8161

• UCREFRP Catalog Number: 8161
• Author: Osmundson, D. B., R. J. Ryel and T. E. Mourning.
• Title: Growth and Survival of Colorado Squawfish in the Upper Colorado River.
• USFW Year: 1997.
• USFW - Doc Type: \
• Copyright Material: YES

688

OSMUNDSON ET AL.

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UPPER REACH

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KILOMETERS

COLORADO

FIGURE I.-Map of Ihe upper Colorado River study area. Transverse Jines indicate boundaries of upper and lower
study reaches (Westwater Canyon was not studied). Upstream-most lines mark sites of physical barriers to fish
movement; no such barriers exist at other Jine locations.

Problems inherent in scale-based aging and
growth estimation for long-lived species, such as
Colorado squawfish, prompted us to refine existing
growth estimates by using new mark-recapture
data. Age estimates so derived also enabled us to
assess rates of adult survival. Our primary objec-
tive was to refine estimates of Colorado squawfish
growth rate by minimizing use of scale analyses
and relying instead on recaptured PIT-tagged fish.
A secondary objective was to estimate adult sur-
vival rate with an approach modified from models
(e.g., Seber 1982) that use declining numbers of
increasingly older fish in the population. A third
objective was to estimate age of Colorado squaw-
fish of various sizes and provide an estimate of
potential longevity. In addition, we discuss the dis-
appearance of very large individuals reported in
historical accounts and mortality factors that may
at present reduce survival rates.

Methods

Study Area

The study area included the occupied range
(about 300 km of river) of the Colorado squawfish
in the Colorado River upstream of the Green River
confluence (Figure 1). Locations are in river ki-
lometers (rk) from the Green River confluence (rk
0.0), converted from river miles mapped by Belk-
nap and Belknap (1974). Further upstream move-
ment is blocked seasonally by a dam at rk 298.1
and all year by another dam at rk 303.0. Upstream
access to the Gunnison River, a major tributary
entering at rk 275.1, is blocked all year by a dam
3.5 km upstream of the mouth. Most adult Colo-
rado squawfish live in the upper part of the study
area; earlier life stages are generally found down-
stream. The study area was divided into upper and
lower reaches based on distribution of life stages..

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