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impact population recruitment (Houde 1987; Bestgen et al. 1997). Winter mortality is often size- selective, but the magnitude of size selectivity varies from year to year. To survive, young-of-year fishes must reach a threshold size by late fall of their first growing season, because overwinter mortality generally removes smaller individuals with low energy reserves (Oliver et al. 1979, Toneys and Coble 1979; Shuter et al. 1980; Post and Evans 1989; Schindler 1999). For larval and juvenile fishes, reaching this threshold size confers a host of advantages during a time when mortality is typically high, including lower probability of starvation due to higher prey capture rates, and decreased vulnerability to predators due to improved swimming ability (Garvey et al. 1998). Although young-of-year fish may reach the threshold size, other environmental factors, such as ice formation, ice jams, cold water temperatures, and winter flow fluctuations, may impose additional threats to overwinter survival. Winter flows in the middle Green River are influenced by releases from Flaming Gorge Dam. It has been hypothesized that winter operations of Flaming Gorge Dam reduces survival of age-0 Colorado pikeminnow because fluctuating discharge and associated changes in water surface elevation modify the characteristics of nursery habitats which causes an increase in fish activity (Carlson and Muth 1989; U. S. Fish and Wildlife Service 1992; Haines et al. 1998; Valdez and Cowdell 1999). Yet, a study by Hayse et al. (2000) found that adverse winter conditions in backwater habitats including ice formation, ice jams, and flow-through conditions were likely more a function of local upstream conditions than winter flow fluctuations initiated at Flaming Gorge Dam. However, higher stage elevations produced by ice jams may inundate nursery backwater habitats and transform them into unsuitable flow-through areas. In addition, as flows decline, fish must search for suitable habitats to avoid being stranded. Winter flows may dismantle ice cover which acts as an insulator, allow the creation of frazil ice, and may result in ice jams that increase river stage and inundate backwaters (Valdez and Cowdell 1999). Each of these stressors may impact overwinter survival of young Colorado pikeminnow by causing fish to redistribute to more suitable habitats at a time that is very costly energetically. iii