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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A total of only 16 Colorado pikeminnow larvae were collected on the Gunnison
River during the five years. Two Colorado pikeminnow larvae were collected just above
the confluence in 1992, one in 1995, and three were collected there in 1996 (Appendix 1,
Table 4). Two Colorado pikeminnow larvae were collected above the Redlands diversion
dam in 1995 and three were, collected there in 1996 (Appendix 1, Table 5). Colorado
pikeminnow larvae were collected in each of the three years the Bridgeport station was
sampled with one in 1994, three in 1995 and one in 1996 (Appendix 1, Table 6).

Colorado Pikeminnow - LOMA Catch Characteristics and Drift Estimates

The entire Colorado pikeminnow reproductive season may not have been sampled
in all five of the study years. Forexample, in 1992 the first year of the study, sampling
was done on June 25 and 26 and continuous sampling began on July 1 (Appendix 2,
Figure 1). No CPM were captured in June but they were captured on July 1, and could
have been present prior to continuous sampling. In 1993 the drift period was later in the
summer and the sampling period appeared to include the beginning of the drifting season
but may have been terminated prior to the end of the pikeminnow drifting period.
Sampling was terminated on August 9, 1993 and 4 Colorado pikeminnow larvae were
collected on August Sth sample (Appendix 2, Figure 3). In 1994 large numbers of native
fish larvae were collected on the first day of sampling, June 27, however the first
pikeminnow was not captured until July 5, nine days later (Appendix 2, Figure 5). It
appears the entire pikeminnow drift season was sampled in 1995 and 1996 (Appendix 2,
Figures 7 and 9).

The drift season (days between first and last capture) of Colorado pikeminnow
varied between 12 and 23 days. The pikeminnow spawning season, estimated from back-
calculated hatching dates, was between 21 and 38 days (Table 3). The earliest date that
Colorado pikeminnow larvae were sampled was in 1992 on July 1 (Table 3). Water
temperature rose to 180C by June 9 in 1992 and the estimated first spawning date was
June 12. In 1994, the other low and early runoff year, water temperatures reached 180C
on June 10 and the first estimated spawning date was June 17. The estimated first
spawning date in 1993 was July 3 when water temperature was 160C. The latest larvae
first appeared in a sample was on August 8 in 1995 (Table 3), which was the year with the
highest flows and coldest water temperature. The first estimated spawn date in 1995 was
July 21, over 40 days later than in 1992 and 1994.

In the two warmer years (1992 and 1994), estimated initiation of spawning (June
12 and 17, respectively) came after water temperatures had reached 180C (Table 3).
These two years also had the lowest accumulated degree-days prior to first hatch, 1456 in
1992 and 151S in 1994. The estimated date for initiation of spawning preceded the date
water temperature reached 180C in the other three years. Spawning was initiated 7 days,
8 days, and 3 days before water temperatures reached 18 C in 1993, 1995 and 1996,
respectively. Initiations of spawning or peaks in spawning were did not appear to be
associated with sudden increases in flow or hydrograph spikes (Appendix 1, Figure 1).