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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LIST OF KEY WORDS

Colorado pikeminnow, larval drift, Colorado River, Gunnison, Aspinall Unit, Colorado River native
fish, IS-Mile Reach, 18-mile reach

EXECUTIVE SUMMARY

This study was part of the Aspinall study and initiated in 1992. The objectives were to
determine the relative abundance of Colorado pikeminnow larvae produced by wild adults in the
upper main stem Colorado River and to determine correlations between flow and habitat availability
upon growing season and size of Colorado pikeminnow young-of-the-year. Colorado pikeminnow
reproductive success was determined by collection of larval fish in drift nets at three locations on the
Colorado river and three locations on the Gunnison river for three to five year periods. The three
stations sampled for all five study years were: the Colorado River near Loma at RK 247; the
Colorado River just upstream of the Gunnison confluence at RK 275; and the Gunnison River (below
Redlands diversion) (RK 3). A station located on the Gunnison River above the Redlands Diversion
at RK 8.8 was sampled for four years and sites on the Colorado River located at RK 270 and on the
Gunnison River at Bridgeport (RK 47) were sampled for three years.

Larval Colorado pikeminnow were collected at all three stations in the Gunnison River, but in
not every year. Larval Colorado pikeminnow were collected but only during one year at the IS-Mile
Reach site, just upstream of the confluence. Pikeminnow larvae were collected during the three years
of sampling at the Middle station located 2.2 mile downstream of the confluence and during all five
years at the Loma station.

The timing of reproduction was earlier during the two low flow years when water
temperatures reached 18C in early June than in the high flow years when it took longer for water
temperature to warm to that level. Larval abundance estimates were made each year for all species
collected. A positive relationship between Colorado pikeminnow abundance and the native fish
abundance was found. Correlations were made between larval abundance and flow variables. A
strong positive relationship (significant) was found between native and Colorado pikeminnow larval
abundance and the mean flow between July 1 and August 15 (drift season). There was a non-
significant (alpha = 0.30) negative tendency between flow and abundance of nonnative larvae in the
drift. The highest flow year produced the highest density and drift estimates for native and Colorado
pikeminnow and the lowest flow year produced the least numbers of native and pikeminnow larvae.

Different trends were found for pikeminnow drift abundance between years at Loma-
Westwater, Westwater-Moab and Loma-Moab. The biggest discrepancy was in the intermediate flow
year of 1996. In 1996 drift density was low (fourth offive years) at Lorna, but high at Westwater
and Moab (second offive years). Also no relationship was found between larval drift densities at
Loma with young-of-year densities sampled in nursery habitats near Moab. The highest pikeminnow
and nonnative cyprinid densities during fall backwater seining were in 1996, the year with the least
number of native and nonnative larvae measured in the drift at Loma.

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