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iv. The net effect of removal was not significant due to increases in catch which were ~ likely a result of immigration, reproduction, and growth of larval nonnative fish. • Colorado River i. No significant differences were observed between treatment and control reaches; no 1 decline in catch rate in treatment areas was observed relative to catch in control areas. ii. Temporary reductions in abundance of nonnative adults were observed at a reach level on the Colorado River within 1998 and 1999. iii. Declines in catch and catch rate were observed in consecutive removal trips on the Colorado River in 1998 and 1999 until the final trip, but in 2000 catch increased on consecutive removal trips. iv. The net effect of removal was not significant due to increases in catch which were a result of immigration, reproduction, and growth of larval nonnative fish. 3) cyprinid removal by seining all backwaters in a river reach will shift the species composition. ~ No significant shifts in species composition were observed. 4) Fall YOY ISMP sampling will be able to detect changes in cyprinid abundance (backwater and reach levels) and species composition from cyprinid removal the previous spring. • Fall YOY ISMP sampling was not able to detect changes in nonnative cyprinid abundance ~ following removal periods. • Fall YOY ISMP sampling was not able to detect changes in Colorado pikeminnow or razorback sucker abundance following nonnative cyprinid removal periods. • Failure to detect changes in nonnative or endangered fish abundance may be due to the statistical power limitations of the ISMP sampling program. 5) Exclusion of nonnative cyprinid adults from portions of flooded tributaries will increase available area for larval native fishes and reduce area available for nonnative cyprinids. • Blocks nets may reduce area available for nonnative cyprinids, but due to multiple block net failures, the usefulness of this technique could not be determined. 6.0 RECOMMENDATIONS • Removal efforts should be pursued by the Program and additional methods evaluated to increase effectiveness, e.g. ~ i. Investigate increasing effort by increasing number and frequency of removal passes ii. Investigate timing by beginning earlier in the spring and/or adding removal efforts in fall to reduce over-winter competition and suppress nonnative abundance in the spring. ~ iii. Investigate additional methods e.g. alternate gear types or chemical treatment. • Where possible, investigators should assess both depletion effects across removal efforts -20-