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136 GREAT BASIN NATURALIST cies were not found, but plains killifish and sand shiners were collected (Seethaler et al. 1979). Data from this study is most similar to that of Carlson et al. {1979). A comparison with the most recent collections {Colorado River Fishery Project 1982) indicates a high degree of similarity except for: (1) the ab- sence of squawfish and the presence of a pos- sible humpback chub in our collections and (2) the higher densities of redside shiners in our collections. It has been noted by Crosby (1975), Lani- gan and Berry (1979), Carlson et al. (1979), and Valdez and Clemmer (1982) that the pro- portion of nonnative to endemic fishes in- creases as the major tributary streams ap- proached their confluences with the Green River. Lanigan and Berry (1979) attribute the higher proportion of nonnatives to a decrease in habitat diversity, whereas Valdez and Clemmer {1982) have suggested that riverine habitats occupied by native fishes are often Vol. 45, No. 1 unfavorable to nonnatives. Longitudinal data collected on the White River, a major tribu- tary to the Green River, (Lamarra and Carter 1981) show a high correlation between the presence of endemic fishes and coarse sub- strates (riffle areas). Conversely, sandy areas with an absence of riffle habitats in the lower stretches of the White River appear to be avoided by the endemic community. Valdez and Clemmer (1982) also report a similar relationship between substrate size and roundtail and humpback chub. Although no longitudinal data were collected in this study, a low proportion of nonnatives to endemics and natives (1:2) was found in the Yampa River site, suggesting a similar correlation. Fish species distribution, food selection, and feeding intensities were determined by means of a diurnal sampling scheme for fish, drift insects, and larval fishes. These data have provided insights into the structure and func- tion of this ecosystem. It is interesting to note TABLE 5. Calculated linear Electivity Index (Strauss 1979) for the dominant four fish species in the Yampa River near river mile 16.5. Electivity comparisons a re made for benthic (A) and drift (B) or ganisms. Data collected 24-26 July 1981. (A) Benthic Redside Red Roundtail Speckled All Orders shiner shiner chub dace species Ephemeroptera -.016 -.017 -.157 -.068 -.085 Plecoptera -.009 -.009 -.008 -.009 -_009 Trichoptera -.042 -.043 .008 -.001 -.012 Coleoptera .000 .000 .000 .023 .006 Diptera .055 .014 .131 .057 .077 Hymenoptera .013 .043 A29 .000 .022 Lepidoptera -.001 -.001 -.001 .000 .000 Hemiptera .003 .015 .000 .000 .003 Homoptera .000 .000 .000 .000 .000 Annelida -,002 -.002 -.002 -.002 -.002 Hydracrina .000 .(~0 .001 .000 .000 (B) Drift Redside Red Roundtail Speckled All Orders shiner shiner chub dace species Ephemeroptera -.069 -.070 -.210 -.121 -.138 Plecoptera -.001 -.001 .000 -.001 -.001 Trichoptera -.001 -.002 .049 .040 .029 Coleoptera .000 .000 .000 .023 .006 Diptera .055 .014 .131 .057 .077 Odonata .000 .000 .000 .000 .000 Hymenoptera .013 .043 ,029 .000 .022 Lepidoptera .000 .000 .000 .002 .OOI Hemiptera .003 .015 .000 .000 .003 Homoptera .000 .000 .000 .000 .000 Arachnida .000 .000 .000 .000 .000 Megaloptera .000 .000 .0(HI .000 .000 Hydracrina .000 .000 .001 .000 .000 ~'. .: