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Objectives: 1) to determine 3-year trends in abundance, distribution, reproductive success, and size structure of non-native, non-salmonid gamefish populations in the Yampa and Colorado rivers. 2) to determine the incidence of predation by non-native, non-salmonid gamefish species upon the endangered fishes. 3) to determine relative habitat use and preferences of adult northern pike versus Colorado squawfish in the Yampa River during pre-runoff, runoff, and post-runoff flow regimes. STUDY AREA The Yampa River is located in the northwestern comer of Colorado (Figure 1). Originating at 7500 ft elevation near Yampa, Colorado in Routt County, the Yampa River is formed at the confluence of two tributary creek drainages in the Gore Range to the east and the Bear River draining the Flattops Wilderness Area to the west. Transitions in aquatic wildlife from coldwater to cool water communities occurs between RM 170 near Hayden and Craig at RM 140, and from coolwater to warmwater communities from Craig downstream to Duffy Mountain (RM 118). Sampling occurred in the Yampa River from RM 170 to its confluence with the Green River in Dinosaur National Monument. The Juniper, Maybell, and Lily Park sampling reaches could be characterized as low-gradient, bottomland topography with the river contained within incised meanders. These three reaches are separated by Juniper and Cross Mountain canyons, which are short, high-gradient reaches of 1.6 and 2.0 river miles respectively. Above these reaches, the Craig sampling reach extends from Hayden, Colorado to RM 118, and consists of low-gradient, bottomland topography for approximately 36 river miles. The river then follows a generally steeper gradient for the next 29 river miles, at which point the low-gradient topography of the Juniper reach begins. Below the Lily Park reach, the river enters Yampa Canyon, which includes the lower 45 miles of the Yampa River within Dinosaur National Monument. METHODS Fish population samplin~ Data for objectives 1 and 2 were obtained by conducting intensive sampling with electrofishing, netting, seining, and angling in four distinct reaches of the river during April through November in 1987 through 1991. Reaches within the study area included Lily Park- Deerlodge Park (RM 45-55), Maybell-Sunbeam (RM 59-89), Juniper Springs (RM 90-118), and Hayden-Craig (RM 118-170). From 1987 through 1989 field seasons, fish samples provided data on abundance, distribution, reproductive success, and size structure of riverine populations of non-native, non-salmonid gamefish species present. Northern pike collected during this sampling effort were also implanted with transmitters for radio-tracking to determine seasonal 2