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

22

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

characterized by high summer flows with much cooler water temperatures, which
apparently explains why fish reproduction was 3 to 5 weeks later than in other years.
Flows in July 1995 ranged from 35,000 to 15,100 cfs and habitats resulting from flows in
this range appeared to have been beneficial for native larval production, but detrimental
for abundance of nonnative species larvae. Other studies have reported that native fishes,
usually in regard to the adult-life stage, seem to tolerate higher flow events than nonnative
species (McAda and Kaeding, 1989; Hawkins and Nesler, 1991; and Muth and Nesler,
1993). This study suggests that this is also the case for nonnative larval fish abundance. If
nonnative fish numbers were reduced in 1995 compared to other years, the increase in
native larval could be associated with reduced competition or predation and therefore be
independent of improved habitat availability due to flow conditions.

The year with the third highest Colorado pikeminnow density and drift estimate at
Loma was 1993, the other high flow year. Colorado pikeminnow drift estimates were
similar in 1992 and 1993 even though flows were much higher in 1993. This suggests that
flow and temperature alone do not regulate larval abundance.

Flow conditions in 1996 were moderate and intermediate to the two low flow and
the two high flow years. If the moderate flow year had produced intermediate numbers of
native or pikeminnow larvae there would have been very strong correlations between flow
and larval production for stations located in Colorado. However Colorado larval catch
rates and native fish and pikeminnow were poor in 1996. In contrast, Trammel (1999a)
found Colorado pikeminnow larval abundance on the Colorado River was above average
in 1996 at Westwater and Moab. Density of native species larvae were above average in
two of the Gunnison River stations in 1996. Therefore catch results were inconsistent
between stations, with the three upper Colorado River stations having low native larvae
production and the lower Colorado and Gunnison River stations with high larval
production. Flow conditions alone apparently do not account for the low numbers of
native larvae collected in 1996 at Loma.

Longitudinal Trends in Colorado Pikeminnow Abundance

.Colorado pikeminnow density increased in a downstream direction for the three
drift sampling stations on the Colorado River in Colorado. Near the confluence of the
Gunnison River, catch rates were zero to near zero between 1994 and 1996. At the
Middle Station just 3.2 miles downstream catch rates increased, ranging from 1.0 to 10.2
CPMl1000 m3 between 1994 and 1996. Catch rates increased at Loma by 67% to 400%
over the Middle Station during the same three-year period. This increase in Colorado
pikeminnow larval numbers in a downstream direction suggests that there was at least one
Colorado pikeminnow spawning site located above the Middle station and at least one
spawning location between the Middle and Lorna Stations.

The onset of spawning occurred earlier at Westwater than at Loma in 4 out of 5
years, indicating spawning is occurring between Loma and Westwater. Also the
Westwater station is located below a suspected spawning site, only a few miles upstream