WSP00173

• File Number: 8240.200.10.A
• Description: UCRBRIP Habitat Restoration
• Basin: Colorado Mainstem
• Date: 10/8/1997
• Author: UCRBRIP
• Title: Final Habitat Restoration Program FY 1998 Work Plan Part 2
• Water Supply Pro - Doc Type: Publication

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study to investigate the viability of hybrid crosses as a measure of the
potential impacts of reintroduced bony tail on humpback chub and roundtail
chub. Because this project calls for following Gila spp. progeny through the
F2 generation, it is critical that the crosses start immediately. With the
conclusions from this project and the results from the Gila Taxonomy Project,
the UBRIP will be able to continue with bonytail reintroduction based on
informed decisions.

Samples of Gila from natural populations generally contain various forms of
bybrids (e.g., G. elegans x G. robusta, G. elegans x G. cypha). The presence
of hybrids, however, does not necessarily reflect the amount of spawning that
occurs between species in the field. For example, the number of hybrids that
reach sexual maturity may be only a small fraction relative to offspring of
pure crosses. To determine the relative fitness of hybrids, therefore,
crosses will be produced in the laboratory. This will allow a direct
comparison of survivorship and performance between pure species and each
possible cross.

The occurrence of interspecific hybridization within the genus Gila has been
well-documented (DeMarais et al. 1992). However, the fitness of hybrids
relative to the pure species has not been ascertained. The risk of long-term
effects of hybridization to pure species of Gila following reintroduction of
bony tail (Gila elegans) into the Upper Basin is directly related to the
fitness of the various Gila hybrids. Naturally reproducing populations of
pure roundtail chub (G. robusta) and humpback chub (G. cypha) occur in the
Upper Basin. The purpose of this study is to determine the potential risk of
reintroduction of bony tail into the Upper Basin on roundtail chub and humpback
chub due to increased opportunity for hybridization.

Working hypothesis:

Hybridization is common in fish, particularly between those that 1) do not
exhibit parental care 2) fertilize eggs externally and 3) have limited habitat
relative to that available historically. It has been demonstrated that Gila
spp. hybrids exist in the Colorado River. If hybrids (F" F2 and backcrosses)
have a significantly lower relative fitness than any pure Gila then it is
unlikely that hybridization among Gila will increase significantly following
reintroduction of pure Gila spp,

IV. Study Goals, Objectives, End Product:

Goals: The goal is to determine the potential risk for Gila species to
hybridize following the reintroduction of bonytai1 in the Upper Colorado River
Basin.

Objectives:

1) quantify the viability of laboratory-produced hybrid ,Gila, crosses relative
to laboratory-produced pure Gila crosses

2) determine the relative fitness of F, backcrosses.

End Product:

I,

A report documenting the viability of laboratory-produced Gila hybrid crosses
,relative to laboratory-produced pure Gila~ The report will have
recommendations for future bony tail reintroduction efforts relative to Gila

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