9299

• UCREFRP Catalog Number: 9299
• Author: Anderson, R. and J. Mumma.
• Title: Aspinall Studies
• USFW Year: 1999.
• USFW - Doc Type: Annual assessment of Colorado Pikeminnow Larval Production in the Gunnison and Colorado Rivers, Colorado 1992-1996.
• Copyright Material: NO

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· Drift density for Colorado pikeminnow was inconsistent at Loma between years with
similar flow and temperature conditions. The two low flow years (1992 and 1994)
did not produce similar numbers of pikeminnow larvae in the drift. Also both native
and nonnative larvae numbers were higher in 1992 compared to 1994 suggesting
increased carrying capacity for all species in 1992.

· 1992 followed a low runoff flow year (1991), while 1994 followed a high flow year.
The occurrence of two low flow years in a row may have contributed to differences
observed between 1992 and 1994, but not enough data is available to support a
multiple year effect.

· Drift densities for Colorado pikeminnow larvae and for native fish as a group were
positively correlated (significant P> 0.05) with each other indicating flow conditions
conducive for native larvae production in general was also conducive to production of
pikeminnow larvae.

· Transport abundance of nonnative species larvae was in general negatively correlated
with discha:ge with the least number of nonnative larvae produced in high flow years.

· The lowest flow year (1994) produced the highest nonnative drift densities at Loma.

· Colorado pikeminnow densities estimates for Lorna and Westwater indicate that
Colorado pikeminnow larval densities drop between these locations which does not
support assumptions of a progressively increasing downstream drift.

· Correlations for Colorado pikeminnow larval densities between Loma and Westwater
and Westwater and Moab were poor, but were fairly good between Loma and Moab.

· Drift density for Colorado pikeminnow was inconsistent between stations in 1996, the
intermediate flow year. In 1996 drift density was lowest at Loma, but was high at
Westwater and Moab.

· Strong positive correlations between Colorado pikeminnow larval densities in the drift
for all sites (Loma, Westwater and Moab) and Colorado pikeminnow YOY sampled in
summer and fall seining were not identified.

· Years with increased larval production may have had positive effects on year-class
strength prior to inundation of the Colorado River (Lake Powell), but in this study
Colorado pikeminnow larval abundance at Loma was not a good predictor of
abundance ofYOY in fall samples.

· Non native cyprinid (NNC) densities in fall and spring backwaters were found to be
strongly associated with runoff flow conditions with low densities in the high years and
high abundance in flow low years.