8185

• UCREFRP Catalog Number: 8185
• Author: Valdez, R. A. and B. R. Cowdell.
• Title: Annual Summary Report-1992\
• USFW Year: 1993.
• Copyright Material: NO

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4.2 SUMMARY

Survival of age-O Colorado squawfish in the upper basin may be low. High mortality of age-O squawfish may
account for low recruitment to the population and continued endangerment of this species. Overwinter
decreases of age-O squawfish CPE, observed in this study, averaged approximately 60% for five year classes.
We feel a large portion of this decrease is due to mortality during the winter months. Downstream transport
and use of habitats other than backwaters were recognized as important considerations, but did not account
for substantial decreases.

The apparent relationship between overwinter decreases in age-O CPE and backwater depth has important
management implications. This relationship needs further investigation and will be tested on data from Reach
1 and Reach 4. However preliminary results showed a decrease in fall to spring CPE's of only 15% for
backwaters greater than 4 ft deep. If habitat parameters prove to significantly influence overwinter survival
then management plans such as flow alteration or habitat improvement could prove valuable to the recovery
of this species.

Another factor to consider in decreased density of age-O Colorado squawfish is the role of spring runoff in
transporting fish downstream. It is likely that a large number of fish are transported downstream, into
Cataract Canyon and Lake Powell by high spring flows. The large number of fish predators in Lake Powell
would greatly reduce the survivability of young Colorado squawfish. Spring runoff is typically in late April and
May, after spring sampling. Therefore runoff did not influence decreases observed in this study, but is
probably another significant source of mortality for age-O Colorado squawfish.

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