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 8. Geometric-mean CPE (fish/m1) of NNC (fathead minnow, red shiner and
sand shiner) during fall ISMP sampling in Reach 1 and 2 (McAda et aI.,
1998), and NNC drift densities at Lorna (Anderson, 1999) and Moab
(Trammel, 1999) with coefficients of determination between NNC density
at Reach 1 and 2, and between backwater and drift sampling in the same
reach.

BACKWATER Estimated
YEAR NNC. Densitv NNC Drift BW-DRlFT
Fish/100m3 Abundant'e Rank
ISMP ISMP Reach 1 Moah
1996 11.6 1 055 191 1-3
1994 8.9 3 325 758 2-4
1992 7.7 976 200 3-2
1993 6.8 4 140555 4-5
1995 3 965 335 5-1
IS1\1P Reach 2 Loma
1992 5.5 597 009 1-1
1994 2.8 512212 2-2
1995 2.6 181 363 3-3
1993 1.4 64015 4-4
1996 1.1 14 683 5-5
R2 (RI-R2) 0.042 (RI-R2) 0.098 roW-DR) 0.776

Colorado Pikeminnow YOY Size and Survival

Mean length ofYOY in summer backwater samples was higWy variable between
years and negatively related to water temperatures with greater size in low runoff years
and reduced size in high years (Table 9). The year with the smallest mean size during
summer backwater seining was in 1995 when only one specimen at 12 mm was collected
on August 19' which was also the same day as the Lorna median drift date.

The year with the largest mean YOY size in summer backwaters was 1994, the
year with the earliest median drift date of July 12th (Table 9).

Assuming the Colorado pikeminnow YOY have a strong habitat preference for
backwaters in the spring, the data (Utah-backwater) strongly indicate improved over-
winter survivorship during low flow years (Table 6). This is probably a function of higher
average size ofYOY in the fall. The credence of the data is suspect, but the year (1994)
with apparently the highest over-winter survival rate (214%) also had the largest average
size in the fall (50.7 mm)(Table 9). The two high flow years, which had above average
drift densities, had the lowest average size (31.6 and 35.0 mm) and also the lowest
densities ofYOY in the spring samples (Table 8). The year with the highest YOY fall
density (1996) and the second highest fall size (43.2 mm) was not sampled in the spring.