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within 20 to 30 d following hatching and zooplankton densities needed to maintain razorback sucker survival have not been reported in main channel habitats. Thus, if restricted to the main channel environment after flows recede below 447 m3/s, young larvae face potential starvation and older larvae could be nutritionally stressed. Fish composition and habitat use in Old Charley Wash Despite the success of catching juvenile and adult razorback sucker, the composition of fish in Old Charley Wash was dominated by nonnative fishes. The wetland attracted and maintained floodplain and lentic oriented fishes from the river. The same fishes in relative abundance were attracted annually either by temperature (Bulkley and Pimenta11983), food (Tyus and Karp 1990), odors, or some combination of gradients. Whatever the attraction, it is clear that large numbers of adult fish from the river sought and remained in Old Charley Wash. Longitudinal and lateral migrations into the floodplain is a regular feature of riverine fishes for reproduction and feeding (Welcomme 1985). Razorback sucker undergo both longitudinal migrations (Tyus and Karp 1990, Modde and Irving accepted) associated with spawning and postspawning movements and lateral migrations on the floodplain following spawning (Tyus and Karp 1990, and this study) into favorable thermal ranges with higher prey densities. At the same time, many nonnative species have become established in the Middle Green River which are primarily lentic habitat species (Tyus et al. 1982) and exist in a marginal capacity in the riverine environment. Because floodplain wetlands are warmer and have greater prey densities than the main channel at the time they become isolated, many lentic-oriented fish such as green sunfish, black crappie, black bullhead, and carp tend to remain in floodplain wetlands. Conversely, many riverine fishes leave floodplain wetlands as water recedes (Welcomme 1985). Adult fishes have been observed to leave floodplain habitats before age-O fishes (Idaho et al. 1970 and Williams 1971, in Welcomme 1985). Similarly, age response to draining was observed during fish collections at Old Charley Wash in 1996 and probably occurred in the spring as natural flows declined. The dominance of nonnative fishes in Old Charley Wash was greater than that observed in the main channel reaches ofthe Green River (McAda et al. 1994) and other Middle Green River tributaries in the Green River subbasin such as the Yampa (Modde and Smith 1995), and White (Irving and Modde 1994) rivers. The composition of nonnative fish in Walter Walker Wildlife Area Gravel Pit (18 ha shallow gravel pit in the Grand Valley, Colorado) was dominated by nonnative fishes (96.1 %) when the gravel pit was isolated, yet, when connected to the Colorado River, the pit consisted of 41 % native fishes (Valdez and Wick 1983). When isolated, lentic-oriented species, i.e. black bullhead, green sunfish and carp, represented a higher percent composition than when the gravel pit was connected to the river. The composition of nonnative fishes in Old Charley Wash included many species or life stages not abundant in the main channel. With the exception of fathead minnows and red shiners, age-O fishes collected in Old Charley Wash are not commonly found in the main channel (McAda et al. 1994). Based on biomass, the fishes that benefited reproductively in Old Charley Wash included green sunfish, black crappie, black bullhead, fathead minnow, carp, and razorback sucker. The former three species exist in riverine habitats, but have been reported only as rare in riverine habitats of the Middle Green River by Tyus et al. (1982). Thus, with the exception of carp and fathead minnows, many of the nonnative species that benefit reproductively from floodplain depressions are not those that are abundant in the main channel. 42