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11 Information on radio-tagged adult Colorado pikeminnow during fall suggests that fish seek out deepwater areas in the Colorado River (Miller et al. 1982, Osmundson and Kaeding 1989), as do many other riverine species. River pools, runs, and other deep water areas, especially in upstream reaches, are important winter habitats for Colorado pikeminnow (Osmundson et al. 1995). Very little information is available on the influence of turbidity on the endangered Colorado River fishes. Oamundson and Kaeding (1989) found that turbidity allows use of relatively shallow habitats ostensibly by providing adults with needed cover; this allows foraging and resting in areas otherwise exposed to avian or land predators. Tyus and Haines (1991) found that young Colorado pikeminnow in the Green River preferred backwaters that were turbid. Clear conditions in these shallow waters might expose young fish to predation from wading birds or introduced, sight-feeding, piscivorous fish. It is unknown whether the river was as turbid in the past as it is today. For now, it is assumed that these endemic fishes evolved under natural conditions of high turbidity; therefore the retention of these highly turbid conditions is probably an important factor in maintaining the ability of these fish to compete with nonnatives that may not have evolved under similar conditions. Osmundson and Burnham (1996) estimated the population of adult and subadult Colorado pikeminnow in the Colorado River (from Palisade to the confluence with the Green River) to be 600-650 individuals during 1991-1994. They estimated an average of 4.0-4.2 fish per mile above Westwater Canyon, and 3.1-3.4 fish per mile below Westwater Canyon. Preliminary estimates from a 1998 survey indicate a population of over 750 subadults and adults in the Colorado River (USFWS unpublished data). Colorado pikeminnow reproduce each year, however, strong year classes that recruit fish to the adult population are relatively rare (Osmundson and Burnham 1998). A distinct increase of subadult fish was found below Moab in 1991 and within a few years these fish were distributed throughout the Colorado River. Osmundson and Burnham (1998) concluded that these fish were the result of one or more strong year classes produced during the mid-1980'x. McAda and Ryel (1999) have identified another strong year-class that occurred in 1996. In both cases, the common hydrologic conditions that led to successful reproduction and first year survival was a spring and summer of moderately high flows following a year of exceptionally high flood flows (MCAda and Ryel 1999).