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PREFACE Four "big river" fishes of the mainstream Colorado River are listed as endangered under provisions of the Endangered Species Act. Critical habitat was designated for these fishes in 1994 end 127 species that are endangered, threatened a~cl-e~~ed, armd proposed or~Eandidates for listing potentially may be affected by that designation. Information about most of these species is scant, but at least two candidate fishes are being affected by the same factors that have caused the decline of the listed fishes. The decline of so many species are indications that changes at the ecosystem level have placed these species at risk. Major environmental change may affect more than one species adversely, and such ecosystem threats require ecosystem-level solutions. When there is an environmental basis for the endangerment of a community or ecosystem, present policy and philosophies of the U.S. Fish and Wildlife Service (Service) encourages consideration of "multispecies" or ecosystem recovery plans. Formulation of ecosystem recovery plans is a relatively new endeavor, and only a few have been. completed. Most recovery plans address only the needs of individual species. Therefore, this plan is different in format and content than single-species recovery plans, and augments, rather than replaces existing plans for the four listed fishes (i.e., razorback sucker Xyrauchen texanus, Colorado squawfish Ptychocheilus ]ucius, humpback chub Gila cypha, and bonytail G. elegans). The goal of the plan is to expedite recovery of the listed fishes by facilitating implementation of existing recovery plans, assisting in relating recovery planning and critical habitat management, and reducing the likelihood of listing additional species at risk by removing environmental threats. This "umbrella" concept of an aquatic ecosystem recovery plan is accomplished by combining a strategic ecosystem plan with existing species-specific plans. Those wishing an overview of the multispecies recovery effort can use the ecosystem plan. Those wishing to work with single species recovery efforts will benefit from the detailed recovery plan for each species. Recovery of the four listed Colorado River fishes is being accomplished by formal recovery implementation programs in important areas of their ranges. It is anticipated that these recovery implementation programs will develop site-specific work plans for the ecosystem recovery under the broad guidance provided in this plan. Recovery implementation for specific species will rely on detailed work plans completed under guidance provided by species-specific recovery plans. v