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

20

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

may be far enough upstream of the dam so that some larvae find suitable nursery habitats
well upstream of the diversion structure.

Colorado Pikeminnow Distribution. Colorado River. Colorado

In the first two years of the study Colorado pikeminnow larvae were routinely
collected at Loma (RM 153.7), but not at the two upstream sites (Upper Colorado RM
171.4 and the Lower Gunnison RM 1.8). This suggested that the source of drifting larvae
(the spawning location) was upstream ofLoma, but below the confluence of the Gunnison
River. Sampling in subsequent years also indicated that Colorado pikeminnow spawning
activities were primarily located downstream ofthe confluence of the Gunnison River. A
sampling station, located at RM 167.8, was added in 1994 to help define the reach of river
where known pikeminnow spawning occurred. This station, referred to as the Middle
Colorado station, and was located 1.1 miles downstream of a suspected spawning site near
Connected lakes (Doug Osmundson, personal communication) and is 3.6 miles
, downstream of the upper Colorado River station and 2.2 miles downstream of the
Gunnison River confluence. The appearance of pikeminnow larvae at this location
indicated the existence of spawning activities between this site and the two upstream
stations. The pikeminnow larval catch rate at the Middle station was less than Loma
(about 60% in 1994 and 1996, and 20% in 1995), indicating a smaller larval population
there, presumably because fewer larvae were produced in the river above this station.

The Upper Colorado station had comparatively low drift estimates for native
species and Colorado pikeminnow in all study years. Colorado pikeminnow spawning in
the 15-Mile Reach was documented by the collection of three larvae at the Upper
Colorado Station in 1995. That was the only year larvae were collected at this site and the
year with the highest summer flows. High summer flows appear to be a contributing
factor for a general increase in larval numbers in 1995 because pikeminnow catch rates
were highest that year at all sites. The Loma 1995 Colorado pikeminnow larval catch rate
200% higher in 1995 compared to the next highest year and was 330% higher than the
average of the four other years.

Osmundson and Kaeding (1989) reported that adult pikeminnow were present in
the 15-mile reach during the 1986, 1987 and 1988 spawning seasons, but that despite
intensive sampling no larvae were collected in those years. They also reported that two
pikeminnow larvae were collected in the IS-mile reach in 1982. In contrast to 1995, 1982
was not a high runoff year. The peak flow in 1982 was 13,400 cfs at the Cameo gauge
and it was 29,100 cfs in 1995. Peak flow in 1993 (22,200) and 1996 (20,700) were than
in 1982, which was in turn higher than in 1994 (11,000) and 1992 (7990). Larval
pikeminnow collections continued to be very rare in the IS-mile for this study, but were
more commonly collected 2.2 miles downstream of the confluence at the Middle station.