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months to grow and accumulate fat reserves before entering their first winter (Thompson et al. 1991). Overwinter survival during first year of life is a primary factor determining year-class strength of most temperate zone fishes (Garvey et al. 1998), including Colorado pikeminnow (Haines et al. 1998). Winter is a period of reduced growth, depletion of energy, and heightened mortality risk. Small changes in the rate of growth or mortality of larvae and juvenile have been shown to substantially impact population recruitment (Houde 1987; Bestgen et al. 1997). The effects of stage fluctuations induced by hydropower operations of Flaming Gorge Dam, should they exist, are presumed much greater at the nursery area in the middle Green River (91 miles below the dam) than in the lower Green River (290 miles below the dam). Winter flows in the middle Green River consists primarily of releases from Flaming Gorge Dam, and it has been hypothesized by several investigators that winter operations influences age-0 fish survival (Carlson and Muth 1989; U. S. Fish and Wildlife Service 1992; Valdez et al. 1999; Haines et al. 1998). As hydropower demands increase, higher flows may inundate nursery backwater habitats and transform them into unsuitable flow-through areas. It has been a speculated that fluctuating winter flows may dismantle ice cover which acts as an insulator, allow the creation of frazil, and may result in ice jams that increase river stage and inundate backwaters (Valdez and Cowdell 1999). As a result, each of these factors may contribute additional stress to the overwinter survival of young Colorado pikeminnow by causing the fish to redistribute to more suitable habitats at a time that is very costly bioenergetically. However, the effects of flow fluctuations on overwinter fish survival and nursery habitats, should they exist, have not been demonstrated in the field. A-2