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

EXECUTIVE SUMMARY

The primary focus of this study was to detennine how well populations of non-native
wannwater gamefish species have adapted to the riverine environments in the Colorado River, and
to evaluate their potential impacts upon endangered fishes from predation and competition.
Limiting factors affecting these game fish species that are imposed by environmental conditions
in the Colorado River were investigated to help define problems posed by these species to the
native fishes. Know ledge of limiting factors and life history traits of these gamefish species may
also indicate recovery measures necessary to minimize negative interactions with the endangered
fishes. Data collected was used to assess population abundance trends, backwater habitat use and
diet relative to existing or potential negative interactions with Colorado River endangered fishes
occurring in the Colorado River, and review potential control options.

Nonnative gamefish were consistently a smaIl percentage and nonnative cyprinids a large
percentage of the backwater habitat fish community collected in September seine samples. Six
gamefish species comprised 1 % of the total number of fish collected by seines in Colorado River
backwaters from 1986-1991. Red shiner and fathead minnow comprised 69-97 %. Green sunfish
was the most abundant game fish species, followed by black bullhead, largemouth bass and channel
catfish. Bluegill and black crappie collections were incidental. The number of backwaters
available for sampling was limited. There were no significant correlations between measures of
seine sampling effort and backwater habitat area. Seine catch rates by year for centrarchid and
ictalurid gamefish species did not show consistent correlations among species to amount of
backwater habitat area seined or available annually. Black bullhead catch rates were significantly
and positively correlated with area seined, but not with backwater habitat area available. The
catch rates of juvenile channel catfish and green sunfish were significantly correlated to backwater
habitat available but not area seined. Largemouth bass numbers collected were not significantly
correlated with area seined or backwater habitat area available. Colorado pikeminnow catch rates
were negatively correlated with area seined and backwater habitat available, but not significantly.
There were no significant correlations between gamefish species or between gamefish species and
Colorado pikeminnow. Colorado pikeminnow catch rates were negatively correlated with
icta1urids and green sunfish, and not correlated with largemouth bass. Nonnative gamefish species
were distributed ubiquitously throughout the study reach at low abundance levels with sporadic
concentrations in a few backwaters each year. Centrarchid fish populations were comprised
mostly of young-of-year and juvenile fish. It is unknown if these young fish recruit into adult
populations or replaced annually from floodplain ponds.

Adult channel catfish and common caIp were the most abundant large-bodied fish species
in Colorado River backwater habitats during April and July, making up 85 % of the catch in 1989-
1990. Differences in adult fish numbers captured in backwaters were noted between April and
July samples in 1989 and 1990, and may have been associated with differences in timing and
magnitude of the spring peak flows and associated water temperatures between years. Adult
Colorado pikeminnow and other native fish species share backwater habitat in April with channel
catfish and common carp, but vacate these habitats by July. Adult channel catfish in the Colorado

vii