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000586 required to effectively kill nonnative fishes. More than likely about 10 ponds will be chemically treated in 1997. Fish and Wildlife. Nonnative fish species would be removed from some floodplain ponds along the Colorado and Gunnison rivers. This would prevent the chron i c escapement of nonnative fi shes into the ri vers where they woul d prey upon endangered fishes or use the river as a conduit to repopulate other backwaters or floodplain ponds. Nonnative fishes of the Family Centrarchidae are orimary tarqets for control. The nonnative fishes in floodplain ponds along the Colorado and Gunnison rivers that are primarily targeted for controli ncl ude various centrarchid species (Martinez and Nesler 1996). Largemouth bass and green sunfish were the most frequently collected piscivores in five randomly selected backwaters of the Upper Colorado River (Valdez and Wick 1983). Juvenile largemouth bass were collected in the Fall of all years between 1979 and 1993 (Appendix D) by seining backwaters of the Upper Colorado River during sampling by the U.S. Fish and Wildlife Service (1979-1985) or during the Recovery Program's Interagency Standardized Monitoring Program (ISMP) from 1986 through 1993 for Colorado squawfish and humpback chub (McAda et ale 1994a,b; 1995). Fewer largemouth bass were collected in backwaters in the Colorado River reach between Fish Ford downstream to the confluence with the Green River (Appendix D). No largemouth bass were collected from backwaters in the Green River in any of those years (1979-1993) during Fall sampling for young-of-the-year Colorado squawfish (Appendix E). Few ponds occur in the fl oodp 1 a ins along the Green Ri ver when compared with the Colorado River. The centrarchid fish species that occurs in the Green River is the green sunfish (Appendix E) and, in some reaches, the smallmouth bass (Salmoides dolomieui). In comparison, numerous floodplain ponds occur along the Upper Colorado River and largemouth bass were stocked in this area by the Colorado. Division of Wildlife until 1985 and by private pond owners (F. Pfeifer, 1994, Personal Communication). Osmundson (1986) concluded that survival of stocked Colorado squawfish (45 - 145 mm in total length) was significantly reduced from predation by largemouth bass that are over 100 mm in total length. Osmundson also reported that fingerl ing Colorado squawfish were more susceptible to largemouth bass predation than several common nonnative fishes (e.g., fathead minnows, red shiners, and young-of- the-year green sunfish) found in backwaters of the Upper Colorado River. The largemouth bass is an effective predator that can have an adverse impact on native fishes (Rivero 1936; Curtis 1942). Langhorst and Marsh (1986) reported that abundant razorback sucker larvae in a Lake Mohave backwater disappeared within 4 weeks after invasion by largemouth bass, green sunfish, bluegill, and some other warmwater fishes. Osmundson (1987) studied the use of ponds along the Colorado River for their potential in rearing Colorado squawfi sh. . He reported that 1 argemouth bass predation by fi sh over 100 mm TL significantly reduced the survival of stocked squawfish. In fact, squawfish were selected over fathead minnows, red shiners, and young-of-the- year green sunfi sh. The preference for soft-rayed forage fi sh (e. g. , minnows) over spiny-rayed species (e.g., sunfish) by largemouth bass has been documented in the literature (e.g., Holton 1977; Lewis and Helms 1964). A bioenergetic study completed for the Colorado Division of Wildlife 17