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ENDANGERED SPECIES recruitment In addition, there also may be potential for commercial harvesting with hoop and block nets, Discussion There is compelling evidence that the decline of the native Colorado River fish fauna and lack of successful recovery is due in large part to adverse interactions with nonnative fishes. In the upper Colorado River basin (and presumably elsewhere) nonnative fishes pose a more seri- ous threat to the native fish fauna than previously thought An important lesson for fish recovery is that non- native fishes should be given full consideration whenever there is any indication that they are a contributing factor in the decline of imperiled fishes. Physical habitat alteration has been attributed to the decline of 91 to 94% of imperiled species in the United States, and nonnative fish interactions may have affected 53 to 70% (Lassuy 1995; Wilcove et aL 1998), Physical habi- tat alteration has been important in causing declines of native fishes, However, adverse nonnative fish interactions appear to be just as important, in causing extinctions of native fishes (Miller et aL 1989; Ross 1997), and fish extinc- tions due to nonnative fishes are not always preceded by habitat alteration (Lassuy 1995), In the Colorado River, fishery management practice has been to introduce hardy nonnative fishes into~the ~ltered physical habitats, Many of these introduced fishes were "preadapted" to the changed habitat (Minckley 1982) and flourished, affecting native fishes by predation, competition, and hybridization. Nonnative fish interactions may now be the major cause for extinction in this physically altered system, The need for reducing impacts of nonnative fish introduc- tions in the Colorado River basin has not drawn much sup- port and the problem has been deferred to other options, such as provision of instream flows and improving physical habitat A general reluctance by fish management agencies to recognize nonnative fishes as a problem is not surprising, because of traditional sportfish management practices (e,g" Everhart and Youngs 1981), In addition, the same agencies responsible for reducing nonnative populations are the ones that introduced the nonnatives in the first place, Happily, more attention is being focused on the nonna- tive fish problem, and in the last few years an emphasis has been placed on evaluating the magnitude of the problem and seeking s.Dlutions, However, elimination or even reduc- tion of nonnative fishes will not be an easy task. At present, we do not know how to initiate effective basinwide fish control programs or how to reduce or eliminate populations of nonnative fishes without adverse effects to the native fishes, Tactical problems, such as the need to develop and test methodologies for reducing or eliminating each trouble- some species, are further compounded by a general lack of ~trategy:, the very concept of "control" has yet to be defined ~ m~arungful recovery terms, Even if the threat of nonna- tive fishes c~n be temporarily reduced, long-term control measures will need to be developed and implemented. In general. recovery9f endangered species is a fonnidable task, in part because it is an emerging field of science, The 22 Fisheries challenging nature of recovering endangered fishes has been clearly illustrated by the results of ESA implementation: no listed fish has been sufficiently recovered that it has been delisted, In over 20 years of recovery effort in the Colorado River basin, more fish have been incrementally listed and in the process, one fish (bony tail) has been extir- pated from nature, It can be argued that lack of a more holistic approach is one of the foremost problems. It is generally agreed that declines of the endangered big-river fishes are due to many of the same problems, However, no multispecies or ecosystem recovery plan has been developed to address common problems although the ESA provides a clear mandate for an ecosystem approach, Instead there has been a tendency to address each species separately, and thus recovery priorities are presumably different than if all spe- cies were considered together, It is assumed that there would be greater support for recovery of several endangered spe- cies in a geographic area than for one species alone, So little attention has been given to reducing the ad- verse effect of introductions on the native species in the Colorado River, or elsewhere for that matter (Lassuy 1995), that the riverine environment has been in a continu- al state of change due to addition and proliferation of new fishes at various locations, We believe that successful recovery of the endangered big-river fishes cannot be accomplished without some nonnative fish control, includ- ing: (1) the prevention of additional introductions, (2) pre- venting escapement of nonnatives fishes from off-channel areas to the river system, and (3) reduction or elimination of populations of introduced nonnative fishes that have become established in the riverine environment Unfortu- nately, all of these measures are unpalatable due to socio- political reasons, and difficult to implement due to a lack of effective control methodologies, At a minimum, future recovery actions should include intensive efforts to produce predator-free zones, with high- est priorities placed on reducing the number of small cyprinids in endangered fish nursery areas and reducing the sizes and numbers of channel catfish in mainstream rivers, We applaud recent recovery implementation efforts in the Colorado River basin in identifying the need for developing control strategies, but little progress has been made in effectively implementing a nonnative fish control program, Even less progress has been accomplished elsewhere. We hope this effort will stimulate others to action. ~ Acknowledgments We thank Henry Maddux, Frank Pfeifer, Charles McAda, Douglas Osmundson, and Ray Tenney for useful discus- sions about nonnative fish control. We are indebted to par- ticipants of the Nonnative Fish Control Workshop for shar- ing their ideas, John Hamill served as project technical officer for the Recovery Implementation Program for Endangered Fish Species in the Upper Colorado River Basin, who provided funds, Catherine Karp and Robert Muth provided comments and suggestions to an earlier draft of the manuscript Vol. 25, No.9