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

1993). Hybridization in Gila is in part a natural phenomenon (McElroy and Douglas 1995).
Given that some hybridization between bonytail, humpback chub, and roundtail chub in the
wild is possible, it may be negligible considering the I) niche separation among these three
species, 2) factors contributing to the bonytail's decline and disappearance, and 3)
preliminary evidence for hybridization.

Kaeding et aI. (1990) and Vanicek and Kramer (1969) suggest ethological mechanisms
may separate bonytail spatially from humpback and roundtail chub during spawning based on
field collection results. In looking at the apparent niches occupied by the three Gila species
in the Upper Basin, roundtail chub appear to be largely a tributary stream species, being
most abundant in the Yampa, White, upper Colorado, Gunnison, and Dolores rivers; while
the humpback chub is largely a deepwater, canyon species, being found in Yampa,
Westwater, Cataract, Desolation-Gray and Black Rocks canyons. This leaves the main stem
rivers and floodplains as an "available" niche for another Gila species, such as bonytail. As
speculation, it is possible the significant loss of floodplain habitat with increasing abundance
of nonnative fish predators and competitors in remaining floodplain habitat could account for
the disappearance of bonytail populations except for the population of large, senile adults
inhabiting Lake Mohave. If the dramatic disappearance of bonytail were the result of
hybridization catalyzed by habitat modification, there has been no evidence of such a
significant event in the genetic material of the remaining Gila populations sampled for such
via morpho-meristic, allozyme, and DNA techniques (Dowling and DeMarais 1993, McElroy
and Douglas 1995). This hybridization event would have, in effect, subsumed the bonytail.
population genome into the genetic material of the other Gila species virtually without a
trace. This is difficult to accept, and makes the supposition that hybridization between
stocked bonytail and humpback chub or roundtail chub to the detriment of the latter two
species also less credible.

In light of the above information, the risk of extinction and permanent loss of wild
bonytail populations is considered to outweigh the risk of potential hybridization from
reintroduction stocking. This position is in conflict with the recommendation in the bonytail
reintroduction plan (Lentsch et aI. 1996), which supports further Gila species hybridization
studies be completed to assess the risks of stocking bonytail in reaches above established
humpback chub populations prior to beginning restoration stocking of bonytail. However,
the Recovery Program has approved stocking of bonytail independent of any hybridization
studies and stocking has occurred in two river reaches in Utah.

Stocking Plan - Colorado pikeminnow (Table 3)

Colorado pikeminnow are designated as the third priority species in this plan. This
ranking results from the relative abundance of Colorado pikeminnow populations in the
Yampa, White, and Colorado rivers compared to razorback sucker and bonytail. A primary
objective of stocking Colorado pikeminnow in Colorado is to restore extirpated and marginaI
adult populations in river reaches upstream of instream barriers. Through stocking we will
strive to establish adult populations that will spawn in available canyon habitat and increase

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