers , (2) Colorado River above the confluence with the Green River, and (3) the Colorado River between the Green River Confluence and Lake Powell. The model was generated to be as specific as empirical data would allow and included parameters on sex ratio, survivorship means and variances for larvae, YOY, 1-, 2- and 3-yr old fish and adults. Initial population values for each age class as well as carrying capacity values for each geomorphic section were also used as initiation values in all models. For purposes of this model, carrying capacity was used to impose documented and perceived He history and habitat constraints on each geomorphic reach and was not intended to represent an actual demographic limit to population growth as commonly used in ecological literature. While the model can be used in a very specific manner (e.g., to model only 2-yr old fish in a particular reach), it can also be parameterized to model the upper Colorado River basin as a single population. ' 13