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Colorado pikeminnow There is more information available for the life history requirements of Colorado ~ pikeminnow than for requirements of the other three endangered species. Because there is so much information, it is both feasible and prudent to divide the presentation into several parts. The first section deals with the requirements of adult pikeminnow when they are not engaged in behavior related to reproduction. The second section covers adults during the relatively short period of time in which they migrate to spawning areas and reproduce. The third section reviews what is known about the ~ requirements of larvae and postlarvae (age-0) from hatching until they leave the nursery areas. The fourth section covers juveniles until they mature and join the adult segment of the population. The final section is a narrative that integrates the main attributes of the life history in the context of the annual hydrograph. ~ Adult. nonspawnino. For most of the year, adult Colorado pikeminnow are not engaged in behavior directly related to reproduction. After spawning, most adult Colorado pikeminnow occupy individual home range areas that are located predominantly in upstream reaches (Tyus 1990), and that may be many miles from the major spawning areas. The other adults will take up residence in home ranges that are close to the ~ spawning areas, or move downstream. Most of the fish tend to remain in the adult habitat areas from late summer until mid-spring (Valdez and Masslich 1989, Wick and Hawkins 1989), when they undertake their spawning migrations (Tyus 1990). In terms of the annual hydrograph, the nonspawning period is associated chiefly with baseflow conditions. Adults spend much of their time in low velocity habitat (e.g., pools, eddies, ~ backwaters, etc.) adjacent to the main channel. At high discharge, low velocity habitats are more likely to be flooded shoreline or overbank areas (Tyus and Karp 1989, Tyus 1990). Adult Colorado pikeminnow in the Yampa, White, and Green rivers occupy a variety of habitats in mid-to-late summer, but are captured most commonly in eddies, pools, runs, ~ and shoreline backwaters, over sand and silt substrates (Tyus et al. 1984, 1987). Visual observations of fish in shallow water indicate that adults also used lower velocity microhabitats behind boulders, flooded vegetation, or other cover, when available. Many radio-tagged fish were located in deep shoreline habitats in the summer and their local movements in these habitats suggested heavy use of the eddy-run interface (Tyus ~ et al. 1987). Physical habitat used by radiotagged adults varied with location; water depths and velocities used by adults were different in the mainstream Green River (mean depth=1.4 m, mean velocity=0.2 mfs) than in the tributary Yampa (mean depth=0.9 m, mean velocity=0.1 m/s) and White (mean depth=0.7 m, mean velocity=0.5 m/s) rivers (Tyus et al. 1984). The high degree of variability among rivers ~ suggests that factors other than depth and velocity may also influence habitat selection. Habitat use during the winter appears to be quite variable and may be influenced by factors such as water level, ice conditions, and food sources (Wick and Hawkins 1989, Valdez and Masslich 1989). The fish remain active all winter, but do not tend to stray 10 •