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<br />ABSTRACT <br />Habitat use and species associations of age-O Colorado squawfish <br />Ptvchocheilus lucius were evaluated in the Green River, Utah, 1979-88. <br />The fish was more abundant (38%) than other species in the summer drift, <br />which included suckers Catostomus spp. (33%), speckled dace Rhinichthvs <br />osculus (14%), and channel catfish Ictalurus punctatus (3%; n = 1,706). <br />However, age-O Colorado squawfish comprised only 3% of summer backwater <br />seine catch, which was dominated by non-native red shiner Notroois <br />lutrensis (87%; n = 102,033). In autumn, age-O Colorado squawfish were <br />captured in 57% of the backwaters seined, but comprised only 6% of a <br />catch (n = 67,179) dominated by non-native red shiner (65%) and fathead <br />minnow Pimeohales oromelas (16%). Colorado squawfish and most other <br />fishes inhabited a wide range of environmental conditions, but were most <br />abundant in backwaters of water depth greater than 15 cm. Red shiner and <br />fathead minnow were abundant in habitats used by young Colorado <br />squawfish, suggesting potential resource sharing. In general, fish <br />abundance was highest in years of warmer temperatures and lower flows. <br />High spring and summer flows were associated with lower standing crops in <br />autumn. We detected no segregation in habitat use between young Colorado <br />squawfish and non-native fish; however proliferation of these introduced <br />fishes should be further evaluated as a factor associated with the <br />decline of Colorado squawfish. <br /> <br />2 <br />