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A total of 47 Colorado squawfish (317-800mm TL) and 13 razorback sucker (500-560mm TL) collected from the Green, White, and Yampa rivers, 1974-1986 were aged by sectioned and polished otoliths and by vertebral centra. Additional museum specimens of six Colorado squawfish (collected 1962-1970) and eight razorback sucker (collected 1926-1982) from various locations were also aged. Colorado squawfish ranged in age from 3 to >31 years and razorback sucker were 20 to >40 years of age. Ages of similar-size Colorado squawfish were generally the same between old (pre-1970) and recent collections. Razorback suckers collected in the Green River were all old fish (average 27 years), presumably hatched before closure of Flaming Gorge Dam, whereas one fish collected in 1961 was only 5 years of age. Although based on a limited number of specimens, this study suggested that Colorado squawfish are recruiting to the existing population and age-size relationships of older fish are similar to the past. Razorback sucker recruitment has been limited or nonexistent in the Green River since the 1960s. (Preliminary Abstract) Study 4: Fish community interactions of endangered and introduced fishes. Report 11- Fish associations and environmental variables in age-0 Colorado squawfish habitats, Green River, Utah. (Haines and Tyus 1990) Age-0 Colorado squawfish Ptychocheilus Lucius were sympatric with seven native and 15 introduced fishes in the Green River, Utah, 1979-88. Larval Colorado squawfish was the most abundant fish captured in summer drift (38%), which included suckers Catostomus spp. (33%), speckled dace Rhinichthys osculus (14%), and channel catfish Ictalurus punctatus (3%). However, ephemeral shoreline embayments (backwaters) used by Colorado squawfish as nursery habitats were dominated by introduced fishes, including red shiner Notropis lutrensis (87%) in summer, and red shiner (65%) and fathead minnow Pimephales promelas (16%) in autumn. Most fishes in backwaters inhabited a wide range of environmental conditions, but were most abundant in habitats deeper than 15 cm. In general, fish abundance was highest in years of lower, more historic summer flows. We detected no segregation in habitat use between young Colorado squawfish and non-native fishes. Hydrologic conditions in the regulated Green River presumably favor the establishment and proliferation of fish invaders from more mesic environments. (Abstract) Report 12- Diets of young Colorado squawfish and other fishes in backwater habitats, Green River. (Muth et al., in prep) Diets of young Colorado squawfish and other fishes in backwater habitats were investigated summer and fall 1987. Analyses of about 1,024 Colorado squawfish stomachs (fish ranging in length from 8-73mm total length) indicated a predominance of chironomid larvae and cyclopoid zooplankton. Of all stomachs containing food (95%), less than 1% (n=10, 21-73mm total length) contained fish or fish parts, suggesting that piscivory by young Colorado squawfish may not become widespread until the fish attain larger sizes. Preliminary analyses of backwater fishes of all sizes from all locations suggests that diet of young Colorado squawfish is overlapped most by native 12