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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"blended stock" being used to restore Colorado and Gunnison river populations.
For downlisting, one Upper Basin population, one population located in either the
Upper or Lower Basin, the Lake Mojave population and the two Green River
populations are necessary. For delisting, these populations and one additional
population in both the Upper and Lower Basins, for a total of seven, is required.

Abundance requirements were specified in the razorback sucker recovery
plan (USFWS 1998), specifying 50,000 adult fish in Lake Mojave, 5,000 adult fish
in the middle Green River, and 2,000 adult fish in the lower Green River. In 1995,
the Lake Mojave population was estimated at about 25,000 fish (Minckley et al.
1991, Marsh 1995). The significant genetic diversity established for this population
(Dowling and Minckley 1993, Dowling et al. 1996) has not been re-evaluated in
light of this decline. Unconfirmed reports from Lower Basin researchers suggest
the Lake Mojave population has declined even further to slightly less than 10,000
fish. Using genetic criteria similar to Colorado pikeminnow with Ne=1000,
Ne/N=0.5, and a sex ratio of 3M;1F, a minimum abundance requirement for
genetic viability would be 2,700 adult fish. One of the recognized threats to this
species is a very high percent mortality of progeny resulting in near complete
recruitment failure. As such, existing razorback sucker adults have a very low
probability of contributing to succeeding generations. A representative mortality
rate for a self-sustaining razorback sucker population is unknown, therefore, until
new information on this parameter is gained, a conservative 100% rate is used
here to determine a buffer to the genetic minimum. This results in an abundance
requirement of 5,400 adult fish.

Utah's razorback sucker stocking plan (Hudson et at 1999) specifies 5,316
adult fish as an end product for both the middle and lower reaches of the Green
River. These stocking objectives were based on Lentsch et al. (1998), which
quantified a viable population size for razorback sucker necessary to maintain
genetic viability and demographic stability. Viability was defined in Lentsch et al.
(1998) as a 95% chance of persistence for 100 years. Utah's stocking plan
approaches the restoration of razorback sucker in the Green River as two
population segments. The razorback populations in the middle and lower reaches
of the Green River will be genetically linked through shared brood stock under the
Utah stocking plan as well as likely transfer of wild adult fish and downstream
displacement of larval fish between the two river reaches. The resultant density of
adult razorback sucker under the Utah stocking plan would be almost 31 fish/mile.
Under Colorado's endangered fish stocking plan (Nesler 1998), an objective of
three adult populations (population segments) was set for the Colorado. and
Gunnison rivers. A stocking objective of 475 adult fish/mile as a target was
established for each population/river reach. This density objective was derived
using fish population biomass data for the upper Colorado River between Rifle and
Palisade, Colorado (Anderson 1997), an average length-weight criterion for adult
razorback at first maturity at age 5, and a target razorback adult biomass of 5% of
the Anderson (1997) total fish biomass estimate. In the San Juan River, the
stocking objective for razorback sucker was set at 10% of existing abundance of