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piscivorous fish species that inhabit backwater habitats year-round. The omnivorous channel catfish ktaluras punctatus is ubiquitous throughout main-channel habitats of the upper Colorado River and is commonly found in flooded backwaters during spring runoff (Osmundson 2002) and sometimes occurs in fall seine surveys of backwaters (McAda et al. 1994); however, only large channel catfish are believed piscivorous (Tyus and Nikirk 1990). To date, non-native northern pike Esox lucius, smallmouth bass Micropterus dolomieui, and black crappie remain relatively rare, being captured only sporadically during various sampling efforts (USFWS unpublished data). The most likely place for young Colorado pikeminnow and razorback sucker to come in contact with non-native, predacious fish is in backwaters year-round and in flooded ponds and bottomlands during spring runoff. Colorado pikeminnow YOY first appear in backwaters in July or August of the year they are hatched and may remain in these habitats through the following June. In July, larvae of a new year-class move into the backwaters as most one-year-old pikeminnow presumably move to other habitats (USFWS unpublished data). Hence, backwaters are used year-round by various sizes (8- 110 mm TL) of age-0 Colorado pikeminnow. There has been a decline in numbers of young Colorado pikeminnow in upstream reaches of the Colorado River (Westwater Canyon, Utah, to Grand Junction, Colorado) during the past decade: capture rates have been very low in recent years compared to those in the late 1970's to mid-1980's (Haynes et al. 1984, Osmundson and Kaeding 1989, McAda and Kaeding 1991, McAda et al. 1994, Osmundson and Burnham 1998). Mean catch rates of largemouth bass and green sunfish in fall seine surveys in the upper Colorado River have been highest in the reach upstream of Westwater Canyon (McAda et al. 1994). The source of these centrarchids is unknown, but reproduction might occur in both off-channel ponds connected to the main channel via irrigation returns as well as in the backwaters themselves. Chemical treatment of ponds, recommended by Tyus and Saunders (1996), has been conducted in the Grand Valley by the Colorado Division of Wildlife (CDOW) as part of Recovery Program activities (Anita Martinez, CDOW, personal communication). While this approach was being tested as a means to reduce potential non-native fish immigration, the project summarized here was 3