9591

• UCREFRP Catalog Number: 9591
• Author: Nesler, T. P.
• Title: Interactions Between Endangered Fishes and Introduced Gamefishs in the Colorado River, Colorado, 1986-1991.
• USFW Year: 2002.
• USFW - Doc Type: 91-29,
• Copyright Material: NO

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

and recruitment of razorback sucker was possible even in the presence of a large nonnative fish
population, though this recruitment was still inadequate to support a viable population.

Uncertainties
Data analyses in this investigation were limited by the small sample size of target gamefish
species collected by seines and the number of backwaters available for sampling. Small sample
sizes of centrarchid fish species limited the strength of inferences that could be made about diet
characteristics and selectivity of fish prey species. Daytime stomach sampling of gamefish
predators with crepuscular feeding habits may have under-represented the frequency of fish prey
in the diet, especially with regard to larval fish. This same factor also probably contributed to the
greater frequency of occurrence of fish prey that were not identifiable due to advanced digestive
action. While nighttime sampling is recognized as the most desirable approach for remedying
these biases, difficulty of nighttime navigation of the study reach by boat to access the limited
number of backwaters available throughout study reach made this sampling option not feasible.

The absence of Colorado pikeminnow in largemouth bass and green sunfish stomachs in
this investigation, coupled with the low relative abundance to absence of young and juvenile
Colorado pikeminnow in the study reach precluded a factual conclusion concerning preference by
these centrarchid species for Colorado pikeminnow as prey. Whether or not juvenile and adult
largemouth bass and green sunfish, as top predators, are abundant enough to shape the
composition and relative abundance of the associated fish species in a backwater through selective
predation, either at sites of local concentration or generally throughout the study area could not
be determined. The ongoing nonnative fish control study targeting removal of juvenile and adult
centrarchids in backwaters may provide new information concerning diet and backwater fish
community response to reduced centrarchid predation.

The mismatch between years of seine sampling for young and early-juvenile centrarchids
and block and shock sampling for late-juvenile and adult centrachids prevented using length
frequency distributions from year to year as an assessment of survival and recruitment of juvenile
fish to adult populations found in backwaters. The riverine versus pond origin of centrarchid
populations found in backwaters will be best evaluated through stable isotope analyses currently
ongoing within the Recovery Program.

Management Optionsl Altematives
In order to be effective, the control of nonnative fISh species should be guided by an
understanding of which species and which life stages in which riverine habitats represent the most
significant limiting factor to reproduction, recruitment, growth and survival for native and
endangered fish species in the Colorado River. In the case of nonnative fish species that provide
desirable fishing recreation opportunities, it is also important to establish a reasonable justification
for control actions with respect to conservation/recovery objectives and sportfishery objectives.

Relative abundance data suggests centrarchid fish species have been, and will continue to
be a minor segment of the Colorado River backwater fish community. The centrarchid population

39