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projections of scales mounted between microscope slides. Annuli were clearly defined, because rapid growth during the warm season was followed by severe winter conditions. Although the sample size was small (N=26), the pattern of weight-at-age was clear (Table 1) and compared favorably with growth patterns from a neighboring population upstream in the Yampa River (Nesler 1990). In model simulations, spawning losses (7.7% wet body mass) were subtracted from pike larger than 660 mm on 15 June (simulation day 45), based on the difference in post- spawning length-weight regression in June and regressions in May and October. The proportional contribution of different prey species in the diet was estimated from stomach samples collected during the spring and fall 1993 in the Yampa River, and in May-June 1994 in the Ouray reach, Green River. Stomach contents were identified to species or the lowest possible taxonomic level. We measured total or standard prey lengths (mm) and wet weights (mg). Diet composition was computed separately for each age class of northern pike, and was used to partition daily rations into estimated consumption rates on Colorado squawfish and other prey in the bioenergetic simulations. Since nearly all prey items in the stomachs were cyprinids, we assigned an energy density of 1196 cal (4991 J) per gram wet body mass to Colorado squawfish and other prey (Hewett and Johnson 1992). The energy density of northern pike was treated as a constant 1049 cal/g (4391 J/g; Wahl and Stein 1991). Model Simulation We ran the model to predict daily consumption over 365 d (1 May to 30 April) for each age class of pike. Growth for each cohort was fitted to spring (May-June) and autumn (October) age-specific body mass estimates. A baseline run, using age-specific diet composition from