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Status and natural history of Colorado pikeminnow.--Abundance of Colorado pikeminnow varies by more than an order of magnitude in the three occupied sub-basins of the Upper Colorado River Basin. The population in the San Juan River sub-basin, which is isolated from the remainder of the upper Colorado River Basin by Lake Powell Reservoir, is relatively small and may be extirpated (Platania et al. 1991, Holden 2000). In the Colorado River sub- basin, abundance estimates for Colorado pikeminnow were conducted from 1991 to 1994 and 1998 to 2000 using multiple-pass capture-recapture sampling. Abundance estimates for the 278- river km (RK) reach from 1991 to 1994 were variable among years but averaged about 600 sub- adult and adult fish (Osmundson and Burnham 1998). Estimates conducted from 1998 to 2000 suggested population abundance increased to about 750 sub-adult and adult fish (Osmundson 2002). Although abundance estimates for Colorado pikeminnow have not been conducted in the Green River sub-basin, that population is thought the largest in existence (Tyus 1991). That assumption is based on the larger area of occupied habitat (over 900 RK) in the Green, White, and Yampa rivers and the higher Green River sub-basin catch per unit effort (catch/effort) rates for Colorado pikeminnow gathered from 1986 to 2000, compared to the Colorado River (McAda 2002). Historically, Colorado pikeminnow achieved 1.8 m in length and 40 kg in weight (Jordan and Evermann 1896; Minkley 1973; Tyus 1991; Quartarone 1995), although the largest individual known since intensive sampling began in the late 1970's was 1,240-mm TL (this study). Large individuals may be 35 to 50 years old, and very slow-growing, based on average growth rates of 10 mm/yr or less for recaptured individuals (Osmundson et al. 1997). Sexual maturity is not reached until five to seven years old at a length of 450 to 550-mm TL (Vanicek and Kramer 1969; Osmundson et al. 1997; Osmundson and Burnham 1998). 12