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Other Life History Attributes Distribution Populations of Colorado pikeminnow require an extensive length of river so that an array of habitat types are provided to meet the changing needs of different life stages. Larvae hatched in cobble-gravel substrates of high-gradient reaches drift 100-200 km downstream to low-gradient reaches where backwaters formed in silt-sand bars provide ideal nursery habitat (Haynes et al. 1984, Tyus and Haines 1991). Insectivory is largely replaced by piscivory during the first year (Vanicek and Kramer 1969, Muth and Snyder 1995). As Colorado pikeminnow mature (7-8 yr), the need for larger forage fish is not met in lower reaches of the Colorado River mainstem where native, large-bodied, prey fish are scarce. Consequently, condition (weight as a function of length) declines prompting many Colorado pikeminnow to disperse to upper reaches and tributaries where native suckers and chubs are more abundant (Osmundson et al. 1998). This progressive dispersal pattern results in relatively segregated life stages and adult densities are surprisingly clumped near the upstream margins of their range. This pattern is generally repeated in the Green River system: there, most YOY and subadults are located in middle to lower reaches of the Green River whereas in the upper reaches, as well as the major tributaries, most fish are adults (McAda et al. 1997). Based on these patterns, it is likely that reaches in the Colorado River upstream of the diversion dams, the subject of this report, were historically used primarily by adults, as is currently the case in the Grand Valley. Reproduction Colorado pikeminnow undergo extensive spawning migrations in the Green River system (Tyus 1991, Irving and Modde 2000) and relatively short ones in the Colorado River (McAda and Kaeding 1991). Spawning occurs as spring flows are decreasing and water temperatures are increasing (Haynes et al. 1984, Nesler et al. 1988, Tyus 1991, McAda and Kaeding 1991, Bestgen et al. 1998, Anderson 1999, Trammell and Chart 1999a). In general, spawning occurs earlier during low runoff years and later in high runoff years (McAda and Kaeding 1991, Tyus and Haines 1991, Bestgen et al. 1998), presumably 15