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

sunfish, and channel catfish will all prey upon Colorado pikeminnow larvae, but this predation
rate decreases with darkness, turbidity, alternative invertebrate prey, and increasing size of larvae.
Largemouth bass and the other gamefish species would also have an abundance of nonnative
cyprinid larvae to prey upon in backwaters. The limited larval drift results indicated Colorado
pikeminnow were 10 percent of the drifting fish that could be deposited into backwater habitats.
If centrarchid feeding activity is greatest during crepuscular periods, then larvae of all fish species
would seem to be the least detectable comPared to larger prey due to their small size and
translucent bodies. Colorado pikeminnow larvae were the smallest of the fish larvae identified in
the drift samples. Given the overall abundance of nonnative fish in backwaters, however,
predation on larval Colorado pikeminnow during daylight hours maybe significant, especially if
cover is lacking. The results of this field investigation were unable to confinn the laboratory
experiment results on predation on Colorado pikeminnow larvae found in Muth and Beyers.

Using laboratory tank experiments to investigate prey choice by largemouth bass,
Osmundson (1987) concluded that largemouth bass selected Colorado pikeminnow and fathead
minnow preferentially as prey over red shiner and green sunfish. These results suggest Colorado
pike minnow juveniles and fathead minnow may have behavioral differences from red shiner and
green sunfish that make them more vulnerable to bass predation. In pond studies, Osmundson
concluded fathead minnow populations that were considered abundant did not buffer stocked
Colorado pikeminnow from bass predation. Colorado pikeminnow used in these experiments were
40-14Smm in the pond experiments and 50-12Omm in the laboratory tank experiments.
Significant predation by bass on Colorado pikeminnow in these ponds may also be attributed to
a difference in behaviors between resident wild prey species (including fathead minnow,
mosquitofish, green sunfish, "red shiner and bluegill) and a stocked naive prey species (Colorado
pikeminnow). Colorado pikeminnow young were stocked at 3,090 fishlha into four small ponds
(0.85-3.16 ha) harboring largemouth bass populations of 11-319/ha with a mean of 85/ha
(weighted for pond size). Osmundson's field results suggest the more that backwaters and flooded
bottomlands act like pond habitats, the more juvenile and adult largemouth bass they may support,
and the less favorable these habitats will be for the survival of an influx of Colorado pikeminnow
larvae or juveniles due to predation. This investigation found few backwaters simulating pond-like
conditions with significant juvenile-adult bass populations. Based on ISMP and other studies, a
population of Colorado pikeminnow So-lSOmm long was lacking in the study area.

Using Osmundson's analysis of fish prey vulnerability to bass predation, it may be inferred
that all fish < 100 nun in length would be vulnerable to a backwater population of juvenile bass
mostly 50-ISO mm in length. The smallest green sunfish found by Osmundson (1987) to ingest
a Colorado pikeminnow was 103 mm, though the size of fish sampled was restricted by the
sampling equipment used toperfOlm nonlethal sampling. Only 21 of 223 green sunfish (9%) in
this investigation were Ll00 mm in length. Osmundson also found that green sunfish ingest prey
up to 60 % of their body length, so a 30 mm long green sunfish (the most abundant size class noted
in ISMP seine samples) could conceivably prey upon 18 mm long Colorado pikeminnow.

Results of this study indicate the potential nonnative cyprinid fish prey base is large

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