9378

• UCREFRP Catalog Number: 9378
• Author: Nesler, T. P.
• Title: Recovery of the Colorado River Endangered Fishes
• USFW Year: 2000.
• USFW - Doc Type: Biological recovery goals and criteria for Colorado pikeminnow, Humpback chub, Razorback sucker & Bonytail.
• Copyright Material: NO

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for the Little Colorado and Colorado rivers in Grand Canyon. Using a similar
approach for the 1991-1997 data set, estimates were 619 fish in 1994 and 455 fish
in 1997 (Table A-5). 95% confidence intervals for these two estimates overlap
greatly. Abundance estimates for 1994 and 1997 were 1,234 and 1,159,
respectively at 76% survival; and 2,039 and 2,495, respectively at 90% survival.
Only at the 90% survival rate did the estimates increase between 1994 and 1997.
In comparison, Pfeifer et al. (1998) estimated this population to be 1,528 fish in
1998 (95% CI: 888-2,750).

For the Westwater population, population estimates for 1993-1997 ranged
from 1,164-3,243 fish under a 59% survival rate, 2,039-4,493 fish under a 76%
survival rate, and 3,287-5,719 fish under a 90% survival rate (Table A-6). For all
three sets of estimates, a steady decline is noted from 1993-1997. Concurrent
with this trend is a steadily rising recapture/capture ratio from 1993-1997. It is
possible that the decline observed is either a progressive improvement in the
accuracy of the population estimates with increasing numbers of marked fish in the
population and number of recaptures in the catch, or a real decline in population
abundance. By comparison, Chart and Lentsch (1999) estimated the humpback
chub population separately in three small reaches (0.25-0.81 mi) within river miles
119-124 of Westwater from 1994-1996 using a Lincoln-Petersen single mark-
recapture estimator. Combining the three estimates for each year resulted in
population estimates of 5,621 fish in 1994, 10,148 fish in 1995, and 5,186 fish in
1996. For most of the individual reach population estimates in Chart and Lentsch,
the 95% confidence intervals were +/- 100% or more of the point estimates.

The remaining population segments of humpback chub have single
abundance estimates for different years. These include: 600 fish in Yampa
Canyon (Nesler et al. 1992 from Karp and Tyus 1990), 1,500 fish in Desolation-
Grey Canyons (Valdez et al. 2000 from Chart and Lentsch 1998), 500 fish in
Cataract Canyon (Valdez et al. 2000 from Valdez 1990), 4,508 fish in Little
Colorado River (Douglas and Marsh 1996), and ~3,707 fish in the Colorado River
in Grand Canyon (Valdez and Ryel 1997).

Length frequency distributions for the Black Rocks and Westwater Canyon
humpback chub populations (Figures 9-10, Table A-7) reflect relatively stable size
structures with the influence of variable year class strength apparent in the size,
number and location of peaks. The Black Rocks population shifted from a bimodal
distribution in 1991 to a unimodal distribution in 1994. In 1991 the bulk of the fish
were in the 300-380mm size groups and in 1994 the bulk of the fish were in the
280-320mm size groups. In 1997, the bimodal distribution was reappearing similar
to 1991, though the bulk of the fish were still in the 280-320mm size groups. By
comparison and unlike Black Rocks, the length frequency distributions of the
Westwater population were dominated annually by smaller fish ~260mm. Shifts in
locations of peaks in the Westwater distribution were noted between 240-320mm.
No lateral movement of the distributions toward the larger size groups, indicative of
declining recruitment, was noticeable.

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