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22 I I I I I I I I I I I I I I I I I I I responsible, since both sites have similar physical conditions for flow and temperature between years. Nesler (1995) proposed it was reasonable to assume normal fUnoffflows would be adverse enough to prevent stable recruitment of small mouth bass in the Yampa River. Sevens apparently has a lack of habitat availability for YOY small mouth bass which indicates Nesler (1995) was probably correct in regard to this section of the Yampa. However large numbers of small mouth bass YOY have been produced in all years, even with normal runoff flows, at Duffy and throughout Little Yampa Canyon. This large section of the river provides a stable source of small mouth bass YOY for colonization to downstream sections like Sevens. Hawkins et aI (1997) reported the occurrence of nonnative fish were fairly rare in the Little Snake River and attributed this to highly variable physical factors such as flows, temperature and turbidity. Hawkins et al (1997) suggested the extreme low flows he observed in the Little Snake River during sampling in 1994, might explain the high percent of native fish population collected during his sampling. Because nonnative fish were not common in their study sites on the Little Snake River, they concluded native fish could tolerate the extreme low flow that year. Those conclusions for the Little Snake are not consistent with results found on the Yampa River during this study. In the Yampa River, native fish versus nonnative composition was found to be primarily a function of local channel morphology or habitat availability, not water quality parameters. Also the Little Snake River fish population was mostly comprised of fish less than 15 cm, not the larger or adult size fish typical of the Yampa, whose habitat requirements need to be a strong consideration in regards to flow assessment. In the Yampa River nonnative species composition for fish less than 15 cm improved greatly in years with low base flows at all sites and it is suspected that this will impact the over 15 cm fish population in the long-term. The low flows in 2000 nearly eliminated all riffle habitats, defined as areas with velocity over 0.6 m/sec, from the Duffy section. Riffles are the principle habitat for speckled dace and aquatic invertebrates, and presumably carrying capacity for riffle obligate species was negatively impacted in 2000 and 2001. The reduced velocities experienced at 2000 flows may not exclude dace from remaining riffles in and of itself, but reduced velocities in riffles made them suitable habitat for YOY small mouth bass. In 2001 it was found that YOY small mouth bass occupied riffle habitats in Little Yampa Canyon. This makes it clear that YOY small mouth bass are very tolerant of low flows and are an efficient predator and/or competitor with species that require habitat with swift currents. It could be that small mouth bass YOY will not occupy riflle habitats when velocities and flow in riflles exceed a certain threshold. However it is not likely that dace can quickly become reestablished in the Yampa River even if high base flows return in the near future. The long- term implications of low base flows are that trophic relationships natural to native fish have been impacted. There appears to be a relationship between YOY bass abundance and reduced YOY of most other species. In 2001 YOY bass could forage on invertebrates in the productive riflle habitat, but not dace or sculpin which explains the YOY bass population increase in 2001. It does not appear that any of the native species are an efficient predator on YOY bass and that larger bass may be their only efficient predator. Northern pike were probably a significant predator on juvenile bass, but pike numbers have been noticeably reduced in 2000 and 2001 either due to unsuitable habitat, forage or management activities (pike removal by the recovery program). It appears that small mouth bass numbers and composition for fish over 15 cm will continue to increase given flow conditions observed in recent years.