7025

• UCREFRP Catalog Number: 7025
• Author: Hawkins, J. A. and T. P. Nesler.
• Title: Nonnative Fishes in the Upper Colorado River Basin
• USFW Year: 1991.
• USFW - Doc Type: An Issue Paper.
• Copyright Material: YES

28

a competitive advantage over native species and that evidence existed but was unable to cite
it Evidence of competitive interaction between headwater trout species was also suggested
as a generally accepted and documented fact

Forty percent of 20 respondents answered yes or "perhaps" to the beneficial effect
segment of the question. Most of these respondents suggested one or more of the
introduced assemblages of red shiner, sand shiner, fathead minnow, and redside shiner
provided food for Colorado squawfish. One respondent indicated this aspect of native fishes
had not been adequately investigated.

Answering this question was more imposing to respondents than intended due to our
inability to provide respondents with the literature synthesis and citation list that was part
of this study. Our intention was to provide our literature base to other experts for their
review. Answering this question would then have provided respondents the means for filling
in gaps in our literature review. The literature base was later provided with a draft copy of
this report to all respondents for review.

Question 6: For problematic introduced fish species, what is your evaluation of their
abundance, and the magnitude of their impact(s )-increasing, decreasing, constant,
fluctuating with environmental variables, etc.? (please identify environmental
influences if known)

For small-bodied fish species including red shiner, sand shiner, redside shiner, and
fathead minnow, one or more of these species were considered by consensus to be
widespread and abundant within many drainages. Thirty-eight percent of the respondents
cited the abundance and impact of these small species to be inversely influenced by flow
regime and associated environmental variables. In most cases, the impact of smaller species
was judged to be constant, but fluctuating in severity with abundance as influenced by flow.
Red shiner was noted as the most likely single, small-bodied species to be exerting a negative
impact

I

I

Northern pike, channel catfish, and common carp were most often listed as specific
problematic large fish species. Four of five respondents considered pike abundance to be
increasing in the upper basin except within the upper Colorado River. Catch rate statistics
were provided indicating northern pike in the Green River have increased since 1975 and
equal the catch rate of Colorado squawfish in 1986-1988. Channel catfish were considered
to be very abundant and fluctuating in year class strength with flow and temperature regime.
The impact of channel catfish was considered to be at a constant level. Common carp were
considered very abundant to the point of ubiquitous, and their impact at a constant level.
Carp were considered to be very opportunistic with changing habitat availability.
Centrarchids, especially largemouth bass, green sunfish, and smallmouth bass; and walleye
were considered to be common in the upper basin with areas of local abundance and impact
One respondent listed that the predaceous impacts of largemouth bass may be relatively
great even though abundance is low. The impact of white sucker was listed as increasing in