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Habitat selection by Colorado pikeminnow Habitats were classified into 6 geomorphological types, as defined in Table 1. Most ~ habitats were associated either with Scour Channels (SC) or Migratory Sandwaves (MS). Other types of habitat such as those associated with horseshoe vortexes (HS) or flooded tributaries (FT) were relatively uncommon. Furthermore, the vast majority were true backwaters rather than isolated pools or areas with flow, so all habitats were pooled together. Selectivity analysis of Colorado pikeminnow use of habitat by geomorphological type ~ revealed a preference for Scour Channel (SC) habitats. In the concurrent Green River nursery habitat study multinomial analysis showed a clear preference for SC habitats, using both the number of occurrences of each habitat type, and the percent that each habitat type comprised of the total sampled habitat area. Although the same analysis of Colorado River data also revealed preference for scour channel habitats in the upper section, in the lower section no significant +~ preference was seen (Tables 14 and 15). Colorado pikeminnow used all the habitats to some extent. In particular, horseshoe vortex habitats were used in a greater proportion than they occurred, particularly in the upper section where they were more frequently encountered. We observed in the field that juvenile pikeminnow seemed to preferentially use HS habitats, but too few juveniles were collected to provide an analysis. The mean catch rate (#CPM/100 mZ) for A Colorado pikeminnow for all sampling occasions was higher in the scour channel habitats than other type (Figure 13) The non-native cyprinids used the SC habitats in at least the same proportion as the habitats occurred. No preference was determined, because NNC were present in almost every habitat. However, total numbers and catch rates ofnon-native cyprinids were higher in the SC ~ habitats than the M5 habitats. In any case, the NNC overwhelm pikeminnow in pure numbers in all habitats (Figure 14). In general, SC habitats were larger in area and volume, and deeper on average and by maximum depth than MS habitats, and provided the majority of the available habitat. Usually, the average maximum SC temperature was less than MS habitats in low/medium water years, ~ and greater in high water years, but this was not a consistent phenomenon. The amount of SC habitat available was less closely correlated with peak flows and sampling flows than total habitat availability. 18 .