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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While it appears that a reduction in frazil ice and jam ice development would be beneficial to nursery
backwaters and hence to the survival of the fish, we caution against reaching the conclusion that low,
steady releases in severe winters would produce these desirable effects. Low volume releases have
not been observed on the Green River during a severe winter, and our understanding of the effective
variables and interrelationships is limited.

Another important consideration is the size and depth of backwaters available during winter periods.
Valdez and Cowdell (1993) found less reduction in overwinter densities of age-O Colorado squawfish
for deeper backwaters, indicating higher survival rates offish in deeper backwater; i.e., fish density
was reduced by 82%,56%,42%,50%, and 15% for backwaters with depths <1 ft, 1-2 ft, 2-3 ft, 3-4
ft, and >4 ft. These results suggest that either deeper backwaters are more persistent under varying
flows or that ice processes have less effect on deeper backwaters.

Further study is recommended that would continue to measure physical and chemical variables in
observed backwaters at various release scenarios and under different winter weather regimes. A
modeling effort is also recommended in which physical, chemical, and biological variables are
considered to evaluate the timing and extent of ice formation under different dam operations.

7.2 Other Considerations

Decreases of39 to 77% in average densities of age-O Colorado squawfish have been reported for the
1987 through 1995 year classes in the primary nursery area of the Green River from Jensen to Ouray
(Figure 7, Valdez and CowdeIl1996). Not enough is known about the life history of this species to
determine if this overwinter survival reflects historic levels, or if survival has been reduced by effects
of dam operations. In the period between ISMP sampling, usually late September to late March, a
number of variables could be responsible for this perceived decrease in fish density, including changes
in habitat use, predation, mortality from stress or starvation, and/or movement or transport from the
area. Sampling of different habitats indicates that a change in habitat is unlikely (Valdez 1990), and
the issue of predation is recognized, but has not been addressed for winter conditions. Kaeding and
Osmundson (1988) stated that the unusually small size of age-O Colorado squawfish at the beginning
of winter might be an important factor affecting .recruitment to adult stocks. Thompson (1989)
determined that age-O Colorado squawfish entering the. winter period at lengths greater than 35 mm
had a higher fat content and presumably a greater chance of overwinter survival.

Movement or transport from nursery areas by age-O Colorado squawfish have not been adequately
evaluated. This could be an important factor in local decreases in densities of fish, since the species
is potomodromous and highly migratory in at least the adult stage (Tyus 1984). Possibly, dispersal
during spike winter flows and spring runoff could be part of the life history strategy by the species
to ~sperse to more downstream reaches with warmer water temperatures and greater food
availability. However, the downstream dispersal path is presently impeded by the presence of Lake
Powell and survival is reduced by the large assemblage of predators in the river and reservoir (Valdez
and Cowdell 1994). This downstream dispersal may be the result of cues provided by spike flows
in winter and early spring which inundate backwaters and force fish to find other habitats.

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