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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Table 3. Colorado pikerninnow larval catch and drift data for Lorna, Colorado
River, 1992 to 1996.

LOMA 1992 1993 1994 1995 1996
# of days sampled 31 40 50 72 61
First date CPM collected I-Jul 23-Jul 5-Jul 8-Aug II-Jul
# of days CPM collected 13 12 15 23 13
# of CPM 52 35 58 226 35
# CPMlI000 m3 4.31 3.16 1.61 8.81 0.98
First spawn date 12-June 3-July 17-June 21-Julv 27-June
Days of Spawnin~ 33 21 38 34 34
Temp. on first spawn date 19.0 16.9 20.0 16.6 16.6
Date of 180C 9-June lO-July lO-June 29-July I-July
Degree days to first spawn date 1456 1552 1518 1799 1533
Mean CPM larvae length (mm) 9.4 9.5 9.3 9.5 9.3
Std. Dev. of mean length 0.56 0.46 0.56 0.72 0.72
Median date of CPM in catch 14-July 29-July 12-July 19-August 2 I-July

Length of Colorado pikeminnow larvae in the sample ranged from 7.5 to Ilmm for
the 5 study years. Mean pikeminnow length at Lorna for the 5 study years ranged from
9.3 mm to 9.5 mm (Table 3), very similar between years due to similarities in age at time
of capture. The median capture date, which represents middle of drift season, varied
between years with the earlier dates in 1994 and 1992 (July 12 and July 14 respectively)
and the latest day on August 19, 1995 (Table 3).

Because the entire drift season was apparently not sampled for some of the five
study years, it was felt that a standardized sampling period should be used to compare
catch characteristics between years and between stations. The standardized season was
set for the 26 days with the highest catch rate of fish larvae per volume sampled. Twenty-
six days was selected because it provided coverage of the year (1992) with the least
number of days sampled. A 26-day period slightly reduces the estimate oflarvae
produced in a year, but it results in an increased average season density (number per
volume) since the days with the lowest catch rates are not included. The total fish drift
estimate was highly variable between years. The highest estimated drift per hour for
Colorado pikeminnow for the standardized period occurred in 1995 and was 268,000
(Table 4). The next highest drift estimate was in 1992 (106,000), followed by 75,000 in
1993 and 28,000 in 1996. The poorest year for catch per cubic meter and drift per hour
for both native fish and Colorado pikeminnow (25,000) was 1994.

Three flow variables [annual peak flow, runoff season (mean of the 3 1 highest
flows), and drift season (mean of flows between July 1 and August 15)] were correlated
against abundance (drift estimate per hour) of native species, nonnative species and
Colorado pikeminnow. The peak and the runoff season variables were highly auto-
correlated (r2 of 0.992). The ~ value for peak flow versus drift season was 0.697, and it
was 0.706 for runoff season versus drift season.

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