8169a

• UCREFRP Catalog Number: 8169a
• Author: Brookshire, D. S., M. McKee and G. Watts.
• Title: Draft Economic Analysis of Proposed Critical Habitat Designation in the Colorado River Basin for the Razorback Sucker, Humpback Chub, Colorado Squawfish, and Bonytail.
• USFW Year: 1993.
• USFW - Doc Type: \
• Copyright Material: NO

include, but are not limited to, alterations adversely modifying any of those physical or
biological features that were the basis for determining the habitat to be critical.
B. Listing History and Proposed Habitat Designations
1. Listing History
The Colorado squawfish and humpback chub were listed as endangered species on March 11,
1967 (32 FR 4001). The bonytail was listed as endangered on April 23, 1980 (45 FR 27713).
On May 16, 1975, USFWS published a notice of its intent to determine critical habitat for the
Colorado squawfish, humpback chub, and numerous other species (40 FR 21499). On
September 14, 1978, USFWS proposed critical habitat for the Colorado squawfish (43 FR
41060). The area proposed included 623 miles of the Colorado, Green, Gunnison, and Yampa
rivers. This proposal was later withdrawn (44 FR 12382; March 6, 1979) to comply with
1978 amendments to the Act (16 U.S.C. 1531 et seq.), that required USFWS to include
critical habitat in the listing of most species and to complete the listing process within 2 years
from the date of the proposed rule or to withdraw the proposal from further consideration.
USFWS did not complete the listing process within the 2-year deadline. The razorback
sucker was proposed for listing as a threatened species on April 24, 1978 (43 FR 17375).
The proposal was withdrawn on May 27, 1980 (45 FR 35410) in accordance with 1978
amendments to the Act that require a determination of critical habitat in the listing of most
species. The razorback sucker was listed as endangered in 1991. Critical habitat for all four
endangered species was proposed on January 29, 1993 (58 FR 6578).
2. Historical and Current Habitat
a. Razorback Sucker
The razorback sucker is part of a unique fish fauna endemic to the Colorado River Basin.
Historical riverine systems provided a wide variety of habitats that razorback suckers
occupied, including backwaters, sloughs, and oxbow Iakes. The razorback sucker was once
abundant throughout the 3,500 miles of the Colorado River Basin, primarily in the mainstem
of the Colorado River and its major tributaries in Arizona, California, Colorado, Nevada, New
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