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

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River demonstrated robust weight gain with increasing length.

Based on length frequency distributions of predators and prey and predator gape sizes,
most small-bodied fish species in Colorado River backwaters were vulnerable to predation by the
sizes of the centrarchid fish species present. Eighty percent of the nonnative cyprinid populations
sampled in backwaters were of suitable prey size to largemouth bass larger than 110 mm in length.
Largemouth bass was the most piscivorous of the nonnative gamefish species present. Largemouth
bass and green sunfISh diets suggest an opportunistic mode of predation. The relative impact of
predation on native fish by nonnative gamefish species in Colorado River backwaters was judged
to be relatively small due to small gamefish numbers or low frequency of fish ingestion, and a
relatively large alternative fish prey base comprised of nonnative cyprinids. Due to their
numerical dominance, red shiner and fathead minnow may be a much more significant competitive
threat to the growth and survival of latval and young-of-year Colorado pikeminnow and razorback
sucker young compared to predation by nonnative game fish species.

Concentrations of centrarchids in some backwaters may present a locaIized predatory
impact to all fishes present. Habitat suitability criteria for centrarchid fish species suggest the
Colorado River mainchanne1 environment is not conducive to their reproduction or smvival. 00-
channel habitats approaching or mreting these suitability criteria of low flow velocity and wanner
water are more likely to support and maintain populations of centrarchid gamefish that may
subsequently contribute escaped fish to native fish habitats. Centrarchids may also tend to
concentrate in floodplain ponds and backwater complexes intended for endangered fish nursery
habitats.

Potential competitive pressure for fish prey between adult channel catfish and adult
Colorado pikeminnow does not appear high based on differences in their relative frequency of fish
ingested and the abundance of nonnative cyprinids as a potential prey base. However, greater
numbers of channel catfish in backwaters and the mainchanne1 compared to Colorado pikeminnow
make competition for available fish prey a possibility. Spawning by channel catfish in the
Colorado River is most likely affected by temperature regime, which may impede catfish
reproduction, and survival and growth of young in some years. Reproduction and recruitment
appears adequate to maintain the catfish population over time. The relative abundance of adult
channel catfish in Colorado River backwaters in July may pose a predation problem for survival
and recruitment of young razorback sucker based on evidence from Lower Basin studies of
channel catfish predation on stocked razorback sucker juveniles.

Nonnative small-bodied cyprinids, common carp, and channel catfish should be a primary
focus of nonnative fish control actions in the Colorado River to reduce their abundance in potential
nursery habitats for Colorado pikeminnow and razorback sucker. Mechanical removal of
nonnative fish from backwaters via seining and mechanical removal of channel catfish and
common carp from summer backwaters via trapping or controlled harvest should be evaluated for
short-term benefits to Colorado pikeminnow, razorback sucker, and other native fish species.
Limiting escapement of centrarchids and other nonnative gamefish from sportfisheries in