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Northern pike were introduced into Elkhead Reservoir, an impoundment on the Yampa River drainage, in 1977 (P.J. Martinez, personal communication) and collected in the mainstream Yampa River as early as 1979 (E.J. Wick, personal communication). Their numbers increased in the upper Yampa River in the early 1980s (Wick et al. 1985) and a downstream movement into the Green River was subsequently documented in 1981 (Tyus et al. 1982b). Northern pike reproduction has been reported in the upper Yampa River drainage, where it has access to the mainstream river (T.P. Nesler, personal communication). Walleye presumably accessed the mainstream Green River by moving downstream from various tributaries. The fish was first reported in Utah in 1951 (Sigler and Miller 1963), and reproducing populations of walleye were established by fish stockings in Duchesne River reservoirs (Fig. 1) in the 1960s and 1970s (G.M. Davis, personal communication). The Green River basin of Colorado and Utah is an important recovery area for four rare and endangered Colorado River fishes (reviewed by Joseph et al. 1977, Carlson and Carlson 1982, U. S. Fish and Wildlife Service 1987). However, over twenty non-native fishes have been introduced into the basin for sport, forage, food, or by accident (Tyus et al. 1982a). Impacts of these introduced fishes on the native fauna are not well understood, but the presence of two large piscivores, northern pike, Esox Lucius, and walleye, Stizostedion vitreum, in areas presently occupied by endangered fishes, is cause for concern. Control of non-native fishes has been identified as a recovery measure under provisions of an interagency recovery program for endangered fish species in the upper Colorado River basin (U.S. Fish and Wildlife Service 1987). Fish introductions in other locations have eliminated 2