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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or lack of growth) are apparent. For the Green River, the bulk of the population is
made up of individuals that are 350-650 mm, resulting in a population size
structure noticeably different from that for the Colorado, White, and Yampa
populations. In contrast, the Colorado, White and Yampa populations appear
comprised of individuals in mostly the 500-750 mm size range. This contrast is
more apparent when the length frequency distributions are condensed to small,
mid, and large size categories (Table A-4). The seven-year averages for these
size categories per river show the Green River with 26% of the fish ~350 mm and
4% ~601 mm. The Colorado, White and Yampa rivers have 4% ~350 mm, 13%
~450 mm, and 1 0% ~500 mm, respectively. The small size category thresholds
were increased in each of these three rivers to be more representative of the size
structure observed. In the large size category, the populations in the Colorado,
White, and Yampa rivers have 29% ~601 mm, 14% ~651 mm, and 8% ~701 mm,
respectively.

Estimation of recruitment percentages for these populations also were
affected by the different population size structures noted above. As a result,
different size groups were used to define recruitment. This was in part a function
of the numbers of fish sampled, but also may be attributed to the presence and
location of nursery and juvenile habitat. On one end of the spectrum is the Green
River, within which an adult population lives in and around nursery and juvenile
habitat. The Colorado River population is comprised primarily of an upstream
adult segment and a downstream juvenile segment. Similarly, the White River is
an upstream tributary adult population located adjacent to the major nursery
habitats of the Green River. On the other end of the spectrum, the Yampa
population is an adult population that is located a considerable distance from the
downstream nursery habitats of the Green River and separated by three
successive canyon reaches (Yampa, Whirlpool and Split Mountain).

Recruitment decreased over the 7 -year estimation period for the Colorado
River population from over 30% in 1991-1992 to 4% in 1995-1997. Given the
abundance and abundance trend observed for this adult population, this
recruitment trend is not a cause for immediate concern. One could expect that
recruitment into a growing population of long-lived fish would eventually be
reduced as this population grows larger and carrying capacity for adult fish is.
reached. Osmundson (1999) suggested this was the case for the Colorado River
population from 1991-1994. Assuming no change in adult mortality and a stable
habitat carrying capacity, recruitment would naturally be low until mortality reduces
a significant portion of the older, larger fish. Under such population and habitat
conditions, low recruitment rates would be a natural part of a stable population.
Under a no-change scenario on the Colorado River, population abundance, length
frequency distribution and recruitment would have to be monitored for 3-5 more
years to establish a population-level decline and cause for concern. Carrying
capacity for adult Colorado pikeminnow in the Colorado River may be enhanced by
range expansion past existing barriers, a reduction in biomass of other nonnative
fish predator species, or an increase in the biomass of the fish prey base. The

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