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ISMP backwater sampling evaluation <br />INTRODUCTION <br />The demise of endangered fishes native to the Colorado River Basin has been attributed <br />mainly to habitat change, and effects of non-native fishes which compete with and prey upon <br />native taxa (Carlson and Muth 1989, Minckley and Deacon 1991). In the upper Colorado River <br />Basin, non-native green sunfish Lepomis cyanellus and largemouth bass Micropterus salmoides <br />may represent a substantial source of mortality for early life stages of endangered fishes because <br />they are predaceous and occupy the same low velocity shoreline and backwater habitat. <br />However, distribution and abundance patterns of non-native predaceous centrarchids in riverine <br />habitats are poorly understood, as are factors that regulate their establishment and dispersal. <br />In the Grand Valley reach of the Colorado River, Colorado, numerous floodplain ponds <br /> <br />adjacent to the river support populations of predaceous warm water fishes. Floodplain ponds are <br />being actively managed to improve fishing opportunity and may represent a chronic source of <br />green sunfish and largemouth bass that escape and colonize riverine backwaters used by rare <br />native fishes (Martinez et al. 2001). A monitoring program that accurately tracked abundance of <br />these non-native centrarchids in riverine backwaters would be a means to determine trends in <br />escapement (Colorado Division of Wildlife et al. 1996). <br />Annual sampling (monitoring) has been conducted in the Colorado River in the Grand <br />Valley since 1982 and the Interagency Standardized Monitoring Program (ISMP) was <br />implemented in 1986 (McAda et al. 1994). The ISMP was developed to monitor population <br />trends of endangered Colorado pikeminnow Ptychocheilus Lucius and humpback chub Gila <br />cypha in the Upper Colorado River Basin (McAda et al. 1994). The young-of-year (YOY) <br />-1-