8155

• UCREFRP Catalog Number: 8155
• Author: Minckley, W. L.
• Title: Executive Summary
• USFW Year: 1985.
• USFW - Doc Type: Native Fishes and Natural Aquatic Habitats in U.S. Fish and Wildlife Service Region II West of the Continental Divide.
• Copyright Material: NO

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humpbacK, and roundtai 1 chubs, and may be a major factor in disappearance
oJ f the fir s t s p e c i e san d d ec 1 i n e 0 f the see 0 n d .: J 0 s e p h tl..!l. 1 9 77; Sm i t h
et:<.1. 1979; 'Jalde: 1980; ')alde: et..!l. 1982). The same potentials for
hYbridization between bony tail and roundtail chub exist in the lower basin,
and must be considered a possible result of any reintroduction program.
The Gila River mainstream in eastern Arizona seems the most logical place
to attempt re-establ:shment of this species.. ROlJn,jtail chub is extirpate,j
or rare in that area and chubs in New Mexico segments of the Gila River
tend to be in smaller habitats far upstream from historical records or
probable penetration b;1 stocked bon/tai1.

Humpback chub, Gila cyoha t1illt?r, 1946 U1ap 9)

Historic occurrence in United States: probably widespread in
can/on-bound segments of the Colorado River. Historic abundance: abundant
to common. Geographic distribution: endemic to larger streams of the
Colorado River basin (Holden and HincKley 1980a). Status: endangered.
Reasons for decl ine or 1 isting: basically unKnown - habitat loss
(dewatering of lower portions of major streams in lower basin); habitat
modification (direct and indirect effects of impoundment, channel ization,
diversion, and regulation of discharges); hybridization with bony tail and
roundtai1 chubs due in part to factors just given and to increasing rarity
in the system; and possible interactions with non-native species.

The late discovery and description of humpbacK chub in 1946 and its
procl ivity for inaccessible canyons of the Colorado River system precluded
collection of data on its ecology, abundance, and distribution until after
major modifications had occurred. The bizzare species remains common in
the lower Colorado River basin only in the lowermost few kilometers of the
Li ttle Colorado River, Arizona (MincKley 1977, 1979; Suttkus and Clemmer
1977; Minckley tl Al. 1981; Keading and Zimmermann 1983). It infrequently
occurs in the adjacent Colorado River mainstream. which presumably is
avoided due to persistently cold waters (10-150 C) from the hypol imnion of
Lake Powell. The river warms only a few degrees in its entire passage
through Grand Canyon National ParK from Glen Canyon Dam to LaKe Head, even
in mid-summer (Kubly and Cole 1979). Similar exclusion of native fishes
downstream from Flaming Gorge Dam on the Green River, Wyoming-Utah, was
documented by VaniceK (1967) and Vanicek et Al. 1970). Colorado squawfish,
bony tail , humpback chub, and razorback sucKer may have suffered the same
fate in the Grand Canyon region. Concentrations of humpbacK chub in the
upper basin are in the most natural habitats, the lower Yampa River, Gray
Canyon of Green River, and Westwater and BlacK Rocks reaches of the
mainstream Colorado (Valdez 1980; Tyus et 11. 1982; Valdez et ~. 1982;
CRBSC 1984).

Interspecific hybridization, already mentioned with reference to
bonyta i 1, seems a maj or fac tor i nf 1 uenc i ng humpbacK chub popu I at ions in the
upper Colorado River basin. Despite ample evidence of a major and perhaps
long-term problem (Holden and Stalnaker 1970; Smit~ et .il. 1979. 'JaLdez
17'80; 'hldez et al. 1982), the e:<tent and sio;ni-fc:<.nce of hybrid influence
on any or a11 Gila of the upper basin iemains to ~e thoroughl;; documented.
The reader is referred to discussions of Colorado squawfish, bonytai J, and
razorback suc~:er for considerations of other factors relating to declines
of big-river fishes, including humpbacK chub.

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