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INTRODUCTION An important recent initiative of the Recovery Program for the endangered Colorado River fishes has been control of introduced fishes. Introduced species have been implicated in the decline of endangered fishes nation wide because of predation or competition for food or space (e.g. Ono et al. 1983; Courtenay and Kohler 1986). A survey of knowledgeable upper-basin biologists by Hawkins and Nesler (1991) identified several species that are believed to have negative effects on the endangered fishes of the upper Colorado River basin; 46% of their respondents believed northern pike Esox lucius to be an important problem for the native fish community. Northern pike have been stocked in several upper basin lakes or reservoirs and have escaped to the mainstem rivers of the basin. In the Yampa River drainage, pike were stocked into Elkhead Reservoir on Elkhead Creek, a tributary to the Yampa. The species escaped from the reservoir and established aself-sustaining population in the mainstem Yampa River and associated floodplain ponds and sloughs (Nesler 1995). Northern pike increased dramatically in abundance in the Yampa River from early escapement to the present. Miller et al. (1982) captured seven northern pike during an intensive river-wide, year-around sampling effort in 1981, but recent efforts by the Interagency. Standardized Monitoring Program have captured as many as 70 in week-long, shoreline electrofishing samples (McAda et al. 1994). These collections were downstream from the major population center for the species which is upstream from Craig, CO (Nesler 1995). The population is large enough to attract considerable interest from sport fishermen. Individuals from this population have moved downstream into the Green River where important habitat for young- of-year Colorado squawfish Ptychocheilus lucius occurs (Tyus and Beard 1990; McAda et al. 1994). Predation on small Colorado squawfish has been documented there (Growl and Lentsch 1996). Northern pike are considerably less abundant in the Colorado River subbasin. Northern pike have occasionally been captured in scattered locations in the river system (e.g. Valdez et al. 1982a, b; McAda et al. 1994; USFWS, unpublished data), but occur in notable numbers only in the Gunnison River near Delta, CO (Burdick 1995). Burdick captured 20 northern 1