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pond was last connected to the river (Chuck McAda, U.S. Fish and Wildlife Service, Grand Junction, CO, personal communication). In addition to their propensity to inhabit man-made gravel pits that are at least ephemerally connected to the river, razorback sucker are most often captured in low velocity habitats in the channel (Figure 2) and wetland ponds connected to the channel (McAda and Wydoski 1980, Tyus et al. 1987). Bulkley and Pimentel (1983) showed that razorback sucker preferred temperatures of 22-25oC in shuttle box experiments. In the potamon reaches of the Upper Colorado River Basin, shallow, backwater and wetland habitats are typically closer to the preferred temperatures than is the river channel, especially in the upstream reaches where razorback sucker are most commonly found. Indeed, Wick et al. (1983) showed that backwaters flooded by spring runoff on the Yampa River were significantly warmer than the channel, thereby offering more degree days for maturation of spawning condition. Naturally functioning backwaters (i.e., not influenced by erratic, regulated flows) also contain food sources, such as zooplankton, invertebrates associated with macrophytes and microbially-rich detritus, needed to mediate growth of razorback sucker (Wick et al. 1982, Wick 1991). The reproductive bottleneck that is preventing recruitment of razorback sucker in the Upper Colorado River Basin is unknown. Clearly, these suckers prefer lacustrine-like environments, owing to their proclivity for low velocity habitats, especially flooded gravel pits and wetlands during high flows. River flow regulation, wetland revetments, diversion dams (which limit migratory pathways, see Figure 1) and presence of abundant native and nonnative predators (also discussed below with regard to similar influences on squawfish) may prohibit the fish from using backwaters and seasonally flooded wetlands in a manner that will allow recruitment to occur annually. Indeed, in Lake Mohave on the Lower Colorado River, where a large population of razorback sucker have persisted for many years, but did not recruit in spite of apparent spawning success each year, the recruitment bottleneck was attributed to predation of larvae and early juveniles by nonnative minnows and sunfish (Marsh and Langhorst 1988, Marsh and Minckley 1989, Papoulias and 10