7775

• UCREFRP Catalog Number: 7775
• Author: Carlson, C. A. and R. T. Muth.
• Title: Inland Fisheries Management in North America, Chapter 15
• USFW Year: 1993.
• USFW - Doc Type: Endangered Species Management.
• Copyright Material: YES

3.10

CHAPTER 15

and Las Vegas dace), 16 are listed as federally endangered, 6 are listed as federally
threatened, and 13 have either been proposed for federal listing or are under
review for federal listing. In all, 35 native fishes (65% of the total number) receive
legal protection under the ESA; except for one, the Gila topminnow, all are
endemic to the system. Several of the remaining native fishes are variously
protected by one or more basin states. Of the federally protected fishes, 29 occur
only in the lower Colorado River basin (Arizona, California, Nevada, and New
Mexico), 2 occur only in the upper Colorado River basin (Colorado, Utah, and
Wyoming), and 4 occurred in both the upper and lower basins. The two extinct
fishes occurred in the lower basin. Researchers have attributed the decline of
native fishes to (1) modification and loss of habitat and (2) introduction of
nonnative species (Stanford and Ward 1986b). Typically, these two factors occur
together and have synergistic adverse effects.
Closure of Hoover Dam in 1935 marked the end of the free-flowing Colorado
River. Since then, the basin has become one of the most altered and controlled
river systems in the United States. Dam construction and regulation have
drastically altered physical and biological features of the river system (Stanford
and Ward 19800), and, as a result, native fishes inhabiting the larger river channels
have been severely impacted. Biologists are just beginning to recognize the
detrimental effects of the spread of nonnative riparian vegetation on stream widths
and, therefore, fish habitat in the basin.
Since the late 1800s, approximately 67 fish species have been successfully
introduced into the basin, raising the total number of species to over 100 (Carlson
and Muth 1989). Most of these introductions resulted from efforts to establish
sport fisheries in and downstream of newly created reservoirs. Several authors
have noted adverse impacts of introduced nonnative species on native fishes of
the basin. Invoking the island biogeography theory, Molles (1980) proposed that,
because the Colorado River basin is an insular system, the native fish fauna is
especially vulnerable to invasion by nonnative fish species. Using local extirpa-
tions of native fishes in the lower basin as examples, he suggested that successful
invasions can cause removal of native fishes through competitive replacement or
predation.
Of the basin's 16 federally endangered fishes, the bony tail, Pahranagat roundtail
chub, Virgin River roundtail chub, woundfin, Colorado squawfish, razorback
sucker, desert pupfish, and Gila topminnow are currently being maintained at
Dexter National Fish Hatchery, New Mexico (Johnson and Jensen 1991). Since
1974, the Dexter hatchery has served as a refuge and propagation facility for
several rare and imperiled native fishes of the American Southwest. Personnel
from the Dexter facility have assisted in development of similar programs
elsewhere in North America and in other countries.
Despite the severely perturbed state of the basin, threats of additional water
development, and in some cases years of research and volumes of data, efforts to
recover the federally protected fishes by direct management of wild populations or
their habitats have been limited. In the lower basin, where river habitat degrada-
tion and fish extirpation has been especially severe, recovery work has empha-
sized acquisition of habitats and brood stocks, propagation, and reintroduction.
Hatchery brood stocks of the threatened Apache trout and endangered Gila trout
now exist, fish are being reintroduced, and several new stream populations have
been established. Since 1981, young razorback sucker have been reintroduced