9417

• UCREFRP Catalog Number: 9417
• Author: U.S. Department of the Interior.
• Title: Final Environmental Assessment
• USFW Year: 1998.
• USFW - Doc Type: Acquisition and Enhancement of Floodplain Habitats along the Upper Colorado, Green, and Gunnison Rivers as part of the Recovery Program for Endangered Colorado River Fishes.
• Copyright Material: NO

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In a study of predation on Colorado squawfish and various non-native fish, the largemouth
baSs showed a strong preference for native fish, eating twice as many young squawfish as
red shiners and as much as 20 times the number of squawfish as green sunfish.
(Largemouth bass, red shiners and green sunfish are not native to the Colorado River
. basin.)

Young Colorado squawfish have been found in the stomachs of channel catfish collected
in the Dolores River, even though squawfish are very rare in the area. Also, researchers as
well as early'settlers reported finding squawfish that had died trying to swallow channel
catfish. Apparently, the barbs of the catfish make it difficult or impossible for squawfish to
swallow and digest this non-native fish..

Utah State University researchers found that in the Duchesne River, 1- to 3-year-old
Colorado squawfish constituted 5 percent of the diets of northern pike - a
disproportionately high percentage, given that squawfish constitute a much smaller portion
of the available food base fu the river. Based on their findings, the researchers concluded
that a single northern pike could consume more than 100 year-old squawfish per year.

(frout tend to inhabit colder stretches of river than the four endangered fish and generally are not
considered a threat.)

. Other factors

Some portions of the upper Colorado River basin have high levels of selenium. Animals need
this naturally occurring element in trace amounts. But in larger concentrations, selenium can
cause severe birth defects in wildlife. Researchers are conducting studies to determine if
, selenium is affecting the ability of endangered fish to survive and produce healthy offspring.

Some native fish have been killed by anglers who did not release the fish properly. Also, some
. anglers may prefer to catch northern pike, channel catfish and other non-native sport fish they
may consider more desirable. After hooking a native fish, they simply have tossed it on the
riverbank to die.

In the mid-l 960s, efforts were made to reduce native fish populations and make way for stocking
non-native sport fish in Flaming Gorge and Navajo reservoirs. This was done with the use of
rotenone, a substance extracted from the root of the derris plant. (Rotenone reduces fish
populations by temporarily preventing fish from absorbing oxygen through their gills.)
Following the use of rotenone, biologists found that although the total number offish had
dropped, the ratios of various species remained about the same. This led them to conclude there
was no lasting effect on the native fish species.

Other factors that may have contributed to the fishes' decline include pollution and parasites.
Among the chubs, hybridization may also be a factor. '