8119

• UCREFRP Catalog Number: 8119
• Author: Valdez, R. A. and B. R. Cowdell.
• Title: Effects of Flow Regulation and Ice Formation on Overwinter Nursery Habitat and Survival of Age-0 Colorado Squawfish in the Green River Below Flaming Gorge Dam - Draft.
• USFW Year: 1996.
• USFW - Doc Type: Logan, Utah.
• Copyright Material: NO

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Findings of age-O Colorado squawfish in the inflow to Lake Powell indicate that the primary dispersal
flow may be early spring spikes that usually occur in late February and early March, prior to the usual
spring runoff, and just prior to the spring ISl\1P sampling period. Valdez and Cowdell (1994)
reported densities of age-O Colorado squawfish of 1.05 fish/lOO m2 in April 1993, which was
comparable to 1.03 fish/IOO m2 in the lower 120 miles of the Green River and 0.92 fish/100 m2 in the
lower 35 miles of the Colorado River. By August, 1993, only 1 age-O Colorado squawfish was found
in the inflow, compared to 51 in April. A similar catch rate of 1.12 fish/1 00 m2 was reported in April
1994, but only 4 age-O fish were found in August, compared to 73 in April. These results indicate
that large numbers of age-O Colorado squawfish are being transported into Lake Powell in early
spring, but few are surviving.

The spike flows that occur in the Green River in February and March are primarily the result of early,
low-elevation runoff. These flows are independent of releases from Flaming Gorge Dam, and they
originate primarily from the Yampa, White, and Duchesne rivers, although other smaller tributaries
also contribute. Spike flows of varying magnitude occurred in every year ofISMP sampling from
1988 through 1995, and with each spike flow, there was a concurrent increase in water temperature
(Figure 8). We hypothesis that the combination of increased flows and warmer temperatures inundate
overwinter nursery backwaters and provide the cue by which these older age-O fish move to more
downstream locations. While we recognize that not all of the age-O fish in the nursery habitats move
downstream, we believe that movement of a substantial proportion of the fish from the nursery areas
contribute to much of the overwinter decrease in density.

The hypothesis of downstream movement by intermediate size Colorado squawfish is supported by
current size distributions and the historic setting. Valdez et al. (1982) and Osmundson et al. (1996)
reported an increasingly greater proportion of smaller Colorado squawfish in the lower reaches of the
upper Colorado River with distance downstream, suggesting a natural tendency for young fish to
move or to be transported downstream. Valdez (1990) also observed large numbers of age-O
Colorado squawfish transported downstream after a large, late-summer rainstorm nearly doubled the
flow of the lower Green River; these fish were transported for about 25 miles to the Lake Powell
inflow.

Recent observations indicate that young Colorado squawfish historically descended to lower, warmer,
more productive reaches of river such as Glen Canyon. Historically, the Colorado River through
Glen Canyon was a gentle, meandering, alluvial reach of river that would have provided an abundance
of nursery habitat for young fish. This area is now inundated by Lake Powell and no longer available
for the fish.

To test the hypothesis of downstream dispersal byage-O Colorado squaWfish during winter and early
spring spike flows, we compared the percentage decrease in overwinter density of fish with the slope
of cumulative flow for the period between samples (i.e., late September to late March). Presumably,
flow periods with steeper slopes would reflect a greater occurrence of flow spikes. We found no
clear relationship in this analysis. However, this analysis may not be sufficiently sensitive to
discriminate the occurrence of short-duration spikes. Flow spikes of only a few days may be of
sufficient magnitude to inundate backwaters and cue fish to move.

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