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

Annual survival rates for fish >_ 550 mm TL were estimated by comparing measured size
distributions with simulated stable age and size distributions; these estimates ranged from
0.83-0.88 (p < 0.05) with the best fit at 0.86. This estimate was later corroborated with
results from the Cormack-Jolly-Seber model which utilized capture-recapture histories of
individual fish. An annual survival rate of approximately 0.86 is not considered low for a
long-lived fish; therefore, low adult survival rate is likely not a major cause of the decline of
this population. However, survival rate does affect the potential for population increase;
hence, any management activity that can increase this rate would likely benefit this population.
Distribution and dispersal patterns
Longitudinal distribution of Colorado squawfish was described in terms of mean size of fish
and mean number of adults captured per net in six river strata. Average length of Colorado
squawfish progressively increased upstream and was most pronounced during the first years of
the study. During 1991-1992, most adults (> 500 mm TL) were concentrated in the upper
reach and very few adults were found in the lower reach. As the study progressed,
demographics changed as 2-3 consecutive strong year classes, hatched during the mid-1980's,
grew and dispersed as they recruited to the adult population. As a result, average length
increased in lower-reach strata and decreased in upper-reach strata. By 1993, there was a
five-fold increase in catch rates of adults in the lower reach.
Subadult and adult Colorado squawfish were often patchily distributed during the spring
sampling period. The proportion of sampled backwaters yielding Colorado squawfish ranged
from 16 to 50% depending on the year and river strata sampled (the river was subdivided into
6 study strata based on changes in average channel gradient: three below Westwater Canyon,
strata 1-3, and three above Westwater Canyon, strata 5-7; Westwater Canyon itself, Stratum
4, was not sampled). Within Stratum 1 (most downstream stratum) as much as 55% of the
total fish captured within a given year came from a single backwater; within Stratum 3, as
much as 89% came from a single backwater; within Stratum 6, as much as 78%; within
Stratum 7, 52%. The highest densities of backwaters occurred in the Grand Valley portion of
the river: 0.78-0.91 backwaters/km in Stratum 6 (18-mile reach) and 0.62-0.75
backwaters/km in Stratum 7 (15-mile reach).
Movements of fish captured one year and recaptured in a subsequent year were compared
among size classes of fish and between those fish initially captured in the lower reach and
those captured in the upper reach. Of those Colorado squawfish initially captured and tagged
in the upper reach, only 16% were later located > 10 km from the previous capture site; of
those tagged in the lower reach, 58% were later located > 10 km from the previous site. Of
those movements > 10 km, 80% were directed upstream for upper-reach-tagged fish; 68%
were directed upstream for lower-reach-tagged fish. Also, for lower-reach-tagged fish, 44%
of movements > 10 km were movements from the lower reach to the upper reach, whereas
none of the fish located in the upper reach during the study period were later recaptured in the
lower reach. Distance moved was inversely related to fish size: displacement of fish < 550
mm TL averaged 33.6 km; for those > 550 mm TL, average displacement was only 7.5 km.
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