9378

• UCREFRP Catalog Number: 9378
• Author: Nesler, T. P.
• Title: Recovery of the Colorado River Endangered Fishes
• USFW Year: 2000.
• USFW - Doc Type: Biological recovery goals and criteria for Colorado pikeminnow, Humpback chub, Razorback sucker & Bonytail.
• Copyright Material: NO

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of a humpback chub population has been that in Hideout Canyon impacted by
Flaming Gorge Reservoir in 1962. It is important to recognize that these seven
population segments have persisted since their discovery. None appear to be in
danger of extirpation due to present environmental conditions. All five of the
population complexes must meet the requirements for abundance trend,
recruitment and population size structure for both downlisting and delisting. This
includes separate goals for these criteria for Black Rocks and Westwater Canyon
population segments.

The same considerations of genetic viability described for Colorado
pikeminnow apply to the humpback chub. As such, an Ne of 500-1,000 fish and
NJN ratio of 0.5 are applied to the determination of a minimum abundance
criterion. Observed sex ratios in humpback chub populations in the Little Colorado
River and Westwater Canyon appear very close to 1 M: 1 F (Valdez and Ryel 1995,
Chart and Lentsch 1999). Given that these observations mayor may not
accurately represent the true sex ratio of the spawning population, M:F ratios of
2M: 1 F and 3M: 1 F were also considered. Under these population combinations, the
target abundance necessary to maintain Ne=1000 fish would be 2,000-2,700 fish.
The target abundance necessary to maintain Ne=SOO fish would be 1,000-1,400
fish. The range of population estimates provided here and in other studies suggest
most of the humpback chub population segments and population complexes have
adequate numbers to maintain an Ne=SOO criterion regardless of the sex ratio.
The Slack Rocks, Westwater Canyon, Little Colorado-Grand Canyon, and
Desolation-Gray Canyon populations have adequate numbers to maintain an
Ne=1 ,000 under the 1:1 sex ratio and Ne/N=O.S.

Lacking better biological data to refine a target abundance for genetic
viability, 2000 adult fish ~17S mm is proposed as a feasible and credible genetic
requirement. Addition of a buffer to this minimum equal to the 24% mortality rate
results in an abundance goal of 2,500 fish. Population estimates for Black Rocks-
Westwater and Little Colorado-Grand Canyon demonstrate abundance estimates
well above this requirement (Table A-S, A-6; Pfeifer et al. 1998, Chart and Lentsch
1998, Douglas and Marsh 1996, Valdez and Ryel 1997). Further population
estimation work will be necessary to determine if the Desolation-Gray Canyon or
the Yampa-Whirlpool-Split Mountain population complexes have the capacity to
meet this abundance goal. It is proposed that two of the five populations or
population complexes meeting the abundance criteria for maintaining genetic
viability are sufficient argument for down listing the species to threatened; and three
populations meeting the necessary abundance criteria sufficient for deli sting.
Further population estimation work is necessary on most of the humpback chub
populations to establish abundance magnitude and trends.

The available habitat and potential carrying capacity of these canyons is
clearly different in terms of river miles, and perhaps also different in terms of the
quality of habitat for humpback chub. If one accepts a 76% survival rate in the
Black Rocks and Westwater populations, then the population estimates suggest

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