7224

• UCREFRP Catalog Number: 7224
• Author: Kaeding, L. R., B. D. Burdick, P. A. Schrader and C. McAda
• Title: Temporal and Spatial Relations between the Spawning of Humpback Chub and Roundtail Chub in the Upper Colorado River
• USFW Year: 1990
• USFW - Doc Type: Transactions of the American Fisheries Society
• Copyright Material: YES

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Transactions of the American Fisheries Society 119: t35-144, 1990
Temporal and Spatial Relations between the Spawning of
Humpback Chub and Roundtail Chub in the
Upper Colorado River
LYNN R. KAEDING, BOB D. BURDICK, PATRICIA A. SCHRADER,
AND CHARLES W. MCADA
U.S. Fish and Wildlife Service, Colorado River Fishery Project
529 25-1/2 Road, Grand Junction, Colorado 81505, USA
Abstract. -Gonadosomatic indices (100 x gonad weight whole-body weight) and the occurrence
of expressible gametes indicated a temporal overlap in the spawning of humpback chub Gila cypha
and roundtail chub G. robusta at Black Rocks, a turbulent, deepwater reach of the upper Colorado
River, Colorado {39°N, 109°W). Radiotelemetry showed that roundtail chub moved throughout
the upper Colorado River during the spawning season, whereas humpback chub were restricted
almost entirely to the 1.8-km Black Rocks reach; however, spatial isolation of the species during
spawning was not demonstrated. A principal component analysis of morphologic measurements
taken from live fish supported taxonomic assignments made in the field and suggested that most
of the few specimens considered hybrids or intergrades by field personnel were actually humpback
chub; however, an analysis based on fin ray counts indicated that this group included roundtail
chub, hybrids, or backcross individuals. The strong affinity of humpback chub for the unique
habitat conditions created by water flowing rapidly among large, angular boulders and shoreline
rock outcrops at Black Rocks and elsewhere suggests that the unusual morphology of this species
is an adaptation to life in such habitats. The limited availability of such habitats might be an
important factor controlling the distribution and relative abundance of the humpback chub, a
species now listed as endangered.
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The humpback chub Gila cypha, roundtail chub
G. robusta, and bonytail G. elegans are native cyp-
rinids of the Colorado River basin (Miller 1946).
Though greatly reduced in numbers and range from
those that occurred historically, humpback and
roundtail chubs persist in viable populations
(Hinckley 1973; Valdez and Clemmer 1982;
Kaeding and Zimmerman 1983), whereas the
bonytail is nearly extinct in its natural habitat
(Kaeding et al. 1986). In the upper basin, habitat
of the widespread and abundant roundtail chub
includes the few disjunct river reaches occupied
by humpback chub (Tyus et al. 1982; Valdez and
Clemmer 1982). The physical and biological char-
acteristics of these few reaches, like those of the
entire Colorado River system, have been altered
by the operation of water-development projects
upstream, by the introduction of nonnative fishes,
and by other human activities (Miller 1961;
Hinckley 1973; Ono et al. 1983).
Interbreeding of fishes, especially cyprinids, has
resulted elsewhere when habitat has been altered
(e.g., Hubbs 1955). Valdez and Clemmer (1982)
used the occurrence of mature "fish with mor-
phologies that apparently intergrade between those
of humpback and roundtail chubs to support the
hypothesis that human-induced habitat alteration
is causing the breakdown of reproductive isolating
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mechanisms between these species in the upper
Colorado River. The possibility of interbreeding
of the humpback chub and roundtail chub in na-
ture has been further suggested by the successful
artificial crossings ofbonytails and humpback chub,
and of bonytails and roundtail chub (Hamman
1981). However, whether the intermediate forms
that occur in nature are actually intergrades, hy-
brids, or extreme variant forms of either species
is unknown. Furthermore, the existence of Ft hy-
brids does not constitute evidence that the species
boundaries are breaking down. Gene exchange be-
tween these species depends on production, sur-
vival, and reproduction of backcross individuals
with fitness similar to parental fish (e.g., Dowling
and Moore 1985).
Additional human alteration of the aquatic eco-
system of the Colorado River will occur, perhaps
most predictably as the result of developments
planned to meet increasing demands for water.
Because the humpback chub is listed as an en-
dangered species (U.S. Office of the Federal Reg-
ister 32:48 [1967];4001), resource management
agencies are especially concerned about the pos-
sible negative effects such developments might
have on the species. Among these concerns is that
of possible disruption of the reproductive isolating
mechanisms of humpback and roundtail chubs,
135