7835

• UCREFRP Catalog Number: 7835
• Author: Osmundson, D. B., et al.
• Title: Studies Of Colorado Squawfish In The Upper Colorado River, Final Reports.
• USFW Year: 1997.
• USFW - Doc Type: Recovery Implementation Program, Project No. 14,
• Copyright Material: NO

Methods
For these studies backwaters were trammel netted during spring runoff each year from 1991
through 1994. Subadults and adults congregate in these low-velocity habitats during runoff
when main-channel flows increase dramatically. Fish were entrapped in backwaters by a
method dubbed `scare and snare'. Using a jon boat and a crew of two people, the open end of
each backwater was first blocked with a trammel net; the boat then entered the backwater by
passing over the net with the motor raised. Once inside, the boat was used to scare fish
toward the net by vigorously motoring back and forth beginning at the far end of the
backwater and working towards the mouth. Nets were pulled as soon as sufficient `scare'
effort was expended (5-15 min depending on backwater size). The entire range of the
Colorado River population upstream of the Green River confluence was systematically
sampled each year, with three complete sampling passes made annually through the upper
reach (upstream of Westwater Canyon) and two annual sampling passes made through the
lower reach (downstream of Westwater Canyon). With each pass, every backwater suspected
of harboring adult Colorado squawfish was netted. Sampling effort was augmented with
electrofishing in some segments where backwaters were few. Additional capture data from
concurrent studies were utilized to the extent possible. Data from fish recaptured during other
studies in 1995 were also used to supplement growth and movement data.
Results and Discussion
Growth, age and survival
Mean annual growth rates of 3- to-6-year-oid fish were calculated from measured lengths of
individuals aged by scale analysis. The highest rate was 82.0 mm/year when fish were three
years old. By the time fish were six years old, average growth rate declined to 32.2 mm/year.
Growth rates for older fish were estimated from changes in total length (TL) of marked and
later recaptured individuals. These annual growth increments were averaged for fish within 50
mm size groups. For fisli 500-549 mm TL, annual growth increments averaged 22.2 mm/yr.
For fish > 550 mm TL, growth increments averaged 8.7 mm/year, but varied (although not
significantly) from 15.0 (700-799 mm-long fish) to 3.9 mm/year (800-899 mm-long fish).
Thus, rate of growth slows markedly after- about age 10, although there appears to be an
increase in rate when fish are between 650 and 800 mm TL; beyond 800 mm TL, growth
apparently slows again. For fish up to 10 years old, results here were similar to those
reported in previous studies. However, measured increments from capture-recapture data
indicated that previous scale-based methods over estimated real growth rates in older fish (>
10 years old).
Based on these new growth curves, ages offish of various length were calculated.
Simulations indicated broad ranges of ages for fish of given lengths, especially fish > 550 mm
TL: it takes 10-22 (X = 15) years for 95% of the fish to reach 600 mm in length; 16-30 (X =
25) years to reach 700 corn; 20-40 (X = 32)years to reach 800 mm.
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