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RESOURCE OVERLAP OF AGE-0 COLORADO SQUAWFISH WITH OTHER FISH SPECIES IN THE GREEN RIVER, FALL, 1980. Charles W. McAda1 and Harold M. Tyus U.S. Fish and Wildlife Service 447 E. Main Street Vernal, Utah 84078 ABSTRACT i Age-0 Colorado sfish (Ptychocheilus lucius) were captured with seines in the Green River during September 1980. Young Colorado squawfish were found in a variety of habitats but were most abundant in shallow backwaters, especially in the afternoon when water temperatures were warmest. Resource use was investigated for spatial, temporal. and dietary components by multiple disc 'nant analysis. A single discriminant function, principal y contributed to by water velocity, accounted for 82% of the explainable variance among fish microhabitat tunes. HL&h spatial overlap in habitat use occurred bet n Colorado squ? wish and red shiner (Notropis lutrensis) and fathead minnow (Pimephales promelas). Little overlap was indicated between Colorado squawfish and channel catfish (Ictalurus punctatus) and flannelmouth sucker (Catostomus latipinnis). Smaller Age-0 Colorado squawfish (22-40 mm TL) fed predominantly 2.n invertebrates ,and some fish; larger Colorado squawfish (40- 9 mm TL) fed heavily on fish -- pre-dominantly red shiner. Dietary overlap of small Colorado squawfish and small (15-55 mm c annel catfish and red shiner was high, but declined as the Colorado squawfish grew and became more piscivorous. There was n9 significant dietary overlap of Colorado s quawfish and fathead minn2w. Although high spatial and dietary overlap was noted between small Colorado squawfish and red shiner, competition between these species was not demonstrated. INTRODUCTION The Colorado squawfish Ptychocheilus lucius was historically distributed throughout the large rivers of the Colorado River Basin. However, its numbers have declined dramatically in the recent past: it is now extirpated from the Lower Basin (Minckley 1973) and is rare in the Upper Basin (Holden and Stalnaker 1975; Tyus et al. 1982). It is currently considered endangered by the U.S. Fish and Wildlife Service and by the American Fisheries Society (Deacon et al. 1979). Modification of free flowing river habitat to a lentic environment through the construction of dams and reservoirs has been a major factor in the decline of the Colorado squawfish (Vanicek et al. 1970; Holden 1979). However, impacts to the remaining free flowing reaches such as reduced streamflow, altered discharge and temperature regimes, water quality changes, and introduction of exotic species are also believed to be detrimental to the Colorado squawfish (Holden 1979; Seethaler 1978). Competition with the red shiner (Notropis lutrensis), redside shiner (Richardsonius balteatus), and other introduced species is considered by some workers as adversely affecting the survival of small Colorado squawfish (Seethaler 1978; Behnke and Benson 1983). However, evidence to support these hypotheses are generally circumstantial. Although competition with introduced species is often 1Present address: U.S..Fish and Wildlife Service, Grand Junction, Colorado 81505 551 2511 Road, Suite B-113, 44