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EXECUTIVE SUMMARY Control of nonnative fish has been identified by the Upper Colorado River Endangered Fish Recovery Program as a primary component in the recovery of four endangered fish species. Historically 12 native fish species lived in the Colorado River in Colorado. These include four listed fish species (Colorado pikeminnow, Ptychocheilus lucius; humpback chub, Gila cypha; bonytail, Gila elegans; and razorback sucker, Xyrauchen texanus). Conversely, 40 nonnative fish species have been collected in the Colorado River in Colorado; thus nonnative fish species outnumber native species by more than three to one. Because riverside ponds may be chronic sources of nonnative fish to critical habitat the goal of this study was to reduce proliferation of nonnative fish species in floodplain habitats and minimize chronic escapement of nonnative fishes from perennial ponds. The primary objective to accomplish this goal was diminution of nonnative fish abundance in riverside ponds and reduction of nonnative fish escapement from ponds. A secondary objective was to detect reinvasion of treated ponds by nonnative fish and identify nonnative fish movement through fish screens. Ultimately, the desired effect of i this study was a reduction in the abundance of nonnative fishes in riverine nursery habitats. An inventory of the study area revealed 729 potential pond sites. Information and education efforts utilized landowner and other interested community contacts to obtain access to 329 of the 729 potential pond sites. Of the 191 ponds that were sampled and found to contain fish in the study area, 147 contained only nonnative fish (21 species) and another 43 contained both native (3 species) and nonnative fish. Only native fish species were collected in only one of the 191 ponds. The total catch from these 191 ponds was 25,393 fish, of which only 387 (1.5%) were native. Of the 191 ponds sampled to determine species composition 86 ponds, totaling 373.8 surface acres, received nonnative fish control treatments (reclamation, screen, water management, black plastic, and re-route irrigation water). The total cost of all treatments was $310,331. The average cost per surface acre for these treatments was ix