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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conducted by numerous researchers and biologists. Population estimates and
length frequency distributions were developed for all riverine populations of the
endangered fish species where available data was deemed sufficient to provide
meaningful results. The purpose of these analyses was to establish the current
status and trends demonstrated by each population, and examine the variability of
population abundance, length frequency distribution and recruitment over the
available range of years. These population parameters are used as recovery
criteria and goals for each were developed using the results of the database
analyses to provide a relevant biological context.

Population estimation: Estimation of population abundance was performed
annually using the Chapman modification of the Petersen mark-recapture method
(Ricker 1975). Confidence intervals for these estimates were calculated from
Poisson frequency distribution tables in Ricker (1975) using the recapture number
as the determining variable.

For each year, the time period during which the target species was sampled
and recapture tags recorded, and the range of river miles in which this sampling
occurred was noted. From the number of the target species captured, the newly
marked, recaptures, total captures, and recaptures in the same year were
recorded. The marked population for a given year was estimated by applying an
annual survival rate to the previous years' cohorts of PIT-tag marked fish for each
year prior to the year of estimation. The newly marked fish in the year of
estimation were added into the estimation of the marked population in subsequent
years. The fish marked and recaptured in the same year were treated the same as
newly marked fish and were not regarded as recaptures for that year's population
estimation. Fish recaptured from previous years' marking were only counted as a
recapture once in the year of estimation, even though some were recaptured
multiple times in the year of estimation.

Two or three survival rates were used for Colorado pikeminnow and
humpback chub in estimating the marked population annually. Different survival
rates resulted in notable differences in the estimates of marked populations, and
thus, the population estimates themselves. For Colorado pikeminnow, 0.70 and
0.86 survival rates were used. The 0.70 survival rate was used to provide a
comparison with estimates derived from the Colorado IRO report (Nesler et al.
1992). The 0.86 survival rate was used based on the work of Osmundson et al.
(1996). For humpback chub, 0.59, 0.76, and 0.90 survival rates were used. The
0.59 rate used in Nesler et al. (1992) was based on age series data from Kaeding
and Zimmerman (1983) for the humpback chub population in the Little Colorado
and Colorado rivers in Grand Canyon. The 0.76 and 0.90 rates were based on
humpback chub mark-recapture data and open population model estimates for the
Grand Canyon population in Valdez and Ryel (1995). For the purpose of
population status assessments, the 0.86 survival rate for Colorado pikeminnow
and 0.76 rate for humpback chub were judged to be the most representative for
each species.