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RESULTS River reach abundance estimates.--A total of 3,800 Colorado pikeminnow was captured. in the 42 sampling occasions (passes, includes fourth pass fish) of the five reaches during sampling from 2000-2003 (Appendix I). Of those fish captured, 3,212 (84.5%) were captured in only one year and not seen in any other year, 532 (14%) were captured in each of two years, 48 were captured in three years (1.3%), and only eight fish (0.2%) were captured in all four sampling years (Table 1). We estimated abundance of Colorado pikeminnow adults and recruits in each of the five river reaches of the Green River Basin for each sampling year to determine spatial abundance patterns and temporal dynamics. A set of eleven models was fit to the data to examine the importance of year-specific apparent survival (S), reach transition probabilities (?&, probability of a fish moving from one reach to another), and capture probabilities (p's, Table 2). The top model in the set of eleven contained 35% of the AIC weight, with the only competing model (# 2) differing in the degree of the polynomial of TL used to model capture probabilities. The third-best model with variable survival over all three annual intervals had a very high and unreliable survival estimate for the 2002 to 2003 interval and was not further considered. Therefore, we made inference from the minimum AIC, model. That model had 81 parameters and included quadratic and cubic effects of TL to model a single S over the 2000 to 2003 period (three length effects, one S, four total parameters), a linear effect of TL to model ?& (a single length effect plus 20 pi's, one each for fish in a given reach moving to any of the other four reaches for each of five reaches; 21 total parameters), and quadratic effects of TL to model p's (two length effects, plus those for five reaches, three sampling passes per year, three or four sampling years depending on the reach, 56 parameters). We held effects of length on estimated parameters constant, which makes the reasonable assumption, for example, that length effects on 25