Nonnative fish control and endangered fish recovery: Lessons from the Colorado River

Native freshwater fish populations are declining in North America and more than 100 fishes are federally listed as threatened or endangered. The Color ado River system in the southwestern United States has been especially affe cted. Most of its native fish populations are in decline, including four "b ig river" fishes, Colorado pikeminnow (Ptychocheilus lucius), humpback chub (Gila cypha), bonytail (G. elegans), and razorback sucker (Xyrauchen texan us), whose populations have declined so precipitously that they are endange red. Physical habitat alteration caused native fish declines, but nonnative 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 move ments 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 nonnative fish control measures has proven difficult. Recovery efforts have identified the need f or developing nonnative fish control strategies and testing methodologies, but no solutions have emerged. A holistic approach, including ecosystem rec overy plans, should be used in systems where more than one species share co mmon problems, such as interactions with nonnative fishes. Nonnative fish i nteractions should be suspected as a potential cause of declining native fi sh populations.