8159

• UCREFRP Catalog Number: 8159
• Author: Trammell, M. and T. Chart.
• Title: Aspinall Studies
• USFW Year: 1999.
• USFW - Doc Type: Annual Assessment of Colorado Pikeminnow Larval Production in the Colorado River, Utah 1992-1996 \
• Copyright Material: NO

temperatures may result in an exaggeration of the significance of the correlations of flow with
the date of 18°C and the onset of spawning. Temperature and flow variables in combination
influence the onset of spawning activity, but do not precisely determine it. The date the river
reaches 18°C should not be considered as a biological threshold to spawning activity, as much as
an index to the onset of spawning. In any given year, by the time the predictive variables would
have been determined, the fish will have begun spawning.
Relationship of flow and temperature variables to abundance of drifting larvae
The flow variables of peak discharge and average June flow were not linearly correlated
with catch rates and transport abundance of Colorado pikeminnow larvae (Pearson's correlations,
Table 5). However, transport abundance had a slight positive association with higher flows
during peak discharge and with high June flows. The moderate to high flow years generally had
higher transport abundance than the low water years, with some exceptions. At the Moab site,
transport abundance appeared to be maximized in the moderate flow year of 1996, and the high
flow year of 1995 had high transport abundance, but the high flow year of 1993 had low
transport abundance. At Westwater, 1993 had the highest transport abundance, although much
lower than Moab, followed by 1996, with the second highest transport abundance, while 1995
had a low transport abundance. The catch rates (volumetric and total transport abundance) of the
Westwater and Moab sites were not correlated with each other, further supporting the earlier
conclusion that the sites were sampling different populations of drifting larvae, which vary
independently of one another.
Temperature variables were also positively related to larval abundance at both the Moab
and the Westwater sites. The correlations of degree days to spawn, date of 18°C, and onset of
spawn to the volumetric catch rate were significant at the Moab site, but much weaker at the
Westwater site. These correlations are probably an artifact of temperature and flow variables
being closely related, and have little biological significance. Total transport abundance was not
significantly correlated with the temperature variables tested.
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