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~ may or may not be connected with the channel at low water, but which can be connected at high water. These piscivorous fishes can displace native fishes and will consume juveniles of the native fishes (Burdick 1996, Osmundson 1987). The third group of nonnatives is a diverse collection of species (including channel catfish, black bullheads, common carp, walleye, and northern pike) that are better adapted for g riverine existence, and which may prey on native fishes in main channel habitat for part or all of the year. Several of these nonnative species that pose problems in the UCRB have been implicated in the demise of native fishes nationwide (ANSTF 1994). The body of evidence documenting the deleterious effect of nonnatives on the native ~ fishes of the Colorado River system is sufficiently compelling to have convinced most experts in the region. Hawkins and Nesler (1991) polled regional fisheries experts and found that 81 % believed nonnative fishes were responsible for significant problems. Maddux et al. (1993) reviewed issues related to the recovery of four endangered Colorado fishes and reported that interactions with nonnatives were the primary factor ~ limiting recovery in some areas. Lentsch et al. (1996a) identified the nature of negative interactions of many nonnatives with the endangered species. The nonnative fish issue has been studied thoroughly (see review by Tyus and Saunders 1996a) and the conclusions are clear that introduced species have played, and continue to play, a significant role in the decline of the native big river fish community. ~ Species,specific Limiting Factors Colorado pikeminnow The specific factors regulating the growth and survival of adult Colorado pikeminnow ~ are not well known. Adults are probably not subject to predation because they are too large for other piscivores to handle. Because the options for reducing adult mortality are limited, the most feasible prospects for increasing the number of adults may lie in improving or increasing their habitat, and this requires an understanding that habitat has biological, as well as abiotic dimensions. Adult nonspawning habitat. Studies in the UCR have shown that the best adult habitat accessible to the extant population is in the reach from Westwater to Palisade (RM 125- 186). This conclusion was derived from data on fish abundance and condition from the upper and lower portions of the Colorado River (Osmundson et al. 1997). Larger fish ~ move out of the lower UCR as they become subadults, and the few that remain tend to be in poorer condition. In contrast, fish occupying the upper part of the UCR tend to improve in condition as they grow larger. The logical inference is that the upper portion (the 15-Mile Reach) offers better habitat conditions for the growth of adults than does the remainder of the accessible portion of the UCR. That does not necessarily mean ~ that conditions in the 15-Mile Reach are optimal. An examination of habitat use by adult Colorado pikeminnow shows that preferences vary seasonally in the 15-Mile Reach (Osmundson et al. 1995). However, results of 28