7025

• UCREFRP Catalog Number: 7025
• Author: Hawkins, J. A. and T. P. Nesler.
• Title: Nonnative Fishes in the Upper Colorado River Basin
• USFW Year: 1991.
• USFW - Doc Type: An Issue Paper.
• Copyright Material: YES

EXECUTIVE SUMMARY

Guidance for the study and management of impacts of nonnative fishes on Colorado
River endangered fishes was addressed through development of this issue paper. A review
of published literature and government reports relating to interactions between native and
nonnative fishes in the Upper Colorado River Basin (upper basin) resulted in a reference
list of 133 papers. A questionnaire was also developed to establish expert opinion and
consensus on various aspects of this issue. Research and management recommendations,
and a strategy for implementation of related recovery efforts are also provided.

Increases in the number and relative abundance of nonnative fish species in the upper
basin have been documented and shown to be related to habitat alteration and river flows.
Predation on endangered fishes in the upper basin has been documented mostly for channel
catfish. Nonnative fish predation was identified most frequently in artificial settings where
native fish were reared in large numbers, in confined areas, or after stocking of hatchery-
reared fish. Studies on diets of native and nonnative fishes have shown dietary overlap
between larval or early juvenile Colorado squawfish and juvenile channel catfish, larval to
adult red shiner, and juvenile to adult green sunfish, but information on availability of forage
items was lacking and conclusive statements about competition could not be made. Spatial
competition is also suggested based on habitat-use overlap between young Colorado
squawfish, red shiner, sand shiner, and fathead minnow, and between adult Colorado
squawfish, channel catfish, northern pike, and walleye. However, in each study, habitat
availability was not documented. Introduction of disease to native fishes from introduced
fishes was not considered a problem; although, Asiatic tapeworm has recently been found
in hatchery-reared Colorado squawfish that were stocked into riverside ponds. Habitat
alteration by introduced fishes and hybridization between nonnative and any of the four
endangered native fishes have not been documented.

Expert opinion suggested that introduced fishes affecting native fishes are a problem
in most Colorado River Basin drainages. Six introduced species of greatest or widespread
concern were channel catfish, red shiner, northern pike, common carp, green sunfish, and
fathead minnow. Most of these species were considered widespread in distribution, and their
impacts on native fishes were considered constant or inversely affected by flow regime.
Expert opinion often identified that Colorado squawfish and razorback sucker, their young,
and all native fish larvae, were negatively affected by predation or competition from
nonnative fishes. Documented evidence ofthese interactions was limited. Research directed
at evaluating predation and competition between the above nonnative fishes and endangered
and other native fishes is needed to answer specific recovery questions. Sport-fishery value
of introduced fishes was recognized largely for reservoir fisheries, tailwater fisheries, and the
Yampa River northern pike fishery. Channel catfish was considered to be supporting the
most widespread riverine fishery, but the fishery was of moderate or greater importance only
in select localities. Riverine fisheries for all other introdl:lced game fish were generally
considered to be small or inconsequential Mortality of endangered fishes from incidental
take by angling was not considered a significant problem by a majority of respondents.

VII

~