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In the Green River sub-basin, adults migrate to two main spawning areas in late spring when snow-melt runoff subsides and water temperatures warm (Tyus 1990; Irving and Modde 2000). Migrations up to 745-RK round-trip to and from spawning areas have been documented by radio-tracking adults in spring and summer, and those fish often return to established home ranges (Tyus 1990; Irving and Modde 2000). Colorado pikeminnow in the Colorado River are much less mobile during the reproductive season and made only local movements to spawning areas (McAda and Kaeding 1991). Colorado pikeminnow typically begin spawning around the summer solstice, when flows decline to near base level and water temperatures range from 16 to 22°C (Haynes et al. 1984; Nesler et al. 1988; Tyus 1991; Bestgen et al. 1998). Eggs are deposited over geomorphically complex riffle habitat and embryos hatch within four to seven days at water temperatures of 18 to 26°C (Hamman 1981; Marsh 1985; Tyus 1990; Harvey et al. 1993; Bestgen and Williams 1994). About five to nine days later, emerging larvae are transported downstream 40 to 200-RK or more to alluvial river reaches, where they rear in low- velocity shoreline areas such as backwaters throughout the summer (Kesler et al. 1988; Tyus and Haines 1991; Bestgen 1996; Bestgen et al. 1998). Variation in abundance of adult Colorado pikeminnow in the Colorado River was thought driven by changes in abundance of young-of-year that rear mostly in the lower portion of the river (Osmundson and Burnham 1998). Recruits move upriver as subadults and eventually establish home ranges (Osmundson et al. 1998), apparently in response to a productivity gradient that increases upstream (Osmundson et al. 1998; Osmundson et al. 2002). Size-structure of Colorado pikeminnow appears stratified in the Green River sub-basin similar to that in the Colorado River (Osmundson et al. 1997), as most juveniles and sub-adults captured were found in the lower reaches of the Green River (McAda 2002). However, movement, recruitment, and 13