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habitat use studies in the UCR should be interpreted carefully because habitat alteration has been extensive and because so few fish were available for study. Therefore, it would be prudent to compare habitat preferences in this part of the basin ~ with those of other populations, especially in the Green and Yampa rivers where there are fewer barriers to fish movement. As indicated previously, habitat use in the unregulated portion of the basin is somewhat variable, and appears to differ from that reported for the 15-Mile Reach. ~ In general, the larger (subadult and adult) Colorado pikeminnow will move into upper river reaches when there is opportunity. For example, adult pikeminnow in the Yampa River frequently travel as far upstream as Craig, and have been reported as far upstream as Steamboat Springs. In the White River, before Taylor Draw Dam was closed, pikeminnow traveled up river as far as Meeker: Historical records from the ~ Gunnison River basin place Colorado pikeminnow in the lower Uncompahgre River, and data from the recently-completed Redlands fish passage structure show that subadults from the lower UCR are now moving into the lower Gunnison River (F. Pfeifer, personal communication). In all three instances adult Colorado pikeminnow were occupying, or seeking, habitat that is comparable to~the Colorado mainstem above Palisade; such adult habitat may be higher quality, in terms of physical and biological features, than that found further downstream. The shortage of high-quality nonspawning habitat may hinder expansion of the number of adult Colorado pikeminnow in the UCR. The conclusion that high quality adult habitat is limited in the 15-Mile Reach in low-flow years also has been reached by the USFWS (Osmundson et al. 1995). Their proposed ~ remedy involves optimizing adult habitat by providing higher minimum flows. Underlying the USFWS proposal are the following assumptions that are standard for instream flow methodologies: (1) the observed pattern of physical habitat use is indicative of requirements, and (2) satisfying the apparent physical habitat requirements will increase the carrying capacity of the riverine environment leading to an increase in ~ the number of adult Colorado pikeminnow, unless the population is really held in check by another limiting factor. Both assumptions merit review. Physical habitat conditions are obviously important, but may not be the primary factor constraining the abundance of adult pikeminnow. The historic prey of adult pikeminnow ~ were native suckers and chubs. These prey species are more abundant in the upper river reaches (e.g., the Yampa above Maybell, the White River above Rangely, and the Colorado River above Palisade; (VV.H. Miller et al. 1982bc, Tyus et al. 1982a, Valdez et al. 1982). Where adult pikeminnow have access to upper river reaches, they tend to be more abundant where the prey are more abundant. Flannelmouth suckers and adult ~ Colorado pikeminnow congregated below Taylor Draw dam after it was closed, presumably because they sought access to °preferered° habitat upstream (Chart and 18ergersen 1992, Trammell et al. 1993). The tendency of the adult Colorado pikeminnow to be distributed in river reaches that contain their preferred prey is well supported by the data, and argues that, for most of the year, physical habitat may be of less direct importance to the Colorado pikeminnow than the distribution of the prey. As 29