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Bv 2003 Age-O small mouth bass was the most common fish <120 mm collected bv shocking - - - and seining in both ri ffle and shoreline habitats. The upstream extent of smallmouth bass has yet to be determined. 2003 sampling found Age-O small mouth bass comprised 97% of the catch for fish less than 15 cm near Hayden, Colorado. The Double Bridges site (RM 148) was upstream of the Elkhead River confluence, and mean length of Age-O smallmouth bass was 66 cm on September 9[h. Also at Double Bridge about 12% of Age-O bass were larger than 75 cm, indicating that even at this upstream location members of the 2003 year-class were likely to recruit. Smallmouth bass was also the most common species collected at Juniper and Maybell for fish less than 15 cm. Mean length for Age-O fish was about 71 cm (September 9[h), Mean length of Age-O smallmouth bass at Lily Park was 75 cm on September 10,2003. Another change in the Yampa River aquatic community during the drought period was the apparent increase in the crayfish population. Crayfish numbers were not quantified, but were much more noticeable in 2003 than baseline years, The increase in crayfish could also have been a response to altered habitats associated with low base flows. Since crayfish were utilized as forage by adult smallmouth bass, a higher abundance of crayfish may have been selectively advantageous for the small mouth bass population. In 2003. the Duffy and Sevens sites were shifting to a small mouth bass, white sucker and crayfish community indicating tropic relationships had simplified during the study period. Habitat diversity was correlated with the diversity of a fish community (Scholsser 1982), The Shannon diversity index confirmed that habitat complexity in the Yampa River had also simplified due to reduced flows in the dry cycle (Anderson and Stweart 2003), The 2002 flows were advantageous for recruitment of nonnative species, There was a large increase in Age-O and Age- I white sucker and carp in 2003 without improvements in numbers of Age-O or Age-I native fish at Sevens. Since white sucker had higher recruitment during 2002 their relative abundance was expected to increase at a faster pace than native sucker. A higher proportion of white sucker could further impact native sucker species through increased hybridization at Sevens, and perhaps at Lily Park. Channel catfish have been identified for negative impacts to native fish populations in the Colorado basin (Tyus and Nikirk 1990). The coexisting populations offlannelmouth sucker, bluehead sucker and channel catfish at Lily Park in 2000 indicated native sucker have not been severely impacted by channel catfish. The lack of round tail chub at Lily Park is consistent with negative impacts attributable to channel catfish. The U. S, Fish and Wildlife Service conducted channel catfish removal in Dinosaur Canyon from 1998 to 2001, which is from 10 to 50 miles downstream of Lily Park. Mark Fuller of the U.S, Fish and Wildlife Service (personal communication) estimated the channel catfish population to be nearly 320 catfishlkm in Dinosaur Canyon, which is about a fourth of the estimate for Lily Park. The total number of channel catfish removed from a 50-mile reach in Dinosaur Canyon in 1998 and 1999 was 4,272 fish (Modde and Fuller 2002), 36 I I I I I I I I I I I I I I I I I I I