8114

• UCREFRP Catalog Number: 8114
• Author: Nesler, T. P.
• Title: Five-year Stocking Plan for Endangered Colorado River Fish Species in Colorado.
• USFW Year: 1998.
• USFW - Doc Type: Denver, CO.
• Copyright Material: NO

This review and evaluation of population status and recruitment potential for the
Grand Valley population of Colorado pikeminnow does not provide much support for the
efficacy of depending upon natural recolonization of the target unoccupied reaches of the
upper Colorado and Gunnison by the existing adult population, or using the Grand Valley
adult population as a donor for translocating sufficient numbers of fish above the instream
barriers in one or both reaches to create an effective population size in a timely manner.
Translocation raises concern over additional risks that include:

1) reproductive success of translocated adult fish taken from a home range; it cannot be
assumed to be the same in the new environment for a species with a complex
spawning behavior. (Conversely, the expectation of reproductive success in stocked
populations must also be tested through monitoring.)

2) the complex reproductive cycle of this species, including factors of imprinting,
chemoreception, learned behavior, or other instinctual behaviors are not understood.

3) reproductive tradeoffs cannot be assumed equal or insignificant between established
patterns in Grand Valley sites and new sites upriver with respect to spawning
behavior, spawning success, hatching success, and larval and young-of-the-year
survival.

Given that the adult Colorado pikeminnow population is about 4 fish/mile in the
Grand Valley reach and 3-3.4 fish/mile in the Utah reach below Westwater, and recruitment
of strong year classes is currently infrequent, depletion of the donor population from either
source appears possible. If one considers the lower ends of the interval estimates provided in
Osmundson and Burnham (161 and 186 fISh) rather than the midpoints, which are within the
95 % confidence limits for estimated population size, than depletion of the donor population
seems likely. Given a survival rate of 0.86 and the translocation of 10 fish/year, it would
require the translocation of 100 adults over a 10 year period to create a new population of 55
fish. Reliance upon occurrence of another strong year class during the period of
translocation effects seems risky for a population considered vulnerable to extinction from
stochastic demographic changes.

This should not be construed as opposition to translocation as part of the strategy for
establishing Colorado pikeminnow populations in the unoccupied reaches of the upper
Colorado and Gunnison rivers. In addition to fish that use the Redlands passageway, it
would also appear desirable to translocate similar-sized fish captured immediately below the
barrier within the plunge pool downstream of the dam. This can be viewed as assisting the
passage of fish that cannot or will not use the constructed passageway and facilitating natural
mechanisms of migration, dispersal and recruitment. Osmundson et aI. (1996) demonstrated
significantly greater non spawning movements upstream for juvenile/early adult Colorado
pikeminnow than for larger, older adults. Though not stated explicitly by Osmundson et al.,
this attribute may be evidence of a population dispersal mechanism by which young fish
recruit into and thus maintain the upstream adult populations. The use of the Redlands

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