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<br />A total of 47 Colorado squawfish (317-800mm Tl) and 13 razorback sucker <br />(500-560mm Tl) collected from the Green, White, and Yampa rivers, 1974-1986 <br />were aged by sectioned and polished otoliths and by vertebral centra. <br />Additional museum specimens of six Colorado squawfish (collected 1962-1970) <br />and eight razorback sucker (collected 1926-1982) from various locations were <br />also aged. Colorado squawfish ranged in age from 3 to >31 years and razorback <br />sucker were 20 to >40 years of age. Ages of similar-size Colorado squawfish <br />were generally the same between old (pre-1970) and recent collections, <br />Razorback suckers collected in the Green River were all old fish (average 27 <br />years), presumably hatched before closure of Flaming Gorge Dam, whereas one <br />fish collected in 1961 was only 5 years of age. Although based on a limited <br />number of specimens, this study suggested that Colorado squawfish are <br />recruiting to the existing population and age-size relationships of older fish <br />are similar to the past. Razorback sucker recruitment has been limited or <br />nonexistent in the Green River since the 1960s. (Preliminary Abstract) <br /> <br />Study 4: Fish community interactions of endangered and introduced fishes, <br /> <br />Report 11- Fish associations and environmental variables in age-O Colorado <br />squawfish habitats, Green River, Utah. (Haines and Tyus 1990) <br /> <br />Age-O Colorado squawfish Ptvchocheilus lucius were sympatric with seven <br />native and 15 introduced fishes in the Green River, Utah, 1979-88, larval <br />Colorado squawfish was the most abundant fish captured in summer drift (38%), <br />which included suckers Catostomus spp. (33%), speckled dace Rhinichthvs <br />osculus (14%), and channel catfish lctalurus ounctatus (3%). However, <br />ephemeral shoreline embayments (backwaters) used by Colorado squawfish as <br />nursery habitats were dominated by introduced fishes, including red shiner <br />Notroois lutrensis (87%) in summer, and red shiner (65%) and fathead minnow <br />Pimeohales oromelas (16%) in autumn. Most fishes in backwaters inhabited a <br />wide range of environmental conditions, but were most abundant in habitats <br />deeper than 15 cm. In general, fish abundance was highest in years of lower, <br />more historic summer flows. We detected no segregation in habitat use between <br />young Colorado squawfish and non-native fishes. Hydrologic conditions in the <br />regulated Green River presumably favor the establishment and proliferation of <br />fish invaders from more mesic environments. (Abstract) <br /> <br />Report 12- Diets of young Colorado squawfish and other fishes in backwater <br />habitats, Green River. (Muth et al., in prep) <br /> <br />Diets of young Colorado squawfish and other fishes in backwater habitats <br />were investigated summer and fall 1987. Analyses of about 1,024 Colorado <br />squawfish stomachs (fish ranging in length from 8-73mm total length) <br />indicated a predominance of chironomid larvae and cyclopoid zooplankton. Of <br />all stomachs containing food (95%), less than 1% (n=10, 21-73mm total length) <br />contained fish or fish parts, suggesting that piscivory by young Colorado <br />squawfish may not become widespread until the fish attain larger sizes. <br />Preliminary analyses of backwater fishes of all sizes from all locations <br />suggests that diet of young Colorado squawfish is overlapped most by native <br /> <br />12 <br />