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Jqoo TllJS and SaMlJders ENDANGERED SPECIES Nonnative Fish Control and Endangered Fish Recovery: Lessons from the Colorado River By Harold M. Tyus and James F. Saunders, III -. ABSTRACT Native freshwater fish populations are declining in North America and more than 100 fishes are fed- erally listed as threatened or endangered. The Colorado River system in the southwestern United States has been especially affected, Most of its native fish populations are in decline, including four "big river" fishes, ~olorado pikeminno~ (Ptychocheilus lucius),ltuJllphrwk chllh (Gila cypha), ppnyt;,;lil (G. elegans), and ~~2!!Jacl$~!!fker (X'yrauchen texanus), whose populations have declined so precipi- tously that they are endangered. Physical habitat alteration caused native fish declines, but nonna- Hve fishes pose a more serious threat to native fishes than previously thought. Nonnative fish control measures needed in the upper Colorado River system, identified in part by a workshop of experts, include: (1) preventing movements of game fishes out of impoundments and curtailing future stockings, (2) reducing numbers of small, nonnative cyprinids in shoreline habitat used as rearing areas by young native fishes, and (3) increasing the harvest of channel catfish (Ictalurus punctatus) and common carp (Cyprinus carpio) in the mainstream. Large-scale implementation of these nonna- tive fish control measures has proven difficult. Recovery efforts have identified the need for devel- oping nonnative fish control strategies and testing methodologies, but no solutions have emerged, A holistic approach, including ecosystem recovery plans, should be used in systems where more than one species share common problems, such as interactions with nonnative f~~h~?: Nonnative fish interactions should be suspected as a potential cause of declining native fish populations, Introduction At least 536 nonnative fishes (representing 75 fami- lies) have been introduced into freshwaters of the Unit- ed States for some practical or aesthetic purpose (Fuller et aL 1999; Heidinger 1999; Li and Moyle 1999), Howev- er, almost all introductions have proven harmful to native communities (Taylor et aL 1984; Courtenay and Robins 1989), As nonnative fishes were introduced, many native fish populations declined, presumably due to adverse interactions that have affected 70% of fishes listed under provisions of the Endangered Species Act (ESA) of 1973 (Lassuy 1995; Us. Fish and Wildlife Ser- vice [USFWS] 1998a). Of 30 fishes that became extinct in the United States during the 20th century, introduced fishes were implicated in 24 extinctions (80%), and intro- duced fishes were the only factor in two extinctions (Miller et aL 1989), Although adverse effects of nonnative Harold M. Tyus is a senior research scientist at the Uni- versity of Colorado at Boulder, James F. Saunders, III is the associate director of the Center for Limnology at the Univer- sity of Colorado at Boulder, Both can be reachedat the Coop- erative Institute for Research in Environmental Sciences, Campus Box 216, University of Colorado, Boulder, CO 80309-0216. Emails are: harold.tyus@colorado.edu, and james.saunders@colorado,edu, September 2000 fishes have been reported for many years, relatively lit- tle has been done to solve the nonnative fish problem (Courtenay 1995), The Colorado River basin of the southwestern United States has been extensively changed by physical habitat alteration and also is among the top five river basins in the United States most affected by introduction of non- native fishes (Fuller et aL 1999), Effects on the 51 fresh- water fish taxa (species and subspecies) native to the Colorado River have been extreme: 2 are extinct, 22 are federally listed as endangered (16) or threatened (6), and 1 fish is a candidate for listing (USFWS 1997, 1998a), Many of the remaining are considered species at risk and protected by state agencies (Carlson and Muth 1989; Rahel et aL 1999), In addition, three estuarine species have been virtually extirpated from the once extensive and productive lower Colorado River delta, The effects of nonnative fish introductions on stream fish communities have not been well documented, but the few studies that exist have reported serious declines of the native stream fishes (e,g" 77% of 31 cases; Ross 1991), The Colorado River basin arguably provides the most complete record for nonnative fish problems than any other large river system in the world, and can per- haps provide some valuable lessons for conservation of native fishes, We explore the role of nonnative species in Fisheries 17 ~~~h