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

population was sampled on both occasions. Tyus and Haines (1991) used a similar method and
reported that estimates based on seine CPUE were unreliable for assessing overwinter survival.
They concluded that a likely explanation for poor performance of the seine-CPUE method was
that capture vulnerability varied on each sampling occasion. Haines and Modde (1996)
demonstrated that capture-recapture provided a more rigorous basis for estimating abundance of
young Colorado squawfish because it accounted for variable capture probabilities.
The objectives of this study were to employ the capture-recapture methodology used by
Haines and Modde (1996) in two 32-km reaches ofthe Green River in order to (1) compare
estimates of Colorado squawfish abundance from capture-recapture and CPUE methods;
(2) estimate overwinter survival of age-O Colorado squawfish; (3) estimate seining catchability
coefficients (probability of capture per seine haul; Ricker 1975) for autumn and spring sampling;
(4) determine extent of downstream movement of marked Colorado squawfish between
1 November and 1 April; and (5) determine the effect of timing and magnitude of spring flows
on dispersal of age-l Colorado squawfish. Results were used to evaluate compliance with
assumptions of capture-recapture methodology, examine the advantages and disadvantages of
capture-recapture and CPUE, and evaluate evidence that links overwinter survival with discharge
magnitude, discharge variability, or fish size.

Study Area

The Green River originates in westem Wyoming and flows into northeastem Utah where
it is impounded by Flaming Gorge Dam (Figure 1). Prom Flaming Gorge Dam, the river flows
unimpeded for 655 km through eastem Utah and northwestem Colorado to its confluence with
the Colorado River. Our study sites were located within two low gradient river reaches (RK 0-
193 and RK 321-513) known to be important nursery areas for young-of-year Colorado
squawfish (Tyus and Haines 1991; McAda et al. 1994). These low gradient (0.2-0.4 mIkm),
alluvial reaches of the Green River consist predominantly of sand and silt substrates (Haines and
Tyus 1990; Tyus and Haines 1991; McAda et al. 1994). One 32-km capture-recapture study site
was located within each nursery reach; the exact location within each reach was based on ease of

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