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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Using each year's estimated population size, a number of fish per river mile,
based on the range of river miles sampled, was calculated as a means of
examining differences among years, rivers (Le. population segments), and
populations. Another calculated ratio was recaptures/total captures (R1C), which
presumably should increase in a population with stable numbers over time given
long-lived individuals with near-permanent PIT tags, and continued tagging. This
ratio could also increase, decrease, or stay the same with an increasing
population; or increase with a declining population, therefore the ratio must be
interpreted in context with concurrent population estimates. The ratio of total
captures to estimated population size was also calculated to examine the potential
magnitude and efficiency of sampling the target population. A ratio of "same-year"
recaptures/total captures was calculated to examine the potential magnitude and
stress impact of multiple captures on individuals within the population.

The use of mark-recapture methods to estimate population abundance
requires certain assumptions concerning geographic and demographic closure,
survival and capture probabilities. The mark-recapture data within the Recovery
Program database is a compilation of all sampling efforts by numerous
researchers and biologists throughout the Upper Basin each year for each river
reach. This approach assumes sampling effort per reach per year was more or
less equal, though the variability in the range of river miles and dates sampled is
evident from Tables A-1,S, and 6. Only a detailed assessment of the sampling
effort by gear, river miles, and dates per project per year per river would enable
verification of equal sampling effort. Exchange of marked fish between river
reaches is known to occur outside of the spawning season, but is assumed to be
negligible for the purpose of achieving geographic closure for population
estimates. Demographic closure is not assumed in this approach and the effects
of mortality and recruitment are accounted for as knife-edge impacts. Survival
between marked and unmarked fish is assumed to be equal and a constant annual
survival rate is applied to adjust the contribution of marked fish from each previous
year in determining the size of the marked population in the estimation year.
Similarly, recruitment to the adult population is assumed to occur primarily at the
end of the capture season, and these new recruits become of the marked and
estimated population in the following year. This is an oversimplification since both
processes act upon the marked and unmarked segments of each population.
continuously.

Lenath freQuency distribution and annual recruitment: Distributions of fish
lengths were derived annually from the length measurement data associated with
each endangered fish captured and recorded in the Program centralized database.
Length frequency distributions were constructed for Colorado pikeminnow by river
and year using SOmm increments ranging from <350mm to >900mm. For the
purpose of estimating recruitment, a minimum size interval associated with
sexually-mature, adult fish was established relative to the minimum size interval
that appeared to be representatively sampled based on available data.
Recruitment is defined here as the percentage of fish achieving the size and

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