9547

• UCREFRP Catalog Number: 9547
• Author: Tyus, H. M. and J. F. S. III.
• Title: An Evaluation of Recovery Needs for Endangered Fishes in the Upper Colorado River, with Recommendations for Future Recovery Actions - Final Report.
• USFW Year: 1999.
• USFW - Doc Type: Glenwood Springs, CO.
• Copyright Material: NO

PART 1. INTRODUCTION
Native fishes of the upper Colorado River (UCR) basin are declining in abundance. The
~ most likely cause of their decline is anthropogenic alteration of the natural environment.
Habitat of the native Colorado River fishes has been changed greatly during the last
100 years by human actions including physical alterations and the introduction of
nonnative species. Alterations to the physical environment are associated primarily
with construction of water development projects that began in the early 1900s (Fradkin
1984, Carlson and Muth 1989). By the 1960s, more than 50 dams and major
diversions had been constructed on mainstream river (Figure 1), and impoundment of
flow by these structures converted many river reaches into lacustrine habitat.
Operation of the dams has altered substantially the natural timing, duration, and
magnitude of annual flood flows in the Colorado River. Flow regulation and the
presence of structures have also caused changes in water temperature, sediment load,
~ nutrient transport, and other facets of water quality (Carlson and Muth 1989). In some
reaches, silt load has been reduced 90% (Fradkin 1984). Most existing mainstream
habitats are now different than the historic habitats in which the native fishes evolved,
and some have been modified so extensively that native fish can no longer survive in
them.
Physical changes in the riverine habitat were accompanied by the introduction and
proliferation of nonnative fish species, including many that are predaceous, highly
competitive, and harmful to the native fish fauna (reviewed by Tyus and Saunders
1996a). Some introduced fishes have become very successful under the
~ environmental conditions that now prevail in the Colorado River system. Although the
native fishes were well adapted to their natural environment, alterations to the physical
habitat may have created conditions that are now more favorable to many of the
introduced species. Even where physical habitat has been altered relatively little,
nonnative fish abundance has increased, and the abundance of native fishes has been
~ reduced. Most habitat used by the native fishes also is occupied now by introduced
species (Minckley 1982, Tyus et al. 1982a, Carlson and Muth 1989).
Changes in the physical and biological characteristics of riverine habitat have
contributed to the endangerment of four native fish species (Colorado pikeminnow,
humpback chub, bonytail, and razorback sucker). These and other fishes native to the
~ main channels of the Colorado River system ("big river fish community") have
disappeared from most of their original habitat. Their endangerment is attributable to a
suite of environmental factors that is essentially the same for all four species. The
problem exists at the ecosystem level because an entire fish community is threatened
and threats include biotic and abiotic factors.
Concern about the decline and endangerment of four species from the big river fish
community resulted in Federal and state listings. Actions to recover species listed
under provisions of the Endangered Species Act are guided by recovery plans prepared
by the U.S. Fish and Wildlife Service (USFWS) for each species. Each of the four