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I I I I I I I I I I I I I I I I I I I The size structure for all three native species in 2001 was similar to earlier years at each site. There were very few to no small bluehead sucker, flaooelmouth sucker, or roundtail chub at Sevens and Duffy on the Yampa. In contrast, the Dolores River had only small fish and mean size was smaller in 2001 than the prior year for all three species. The Colorado River had all sizes of these species. The factors that make the Colorado River more conducive for native fish include adequate instream flow that provides habitat availability for all age-groups. Nonnative fish predators were not impacting survival ofYOY and recruitment of adult fish and also white sucker and white-native sucker hybrids are not significant in the Colorado River as is the case at Duffy. Size structure in the Yampa River would probably be more similar to the Colorado River if non-native predators and suckers were not present. The Yampa River has experienced two consecutive low flow years and during this period a shift from northern pike to smallmouth bass as the primary predator was observed. As this trend continues, the size structure of native fish may continue to change. The Dolores River probably had large adult size fish prior to regulated flow, but adults were rare at the study site. There was a high number of yearling fish in the 2001 sample showing adult fish must be reproducing successfully somewhere in the Dolores River. A very wide range in size structure for native fish was found between the rivers and sites and this will be studied in terms of habitat differences between them. A small number ofYOY and yearling (< 15 cm) white sucker were present at Sevens in 1998 (14) and 1999 (15), but in 2000 the number ofYOY (119) was much higher. The 2001 sampling found a strong mode at 18 cm for at Sevens (Figure A19) showing a strong 2000 year- class as age-one fish. The same is also the case at Duffy, a strong white sucker 2000 year-class was found in both 2000 as YOY in 2001 as age-one (Figure A20). Conversely, yearling bass (13 to 20 cm) were rare at both stations in 2000. The could indicate a predator prey relationship; few yearling bass and weaker predation in 2000 allowed a strong white sucker year-class to develop in 2000. In 2001, yearling bass were common at both Sevens and Duffy and in that year few yay white sucker were found. White sucker between the sizes of 19 and 35 cm have been rare at both Sevens and Duffy during the study period. Ifwhite sucker move into this size-group in 2002, that would be very strong evidence that the northern pike population was been effectively reduced in that year. Most of the white sucker on the Colorado River (Figures A21 and A22) were taken from slow backwater habitats, not the main channel as on the Yampa River. White sucker YOY numbers were very strong in 2001, indicating beneficial aspects to the lower flows in 2001 for their reproduction. Also, yearling white sucker were common in 2001. In general, main channel habitats on the Colorado River have faster currents and most white sucker were taken from backwaters. This suggests that adult white sucker are not significant competitors for resources with adult native suckers. However, the white-flannel mouth (Figures A25 & A26) and white- bluehead (Figure A29 & A30) are found in the main channel and probably are a competitor, but hybridization does not appear widespread at this time. Size structure of the white-flannel mouth cross (Figures A23 and A24) and the white- bluehead sucker (Figures A 27 and A28) cross at the Sevens and Duffy (Yampa) sites has consistently shown the impacts of northern pike predation from 1998 to 2000. Their size structure in 2001 was again comprised primarily of only large individuals. Large size has apparently been their only defense against large predators, but large size is a disadvantage during low flows when habitat availability for large fish is restricted. Since the white sucker group has been the largest taxon at Duffy, this group should be most reactive to significant environmental 31