9388

• UCREFRP Catalog Number: 9388
• Author: Maddux, H. R., W. R. Noonan and L. A. Fitzpatrick.
• Title: Overview of the Proposed Critical Habitat Designation for the Four Colorado River Endangered Fishes.
• USFW Year: 1993.
• USFW - Doc Type: Salt Lake City.
• Copyright Material: NO

PART 11- Biological Overview

This biological overview provides a brief summary of the life histories and habitat
requirements for each species, It provides a synopsis of the Service's Biological Support
Document and readers may wish to refer to that document for detailed information on the
biological aspects of the critical habitat designation.

BACKGROUND

The four endangered fishes are endemic to the Colorado River Basin (Basin), which consists
of portions of seven Western States. The Basin drains approximately 242,000 sq. miles
within the United States and has been politically divided into an Upper and Lower Basin.
The Upper Basin consists of portions of the States of Colorado, New Mexico, Utah, and
Wyoming. The Lower Basin consists of portions of the States of Arizona, California, and
Nevada. An additional 2,000 sq. miles of the Basin lies within Mexico.

Historically, the native fish fauna of the mainstream Colorado River was dominated by
minnows (cyprinids) and suckers (catostomids; Minckley et al. 1986). However, four of
these, the razorback sucker, Colorado squawfish, humpback chub, and bony tail, are now listed
as endangered species under the Endangered Species Act of 1973. These fishes are
threatened with extinction due to the combined effects of habitat loss (including regulation of
natural flow and changes to temperature and sediment regimes); proliferation of introduced
fishes; and other man-induced disturbances (Miller 1961; Minckley 1973; USFWS 1987;
Carlson and Muth 1989).

Colorado squawfish populations survive only in the Upper Basin, where their numbers are
relatively high only in the Green River basin of Utah and Colorado when compared with
other rivers in the Basin (Tyus 1991). Razorback sucker and bony tail populations throughout
the Basin consist predominately of old adult fish, and they persist only because of the
longevity inherent in these species (USFWS 1990a; Minckley et al. 1991). Humpback chub
populations in the Little Colorado River, Black Rocks, and Westwater in the Colorado River
appear relatively stable in number of fish, but declines have been apparent in other locations
(USFWS 1990b).

Conservation of these four species will require the identification and management of water
resources and habitat areas that are considered important to any fish species, such as
spawning areas and nursery grounds. However, because the four endangered fishes are
present in such low numbers, basic life history and habitat use information has been difficult
to obtain. Some areas used by Colorado squawfish and razorback sucker for spawning have
been located by radiotracking, tagging, and collection of eggs or larvae (Tyus and Karp 1990;
Tyus 1990). Spawning information is less available in other places where these species are
more rare. Actions that have resulted in a lack of reproduction andlor recruitment have been
hypothesized as factors in their endangerment (USFWS 1990a, 1990b, 1991). In this case,
not only would a lack of successful recruitment lead to small numbers of fish, but over time,

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