8114

• UCREFRP Catalog Number: 8114
• Author: Nesler, T. P.
• Title: Five-year Stocking Plan for Endangered Colorado River Fish Species in Colorado.
• USFW Year: 1998.
• USFW - Doc Type: Denver, CO.
• Copyright Material: NO

of bonytail inhabiting larger riverine habitat like the Colorado River near Moab,
Utah, where floodplain habitat was more likely to exist.

It is obvious the above two reaches are quite different from a habitat perspective. Similar to
other aspects of setting stocking objectives for bonytail, considerable guesswork is involved.
Depending upon whether this species is a generalist in its habitat needs or not, which
monitoring the above two reaches will provide guidance, river reaches in Colorado
designated for bonytail stocking may be revised considerably.

Further, the following species interactions and recovery actions are anticipated:

1) Biomass and production occupied by common carp and other nonnative fish in the
Grand Valley may be reduced by control efforts, lessening competitive pressure
within floodplain habitats, and perhaps expand expansion of bonytail biomass.

2) With successful establishment of an adult bonytail population, general abundance of
other native species may be reduced as a result of some competition.

Achievement of the stocking objective would result in an adult population of 20,000
4-6-year-old (adult) fish in each reach, and two geographically distinct populations in the
Upper Colorado River subbasin. Due to the uncertainties associated with habitat needs of
bonytail, specific definitions of their probable distribution and abundance cannot be presented
as is being done for razorback sucker and Colorado pikeminnow. Clearly, monitoring will
be required to refme the speculative objectives that serve as a starting point. Interim
management objectives (IMO) for bonytail (Lentsch et al. 1998) indicate at least 2,000 adult
fish are necessary as an effective population size to maintain long term genetic diversity.
Using a demographic model developed by Crowl and Bouwes (1998), simulation results show
a mean population or subpopulation size of 4,800 bonytail (:t800 fish) are necessary to be
95 % confident the population will be maintained over a 100 year period. No river reaches
were designated within the IMOs to direct reintroduction efforts, so it may be interpreted that
the IMO objective for this species is one population of about 5,000 adult fish anywhere in
the Upper Colorado River Basin. The stocking objectives here are considerably greater than
the IMO population objective, though neither can be judged as more reasonable than the
other due to the species' unknowns.

Reintroduction of bonytail into the upper Colorado River Basin has been assessed by
several studies (Chart and Cranney 1992, Meyer 1992, Wydoski 1994, Lentsch et al. 1996).
Recommendations included the necessity of reintroduction of bonytail in the wild to
determine it's ecological requirements, but concerns have been expressed about reintroducing
bonytail because of the potential hybridization with the other Yili species, especially
humpback chub. While acknowledged as a potential risk associated with reintroduction and
establishment of bonytail near existing humpback chub populations (e.g. Black Rocks), this
risk appears to be overemphasized based on the evidence of historic hybridization within the
Gila complex that predates human modification of the river system (Dowling and DeMarais

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