Fish Community Responses to Mechanical Removal of - Nonnative Fishes in a Large Southwestern River

Establishment of nonnative fishes has contributed to the decline of native fishes worldwide. Efficacy of mechanical removal of nonnative fishes in large streams has been difficult to ascertain, and responses by native fishes after removal is equivocal. We summarize results of efforts on the San Juan River, New Mexico, Colorado, and Utah, to suppress nonnative Channel Catfish and Common Carp densities through removal via electrofishing. We assessed spatial and temporal trends in the densities of abundant fishes in relation to removal of nonnative fishes. Common Carp densities declined river-wide after removal but Channel Catfish densities only decreased in upper reaches. Sources of Channel Catfish juveniles and barriers to nonnative fish movement likely influenced the effectiveness of removal. Responses of native fishes to removal were not evident in most species and size classes. Results show that nonnative removal can be partly successful, but the complexity of large river systems limited the ability to completely remove Channel Catfish and document a positive response of native fishes. Nevertheless, these removal efforts coincided with increasing numbers of endangered species through a stocking program. We suggest that continued monitoring and experimentation will help managers untangle the efficacy of the program and its benefits for native fishes.