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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Osmundson and Burnham (1996) as part of the lower estimate and Appendix
Table A-1 as part of the upper estimate.

From the densities estimated in Table A-1, the Green River mainstem is
capable of supporting a minimum population of 2,700 Colorado pikeminnow at4.6
fish/mi., or even a population of 5,800 fish at 10 fish/mi. On the other hand, the
Yampa and White rivers appear to support much lower densities. These tributary
populations are linked genetically with Green River population based on the mixing
of spawning fish from each population at the Yampa and Desolation-Gray Canyon
spawning sites and the dispersal and recruitment of fish to all three populations
from the same Green River nursery habitats. They are relatively small adult
populations that are unlikely to be able to maintain genetic diversity without this
link to the larger Green River population. The Colorado-Gunnison river system
may be able to support a population of 2,700 adult Colorado pikeminnow at a
density of 8.9 fish/mi., but is more likely capable of supporting a population in the
1,500-2,000 range. This is feasible if Ne=500 and NJN=0.5, resulting in a required
population size of 1,300-1,600 fish. Based on the two feasible sex ratios and Ne of
1,000, the San Juan does not appear likely to be able to support a population of
much more than 2,250 fish at 10.1 fish/mi. If one accepts that the San Juan
system is similar to the Colorado River in that its Colorado pikeminnow population
would be distributed as an upstream adult population and a downstream
nursery/juvenile population, and that the carrying capacity of the San Juan is more
similar to the Yampa and White rivers, then it should not be expected to support an
adult population of more than 1,000-1,300 fish. This target population range does
satisfy the lower Ne criterion of 500 fish. If 2,700 adult fish is a minimum
population required for Colorado pikeminnow to maintain a genetically viable
population, then the prospects for recovery of the San Juan population are
diminished. However, the population characteristics of the pikeminnow population
that affect the population size necessary to maintain Ne are not confirmed in
research.

Given the above genetic considerations, it is proposed to further augment
the downlisting and delisting requirements for population abundance for each
population by adding a buffer equal to the annual mortality rate of 14%. This
results in population abundance minimums of 3,100 adult fish in the Green River, .
1,600 fish in the Colorado River, and 1,200 fish in the San Juan River.
Considering the available habitat in river miles, these abundance minimums equal
5.3-5.4 fish/river mile for all three subbasins. For each population, adult fish size is
defined as ~350 mm, and the lower boundary of the 95% confidence interval must
be equal to or less than 50% of the point estimate. This means the lower end of
the confidence interval around these point estimates must be ~1,550 fish for the
Green, ~800 fish for the Colorado, and ~600 fish for the San Juan. Two of the
three populations must meet this abundance requirement. By way of comparison,
existing low end confidence intervals for the population estimates in Table A-1 are
30-55% of the point estimates for the Green River, 40-48% for the White River, 46-
53% for the Yampa River, and 21-36% for the Colorado River. Using the

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