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accompanying changes in available habitat. In the Colorado River during NNC removal, flows in 1998 ' were between 5,000 and 11,000 cfs, and flows in 1999 were between 5,600 and 12,000 cfs, while flows in 2000 were consistently below 4,000 cfs. Trammell and Chart (1999c) found that flows below 4,000 cfs maximized backwater habitat area in the Colorado River, making sample size from that year disproportionately greater than previous years. Perhaps more importantly, the same control habitats ~ were not available from one year to the next in the Colorado River and discharge-related changes in surface areas of individual treatment backwaters may have affected sampling efficiency. Similarly, area sampled and discharge was also variable in the Green River. Regardless of changing discharge, many ~ individual backwaters could not be completely depleted due to depth of water and the abundance of vegetation, which allowed fish to avoid the seine. Variation in discharge probably exacerbated these components of sampling error. ~ In neither the Green nor the Colorado rivers was there a significant increase in the catch or catch rate of razorback sucker or Colorado pikeminnow as measured by the ISMP sampling programs. Thus, temporary reductions did not appear to result in increased recruitment of razorback sucker or Colorado ~ pikeminnow, which may be the ultimate measure of success of the removal efforts. The lack of significance may be partly due to limitations of the statistical power of the 1SMP to detect changes; however, other lines of evidence also suggest recruitment increases were not observed in these years. ~ In the Green River, no wild juvenile razorback sucker were observed during the intensive electrofishing conducted for the Colorado pikeminnow population estimate in the Lower Green River from 2000 to 2002 (Paul Badame, personal communication). Relatively large razorback sucker larvae (19 mm) were collected in the treatment areas during a related study in 1999, perhaps indicating improved growth, but no YOY or juvenile razorback sucker were captured in the fall or in subsequent years sampling in any of the concurrent sampling programs (including nonnative removal, fall ISMP, spring ISMP, razorback sucker evaluations, and bonytail monitoring.) During the fall YOY ISMP sampling for Colorado pikeminnow, the catch and catch rates of Colorado pikeminnow were not significantly greater during the removal years than the long term average (1986-1997), or of recent sampling years 1991-1997. Although the order-of-magnitude increase in age-1 Colorado pikeminnow in 1999 over 1998 in the treatment areas ~ -17-