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held equal in Huggins models. Thus, to the extent possible, we tested for and found no effects of heterogeneity other than the effect of fish body size. Another relevant assumption in this study is that animals mix freely between concentration habitat (backwaters, shorelines, eddies, main channel) and adjoining areas between sampling occasions such that all animals in the population are available for capture. A corollary assumption is that sampling effort was distributed over most occupied habitat. There is evidence that mixing of Colorado pikeminnow does occur between concentration areas and other habitat types among sampling occasions in the Colorado River (Osmundson and Burnham 1998). They found high probabilities of capture within concentration backwater habitat in a single sampling occasion, but relatively low probabilities of recapture in those same locations between occasions. The logical explanation for this capture pattern is that many fish moved into and out of concentration habitat between sampling occasions. In the Green River Basin during this study, we demonstrated similar mixing because initial capture probabilities were equal to recapture probabilities among the short-term sampling occasions (i.e., Pk = CO- If fish were not mixing, we would expect that recapture probabilities would be much higher than initial capture probabilities, because the same shoreline habitat was sampled during each pass. We probably had poor sampling efficiency in very deep pools (> 2-m deep), but the amount of that habitat type relative to shallower, easier-to-sample areas where Colorado pikeminnow typically reside was small (Tyus and McAda 1984). Thus, short-term fish movement patterns, analysis of capture and recapture rates, and our relatively complete sampling coverage of occupied habitat likely minimized bias due to incomplete mixing of marked and unmarked animals. We also assumed that recognition of marked Colorado pikeminnow was high (all were scanned) and that tag loss was low. Although differences in capture rates may exist because of 42