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The annual spawning period for Colorado pikeminnow has been determined in the Green River system from migrations of radio-tagged fish, collections- of ripe fish, and back-calculated dates of larval emergence. Average temperatures during the spawning period were in the range of 22-25°C (Haynes and Muth 1984, Nesler et al. 198$, Tyus 1990). The spawning period, which typically lasts 4 to 5 weeks, generally occurs earlier in low-water years, and later in high-flow years, presumably in response to varying flow and temperature conditions (Vanicek and Kramer 1969, Tyus and Karp 1989). USFWS data from 1981 to 1988 indicated that spawning occurred when flow was decreasing and temperature was increasing following spring peak runoff (Tyus 1990). This ~ generally occun~ed 26 days (range: 17 - 33 d) following migration. Spawning of Colorado pikeminnow is not triggered by a single flow or temperature event, but by the interaction of abiotic and biotic influences (Tyus 1990). The sex ratio of ripe Colorado pikeminnow on spawning grounds shows a consistent ~ bias toward males. Captures in the Green River basin yielded about 15 males for each female (Tyus 1990). A similar bias has been reported for the northem pikeminnow. Patten and Rodman (1969) used scuba to observe spawning of northern pikeminnow and reported the number of males exceeded the number of females by a factor of about 50 to 200. Casey (1962) also reported that male northem pikeminnow outnumbered ~ females. Most capture data suggest that a biased sex ratio is typical in the genus PtYchocheilus. The reason for the biased sex ratio has not been established firmly, but may be the result of spawning behavior or sex-related differences in the age of first reproduction. Radiotracking data show that adults, especially females, do not return every year to the same spawning bar, and that no Colorado pikeminnow have been ~ found spawning on more than one spawning bar. The clear implication is that females do not spawn every year. If females spawn less often than males, there will be fewer ripe females than ripe males in a given year, other things being equal. A sex-related difference in the age of first reproduction may also contribute to a biased sex ratio. If it is assumed that the sex ratio is 1:1 at hatching, and that mortality is not ~ sex-related, the sex ratio for ripe fish will depart from 1:1 if females mature later than males. Application of age-length data from Hawkins (1991) to ripe Colorado pikeminnow collected on spawning grounds shows that males begin reproducing at approximately age eight and females begin reproducing at approximately age ten. For the two years during which maturation of females is delayed relative to males, females ~ are still exposed to mortality. Thus, the number of mature females will be less than the number of mature males. Larvae and Postlarvae. Larval Colorado pikeminnow emerge as sac-fry from cobble bars and begin drifting downstream rapidly (Haynes et al. 1984, Nesler et al. 1988). ~ The yolk sac contains enough energy reserve to support the new fry for several days, after which exogenous feeding becomes necessary. The larvae tend to frequent shorelines with lower velocities and eventually become concentrated in shallow backwater habitats of alluvial reaches (Tyus et al. 1982b, Haynes et al. 1984, Tyus and Haines 1991, Valdez et al. 1982). Once in the backwaters, larvae begin feeding on 15