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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sexual maturity necessary to be part of the adult population. It is assumed that
these fish are completely vulnerable to capture by net and electrofishing sampling.
The percent of this size group to the total number of fish used in the distribution
represented annual recruitment, and was estimated for each year a length
frequency distribution could be constructed. This approach is an indirect means of
characterizing and maintaining stability in the population in terms of successful
reproduction of young and survival to the adult stage.

Length frequency distributions were condensed into three major size
categories to characterize the size structure of the population. These size groups
included a small-size category for fish less than recruitment size, a mid-size
category for the bulk of the adult population, and a large-size category for the older
adults. The purpose for designating these categories was to describe and
maintain the observed size structure of the population. This use of length
frequency categories was also an indirect means of maintaining stability in the
adult population in terms of age composition, growth and survival. A percentage
range was provided for each size category for each population using rolling
averages of annual estimates as a means of accounting for natural variability
between populations and smoothing year-to-year variability in reproduction, year
class strength, and mortality/survival within populations. These rolling averages
were based on the observed annual length frequency distributions. Where the
number of annual estimates permitted, rolling five-year averages for Colorado
pikeminnow and three-year averages for humpback chub were examined for each
size group. The use of five and three-year rolling averages for Colorado
pikeminnow and humpback chub, respectively, was part arbitrary and part a
reflection of differences in maturation ages between the two species. The annual
variation observed is likely a result of a combination of natural and sampling
variability. The recruitment criteria are also presented as rolling averages to offset
the impact of annual variation and provide a more coherent criterion.

Other Criteria: Along with measurable biological criteria, one of the
benchmarks of recovery is demonstration of adequate habitat protection. For
these fish species, the primary habitat concerns are flows, flow regime, flooded
bottomland/backwater nursery habitats, instream passage and non-native fish
populations. Given uncertainty in what is needed in these habitat areas to achieve.
and maintain biological recovery goals, institutional agreements controlling
nonnative fishes stocking, flow maintenance and other habitat management
provide a means for developing protective measures for perceived and existing
threats without implicating downlisting requirements, statutes, or regulation for
experimental measures. Recovery goals should be sufficient to sustain
populations in the face of threats that will continue to exist if they cannot be
removed or reasonably reduced. Evaluation of population parameters in existing
populations that appear stable or improving under existing environmental variables
and threats is a logical basis for determining recovery goals for these fish species.
If populations of the listed fish species are documented as meeting the biological
goals necessary for delisting, then it is reasonable to assume that the necessary