7124 ‎(2)‎

• UCREFRP Catalog Number: 7124
• Author: McAda, C. W. and R. S. Wydoski.
• Title: Technical Papers
• USFW Year: 1980.
• USFW - Doc Type: \
• Copyright Material: YES

The Walker Wildlife Area (station 8) contains a
flooded gravel pit (25.6 ha, 0.5-2.0 m deep) connected
to the Colorado River near Grand Junction, Colorado.
No aquatic vegetation occurs in the pit, but the flood-
ing of tamarisk along the shoreline in spring provides
cover for small fish. The bottom of the pit is composed
primarily of gravel covered with thick layers of silt.
The shoreline is lined with cobbles 5 to 10 cm in diam-
eter. Acanal 6 m wide that permanently connects the
pond with the river gives fish free access to the main
river.
The Colorado River near the Walker Wildlife Area is
80-120 m wide and has alternating stretches of slow
and fast water. The banks are covered primarily with
tamarisk and cottonwood trees. The removal of large
quantities of irrigation water upstream from the study
area substantially reduces streamflow during the
summer. Several irrigation return flows enter the river
in this study section.
General Materials and Methods
Water temperatures were monitored at stations 3
and 8 with a Ryan 30-day thermograph. Water tem-
peratures at other stations were taken with a pocket
thermometer on the dates when fish were collected
there. Turbidities were measured with a Jackson tur-
bidimeter, and water velocities with a Gurley current
meter.
Fish were sampled from August 1974 through
November 1976. Shelf and floating ice prevented effec-
tive sampling during the winter. Most fish were col-
lected in trammel nets 26 to 90 m long, with an inside
mesh of 2.5 cm (bar measure) and an outside mesh of
25 cm. Suitable areas were sampled with a 30-m bag
seine with 2.5-cm mesh. Small fish were captured with
a 5-m seine with 3-mm mesh. Swift, shallow-water
areas were sampled with an electroshocker (120-V DC
generator with a variable voltage pulsator) mounted
on a 3.6-m aluminum boat. Electrofishing was rela-
tively ineffective during periods of high water because
of the swift current and high turbidity. Efficiency of
the trammel nets was also reduced at these times by
the large amounts of floating debris and high water
velocities.
Endangered or threatened species collected were
returned to the river, although a few died from injuries
received during capture. Razorback suckers that died
were frozen and returned to Utah State University for
enzyme analysis. Razorback suckers were sexed before
release by external characteristics determined from
examination of preserved specimens. The last ray of
the anal fin is curved in mature male razorback
suckers but is straight in mature females. The fish
were weighed and measured, and a scale sample was

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Fig. 2. Distribution of the razorback sucker in the Colorado
River basin: historical (all circles) and present (solid circles
only-. (Question mark indicates uncertain record.)
removed. All razorback suckers caught were tagged
with serially numbered Floy anchor tags and eight
with ultrasonic transmitters, before they were released
near the point of capture.
Distribution and Abundance
The range of the razorback sucker has been mark-
edly reduced over the years (Fig. 2) primarily because
of man-made alterations of the Colorado River system.
Early observers reported the species in large numbers
throughout the lower basin. Everman (1916) stated
that razorback suckers were "rather abundant" in the
Salton Sea when it was formed after the breaking of a
dike in 1905. Chamberlain (1904) reported that they
were "rather common" in the San Pedro River and
were sold at nearby Tombstone, Arizona, as "Buffalo"
(the San Pedro is now dry during most of the year-.
Chamberlain (1904) collected a single specimen from
the Salt River in central Arizona, near the present site
of Roosevelt Dam. Miller (1961) reported, however,