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long as the physical habitat is appropriate for maintenance of the native prey population, it is probably adequate for adult Colorado pikeminnow. ~ It is of considerable significance to the recovery effort that the preferred prey items are less abundant in the UCR below Palisade. Adult Colorado pikeminnow may be congregating in the 15-Mile Reach simply because it receives input of prey species from upstream reaches and because it is as close as those fish can get to habitat that contains the preferred prey. The possibility that adult Colorado pikeminnow abundance is prey-limited in the 15-Mile Reach has significant ramifications for the recovery effort. ~ It suggests that, unless food supply can be increased, improvements to physical habitat may not do much to increase the number of fish present. The area with suitable prey for the large adult pikeminnow will be extended greatly when barriers to Debeque Canyon are made passable. ~ Adult migration and spawning habitat. Lack of access to spawning grounds has been implicated in the decline of the Colorado pikeminnow (Joseph et al. 1977, Tyus 1984). Little is known, however, about the historical distribution and abundance of spawning sites. Spawning habitat has been located in the Yampa, Green, and San Juan rivers, and the presence of larvae shows that spawning also occurs in the UCR and the lower ~ Gunnison River. Habitats in the Yampa and San Juan rivers appear to conform to a specific geomorphologic profile. The hydraulic and sedimentologic conditions necessary for the creation and maintenance of habitat with those characteristics have been defined recently by studying bar-forming events at two Colorado pikeminnow spawning sites in the lower Yampa River (Harvey and Mussetter 1996, Harvey et al. ~ 1993). At present, there is no obvious reason to suspect that the quality or quantity of spawning habitat in these three rivers is limiting reproduction of the pikeminnow. Less is known about spawning habitat in the UCR, in part because there have been only limited observations of adult pikeminnow occupying spawning ground, and no running ripe females have been collected (D. Osmundson, personal communication. ~ Pikeminnow continue to spawn in the UCR despite the fact that some of the possible spawning sites have been severely altered by land and water development (Valdez et al. 1982, McAda and Kaeding 1991 b). To the extent that habitat in the UCR may conform to the geomorphic profile that has been established for other spawning sites, it may be possible to infer the location of the habitat. Supplementary data on the ~ distribution of larvae and ripe adults may help narrow the possibilities for spawning sites. It will be very difficult to determine if spawning habitat may limit reproduction or recruitment in the UCR until the location of the habitat is confirmed. In addition to possible spawning sites in the Grand Valley and in the lower Gunnison River, there is physical habitat in Debeque Canyon that appears to match the proposed ~ geomorphologic profile (see Anderson 1996). However, the site in Debeque Canyon is not yet accessible to adults in the UCR because there are barriers to migration. Adult population size. The minimum size required for maintaining a "viable° natural population of any of the endangered fishes should be an important consideration for 30