9557

• UCREFRP Catalog Number: 9557
• Author: Jackson, J. A. and J. M. Hudson.
• Title: Population Estimate for Humpback Chub ‎(Gila cypha)‎ in Desolation and Gray Canyons, Green River, Utah 2001-2003.
• USFW Year: 2005.
• USFW - Doc Type: 22k,
• Copyright Material: NO

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EXECUTIVE SUMMARY
i Humpback chub population estimates were determined for Desolation and Gray canyons
(Deso/Gray), on the Green River, by conducting multiple pass capture-recapture sampling.
Three sampling passes were conducted in 2001 and 2003, and two passes were conducted in
2002 due to low water. In 2003, sampling was moved to the fall to avoid capturing Colorado
pikeminnow that use Desolation Canyon for spawning. Twelve sites were sampled throughout
r the two canyons. Included within these sites, were four long-term sites sampled annually by
Utah Division of Wildlife Resources since 1989. Analysis of point estimates generated by
capture-recapture data demonstrated that the adult humpback chub population was composed of
1,254 individuals in 2001, 2,612 individuals in 2002, and 937 individuals in 2003. Many factors
were at play during the three-year period that may have contributed in the variation in point
r estimates among years. Non-biological factors complicating the validity of the estimates include
moving sampling from summer to fall and low water conditions at the time of sampling. Factors
that may have directly affected the humpback chub population biologically include, the Rattle
Complex fire and subsequent fish kill in late summer 2002, and the apparent increase in
smallmouth bass occupying Deso/Gray. The authenticity of the point estimates will be more
clearly analyzed when population estimates are conducted consistently during the fall months of
2006-2008.
Population estimates for the juvenile portion of the population could not be determined since
very few individuals were collected each year. Numbers of humpback chub juveniles collected
by all methods during the study period were: 5 in 2001, 24 in 2002 and 4 in 2003. To determine
mean recruitment for juveniles into the adult population, we assumed that individuals from 200-
220 mm would be first year adults. Point estimates for this size class were derived by taking the
proportion of these fish relative to the total numbers used in the adult population estimate.
Therefore, estimates of first year adults were 163 (95% CI=149-192) for 2001, 297 (95%
CI=275-343) for 2002, and 134 (95% CI=123-159) for 2003. In order to increase the catch of
juvenile humpback chub we recommend the continuation of electrofishing to the extent possible,
utilization of hoop nets and minnow traps, and incorporation of smaller mesh trammel nets.
Catch rate data incorporated to analyze changes over time at four long-term sites illustrated that
catch rates for humpback chub have not changed substantially since 1989. Furthermore,
10 examination of pre-study and study period data suggest that catch rates can be extremely variable
and possibly dependent on environmental factors during sampling.
This study represents the first sampling period since finalization of the amended recovery goals
for humpback chub in 2002 and is valuable in the initial evaluation of population size of
humpback chub in Deso/Gray. Furthermore, it serves to illuminate any necessary adjustments to
r the sampling design before population estimate sampling for humpback chub in Deso/Gray
resumes in 2006.
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