7066

• UCREFRP Catalog Number: 7066
• Author: Behnke, R. J. and D. E. Benson.
• Title: Endangered and Threatened Fishes of the Upper Colorado River Basin.
• USFW Year: 1980.
• USFW - Doc Type: Bulletin 503A,
• Copyright Material: NO

suitably diverse habitat occurred, all three species
might be found together.
The optimum habitat of bonytail chubs, based on
former collections when they were abundant, appears
to be the open river areas of relatively uniform
depth and current velocity. This type of habitat
typically consists of a shifting sand bottom water
depths of 3 to 4 feet, and a relatively constant,
moderately swift current. The streamlined body and
large fins of the bonytail chub seem to make it well
adapted to live in this type of habitat.
The bonytail chub is a relatively long-lived
species. It does not spawn until it reaches an age
of 5 to 7 years; like the other chub species, it
spawns when the water reaches about 650 F. Little is
known about the life history of the bonytail chub be-
cause it rapidly disappeared before intensive studies
were made. It feeds on insects, often terrestrial
insects taken on the surface of the water. Fragments
of debris and algae in the stomachs of the relatively
few specimens examined suggest that the bonytail chub
may feed intensively after a sudden storm cause flood-
waters to wash food out of tributaries into the main
river channels. The maximum size attained by the
bonytail chub is, in general, 16 to 18 inches. How-
ever, small numbers have continued to exist in the
lower basin reservoirs, Lake Mohave and Lake Havasu,
where they may attain a large size. A specimen about
3 feet long and weighing 8 pounds was reported caught
by an angler in 1975 from Lake Mohave.
Present and Past Distribution
The original distribution included the large-
river environments of the entire basin from Mexico to
Wyoming. In the Gila River, Arizona, bonytail chubs
were last recorded in 1926. They declined in the
lower basin after the construction of lakes Mead,
Havasu, and Mohave. Although they persisted in large
numbers in these reservoirs for several years, and
large numbers were observed spawning in Lake Mohave
in 1954, their numbers continued to decline, because
its spawning did not result in the survival of young
fish. The bonytail chub was still abundant in the
Green River until after the completion of Flaming
Gorge Reservoir in 1963. By the late 1960's bonytail
chubs became very rare. Except for the few specimens
that may yet persist in lakes Havasu and Mohave, the
only bonytail chubs reported in the last 3 years were
from the Green River in Utah. If. it were not for the
stark example provided by the passenger pigeon, such
rapid disappearance of a species once so abundant
would be almost beyond belief.
Causes of Decline
The lack of successful reproduction in reser-
voirs explains the disappearance of bonytail chubs
from the segments of their former range that were
converted into impoundments. Their absence from the
Colorado River of Colorado and Utah and from most of
the Green River where apparently suitable habitat
still exists is not so easily explained. There is
little in the way of documented evidence about the
occurrence or abundance of bonytail chubs in the
Colorado River in Colorado and Utah, but it is as-
sumed that they were common in this large-river
environment. The open river or "run" type of habitat
does not seem to be extensively used by non-native
fishes. Thus, the "bonytail niche" would be expected
to be less impaired than the niches of some other
native fishes. Yet, the bonytail chub has suffered
greater declines than any other native species and
is now the rarest member of the original fish fauna.
Controlled water releases from Flaming Gorge
Dam eliminated the great seasonal peaks of high and
low flows of the original Green River and also cause
daily fluctuations due to power generation. These
changes in flow undoubtedly have influenced subtle
changes in channel configuration and altered optimum
bonytail chub habitat. Although there are no large
dams on the Colorado River above Lake Powell (except
in the headwaters), tributary reservoirs such as the
Curecanti Project on the Gunnison, Ruedi Reservoir on
the Frying Pan River, and Dillon and Green Mountain
reservoirs on the Blue River alter the historical
flow regime in the Colorado by reducing the peak
spring flows. Large amounts of water are diverted
for irrigation, and the return flows are diminished
and the water quality altered.
The extent of the quantitative and qualitative
changes in the flows of the upper Colorado River
basin can be understood from the following facts and
figures. The annual average virgin flow of the upper
Colorado River basin at Lee's Ferry was 14,900,000
acre feet of water. By 1975, 3,823,900 acre feet of
water (26% of the average virgin flow) were lost to
the basin by consumptive irrigation use, transmountain
diversions out of the basin, reservoir evaporation,
and by cities and industry. Due to reservoir
storage, the peak flushing flows of May and June are
now at all time low discharge levels in the Green and
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