8105

• UCREFRP Catalog Number: 8105
• Author: Haines, G. B., D. W. Beyers and T. Modde.
• Title: Estimation of Winter Survival, Movement and Dispersal of Young Colorado Squawfish in the Green River, Utah.
• USFW Year: 1998.
• USFW - Doc Type: Recovery Program Project 36,
• Copyright Material: NO

Introduction

The Colorado squawfish Ptychocheilus lucius is a large piscivorous cyprinid once
abundant in major tributaries throughout the Colorado River basin. It has, however, been
extirpated throughout much of its historic range following conversion of riverine habitat to
artificial impoundments, construction of migratory barriers, and introduction of nonnative fishes
(Tyus 1991a). The largest remaining populations of Colorado squawfish exist in the Green River
subbasin (Tyus 1991a), which is the least impacted of the large tributaries of the Colorado River.
Although the adult population in the Green River subbasin appears stable and persistent (McAda
et al. 1994), survival of young fish in nursery habitats composed of low velocity shoreline
embayments and backwaters is variable (Tyus 1991a; Tyus and Haines 1991) and may be
influenced by upstream releases from Flaming Gorge Dam (Carlson and Muth 1989; Valdez and
Cowdell 1996). The closure of Flaming Gorge Dam on the Green River in 1962 and its
subsequent operation is one potential cause of variable recruitment (Carlson and Muth 1989;
U.S. Fish and Wildlife Service 1992). In 1992 the U.S. Fish and Wildlife Service issued a
Biological Opinion (U. S. Fish and Wildlife Service 1992) that recommended discharge pattems
from Flaming Gorge Dam that more closely mimic the pre-impoundment hydrograph of the
Green River. Because little was known about the effects of winter and spring flows on young
Colorado squawfish the recommendations for winter and spring flows were largely based on
knowledge of adult Colorado squawfish. In 1992 the U.S. Bureau of Reclamation initiated the
Five-Year Flaming Gorge Research Program (Crist and Williams 1994), which proposed to
evaluate biological and physical responses of the Green River ecosystem to recommended flows
and obtain information about the effects of winter and spring flows on endangered fishes.
Several studies in the Five- Year Flaming Gorge Research Program emphasized study of
effects of environmental conditions on young-of-year Colorado squawfish. One study (Valdez
and Cowdell1996) estimated overwinter survival of young- of-year Colorado squawfish by
measuring relative densities (individuals/l00-m2 seined) in autumn and the following spring
based on catch per unit effort (CPUE). This method assumed that fish were equally vulnerable
(i.e., same probability of capture) to seining in the autumn and spring and that the same

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