7229

• UCREFRP Catalog Number: 7229
• Author: Kaeding, L. R. and M. A. Zimmerman
• Title: Life History and Ecology of the Humpback Chub in the Little Colorado and Colorado Rivers of the Grand Canyon
• USFW Year: 1983
• USFW - Doc Type: Transactions of the American Fisheries Society
• Copyright Material: YES

BIOLOGY OF HUMPBACK CHUBS IN THE GRAND CANYON
An annual growth cycle of humpback chubs
in the Colorado was indicated by scale charac-
teristics. New annuli were evident on many scales
in April-May 1981, and crowded circuli were
found at scale margins in October-November
] 980 and 1981. However, we believe that age
estimates derived from the scales of humpback
chubs from the Colorado are not reliable be-
cause some fish formed an annulus near the end
of their first year of life whereas other fish did
not. Evidence supporting this belief is provided
by fish from the well-defined 38-107 mm length
class collected from the Colorado in April-May
(Fig. 3). We believe that all of these humpback
chubs were yearlings because they were too large
to be age-0; the smallest fish was twice the length
of the largest known age-0 fish collected con-
currently (an 18-mm metalarva from the Little
Colorado waters of C 4). Collected during the
time of annulus formation, the small yearling
fish would not have formed an annulus that year
because the fish had yet to develop scales or had
scales too small to show circuli, whereas the
larger humpback chubs of this length class had
developed scales that showed one clear new an-
nulus. We attribute poor early growth of small
Colorado River humpback chubs to low water
temperatures.
Reproduction
The onset of female sexual maturity occurred
at lengths of about 250 to 300 mm (Fig. 4). Fish
of this length were about 3 years old in the Little
Colorado but perhaps older in the Colorado.
The sparse data from males killed accidentally
suggest that the onset of male sexual maturity
occurs at about the same lengths (Fig. 4). How-
ever, smaller mature males were seen in the
field. Of the males collected in April 1981, 15
of 19 (79%) between 200 and 249 mm long and
26 of 35 (74%) between 250 and 300 mm long
expressed milt when external pressure was ap-
plied. The smallest male with milt was 205 mm
long.
Seasonal fluctuations in the gonadosomatic
index and mean ovum diameter of humpback
chubs from the Little Colorado were similar
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150 250 350 450
Total Length (mm)
FIGURE 4.-Relationships between gonadosomatic index
(IOO~gonad weight)whole-body weight) and fish length,
and between mean ovum diameter and fish length, for
humpback chubs from the Little Colorado and Colorado
rivers.
(Fig. 5). Gonad development in preparation for
spawning was rapid between December and
February-April; sharp declines in indices dur-
ing April-May indicated that spawning had oc-
curred during this period. Seasonal gonad de-
velopment of Little Colorado River males longer
than 250 mm paralleled that of females. Milt
was expressed from 25% of 12 males in Feb-
FIGURE 3.-Length frequency distributions for humpback chubs collected from the Little Colorado River, confluence, and
Colorado River. Results of analyses of annuli on scales from humpback chubs are given below respective length frequency
distributions.