9383

• UCREFRP Catalog Number: 9383
• Author: SWCA, I.
• Title: Recovery Goals for the four Colorado River Endangered Fish Species.
• USFW Year: 2000.
• USFW - Doc Type: Logan.
• Copyright Material: NO

Wilkinson 1989). Others (Dowling and DeMarais 1993) suggest that introgressive hybridization
the common evolutionary history of the Colorado River Gila, resulting in high
ic plasticity and adaptability to the rigorous physical habitats present in the Colorado
River Basin. Evidence of intergrades was reported prior to extensive human alterations to the
basin (Miller 1946).
Since only two of the three congeneric and sympatric species of the Gila complex are
federally listed as endangered (i.e., humpback chub and bonytail), fish managers are compelled to
distinguish these f] e non-listed roundtail chub. Morphologic variation in these species has
led to confusion in . dentification, especially for young fish (Douglas et al. 1989, 1998).
This confusion has prec uded accurate assessment of life history characteristics attributable to
one species and definitive estimates of abundance (Chart and Lentsch 1999).
Proportions ofG. cypha, G. robusta, G. elegans, and Gila spp. from each of the six
populations are shown in Table 2. Proportions of these forms in Black Rocks and Westwater
Canyon vary primarily because of increased invasion of these canyon areas by G. robusta during
low water years (Chart and Lentsch 199 Despite this variation, overall average proportions of
G. cypha:G. robusta:Gila spp. for Bla s and Westwater Canyon are similar as 48:45:7 and
44:45:11, respectively. Proportions in the LCR and Colorado River in Grand Canyon are shown
as 100% G. cypha because the known genotype is primarily of this form (Dowling and DeMarais
1993), and recent samples show little evidence of other phenotypes in this population.
Proportions of 48:23: 13: 18 in Cataract Canyon include G. elegans, and indicate a large diverse
complex of Gila associated with this population (McElroy gla~ 1995). The proportion of
14:86:0 in Yampa Canyon shows a large percentage of G. ro relative to G. cypha and little
or no intergradation between these forms. We assume that greater invasion by G. robusta into
habitats occupied by G. cypha is a result of anthropogenic regulation in river flows (i.e., reduced
magnitude and frequency of high flows; Chart and Lentsch 1998). This increased sympatry by
the two species could lead to a higher than historic incidence of hybridization. This indicates that
in order to maintain approximately historic hybridization rates, it is essential to provide sufficient
frequency and magnitude of high spring flows to minimize invasion of these canyon
robusta and G. elegans.

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