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4 Limited recruitment may be occurring in both the upper and lower basins; however, any recruitment that occurs is not sufficient to maintain these populations. The few adult razorback sucker remaining in the upper Colorado River presumably spawn or attempt to spawn; no immature razorback sucker have been collected in 28 years. Taba et al. (1965) collected eight immature razorback sucker (85-115 mm) near Moab, Utah between 1962 and 1964. Although the causes are not known, recruitment failure may be due to deterioration of water quality or predation on eggs, larvae, and young razorback sucker by non- native fish. In addition, the reduction of high spring flows has altered the natural flooding cycle, and some investigators (McAda 1977; Tyus and Karp 1989; Osmundson and Kaeding 1991) attribute lack of recruitment, in part, to the reduced availability of inundated bottomlands which provided spawning or nursery habitat. Because recruitment has failed, remaining populations are composed almost exclusively of old adults. These populations are largely restricted to the upper basin, with the exception of populations inhabiting lentic habitats in man-made impoundments in the lower basin. 'Concentration areas' for adult razorback sucker are 1) Lake Mohave in the lower basin (Bestgen 1990), 2) the upper Green River from the mouth of the Duchesne River upstream to the Yampa River, and 3) the upper Colorado River from Grand Junction, to Clifton, Colorado (USFWS 1987; see Appendix 6.3 of that document)(Figure 1). Data on the relative abundance of adult razorback sucker collected during the Interagency Standardized Monitoring Program (ISMP) for adult Colorado squawfish demonstrate the rarity of the razorback sucker in the upper Colorado River, between Lake Powell, Utah and the Price Stub Diversion Dam near Palisade, Colorado. Only one razorback sucker was collected from 1986 to 2 4