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differences in crew experience or effort, effects of such on abundance estimates should be minimized because capture probabilities were estimated for all sampling occasions, reaches, and years. Abundance estimates.--Consistent declines in abundance estimates over time and in every major river reach suggested a river-wide reduction in abundance of Colorado pikeminnow in the Green River Basin from 2000 to 2003. Given the absence of obvious violations of assumptions for abundance estimation models, and that river-wide estimates were relatively precise, abundance estimates presented here are considered reliable. Abundance of Colorado pikeminnow recruits was much lower than for adults and suggested that recruit abundance was not sufficient to maintain adult abundance at a stable level over the period of study. This is true because the proportion of recruits to adults in the population was much less than the proportion of adults that apparently died each year. The I analysis also supported the hypothesis that reduced recruitment was a main reason for the apparent decline of adult Colorado pikeminnow in the Green River Basin. Recruitment dynamics for Colorado pikeminnow are episodic, occurring at intervals of several years in the Colorado River (Osmundson and Burnham 1998), and such may also be the case in the Green River. Estimates for the Yampa River suggested a stable number of adult Colorado pikeminnow in 2000 and 2001 but abundance declined slightly in 2002 and more in 2003. Based on declines in capture rate from 2000 and 2001 (82 and 120 individuals captured, respectively, see also Fig. 13) to 33 and 31 in each of 2002 and 2003, respectively, and the presence of large, predaceous northern pike in the Yampa River, expectations may have been for lower abundance estimates. However, in each of those latter two years, not a single fish was recaptured from samples taken that year (no within year recaptures). The low probabilities of capture resulted in abundance 43