7715

• UCREFRP Catalog Number: 7715
• Author: U.S. Fish and Wildlife Service, C. R. F. R. T.
• Title: Bonytail Chub Recovery Plan.
• USFW Year: 1990.
• USFW - Doc Type: \
• Copyright Material: NO

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(Holden and Stalnaker 1970). The dorsal fin orlgln is nearer to the tip of
the snout than it is to the base of the caudal fin. Pharyngeal teeth are
typical for large-river chubs at 2,5-4,2.

A combination of characters are used to differentiate adult bony tail,
humpback, and roundtail chubs. Dorsal/anal fin ray counts are usually 10-10
in bony tail chub, 9-9 in roundtail chub, and 9-10 in humpback chub. The
number of gill rakers on the anterior row of the second arch is usually
18 (15-21) in bony tail chub, 15 (13-17) in humpback chub, and 13 (12-15) in
roundtail chub (R. Muth, Colorado State University, pers. comm.). Bony tail
chub have a much narrower caudal peduncle than roundtail chub with the ratio
of head length to caudal peduncle depth generally greater than five. Similar
ratios for roundtail and humpback chub are less than five. The nuchal hump in
.adult bony tail chub rises smoothly from.a concave skull while those of adult
humpback chub arise more abruptly from the skull.

It was previously suggested (U.S. Fish and Wildlife Service 1984) that the
humpback chub could be differentiated from the bony tail chub by the presence
of an overhanging snout in the former species. However, Karp and Tyus (1989)
recently ranked the degree of overhanging snout in these two fishes, from the
Yampa River, using methods described by Douglas et al. (1989), and found that
this character was not useful in separating them. These findings remain
consistent with the original description of the humpback chub by Miller
(1946), who noted the similarity in overhanging snout in humpback and bony tail
chubs, but not the roundtail chub.

Although young bony tail chub may be differentiated from young roundtail and
humpback chubs by certain characteristics, such as fin ray counts and the
presence of larger eyes (Smith et al. 1979), positive field identification of
juvenil e Gil a is diffi cult and often cons idered "tentat i veil even by
knowledgeable investigators. Tyus et al. (1982a) reviewed a combination of
techniques for identifying bony tail chub from the Green River, including
x-rays of vertebrae, gill-raker counts, and other morphological
characteristics. In the laboratory, Muth (1988) differentiated among young
bony tail, humpback, and roundtail chubs based on x-rays of total vertebrae or
myomere counts. However, identification of Gila species has posed a problem
in the Upper Colorado River Basin (Upper Basin) because the three congeneric
species may be sympatric and specimens may have morphometric characteristics
that overlap (Suttkus and Clemmer 1977; Smith et al. 1979; Valdez and Clemmer
1982). Natural hybridization among the three species of Gila has been
suggested by several investigators (Holden and Stalnaker 1970; Robert Miller,
University of Michigan, pers. comm.; Minckley 1973; Smith et al. 1979) and
artificial spawning has produced bony tail x roundtail chub and bony tail x
humpback chub crosses (Hamman 1981). The significance of individuals that
display overlapping morphologic characteristics in the Upper Basin remains
unclear and the possibility of a polymorphic Gila complex (Holden and
Stalnaker 1970; Tyus et al. 1986) is an issue that must be addressed.

The Service is currently developing plans to investigate and clarify the
taxonomic status of the humpback, bony tail, and roundtail chubs in the
Colorado River basin. The work will include technical experts and field
researchers and is intended to determine definitively whether the humpback,
bony tail, and roundtail chubs are distinct species. If they are distinct, the

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