Creating Isolated, Predator-Free Backwaters for Native Fishes Along the Lower Colorado River

Razorback sucker Xyrauchen texanus and bonytail chub Gila elegans are native fishes of the lower Colorado River and are currently listed as endangered under the terms of the Endangered Species Act (ESA).  Their initial declines were attributed to anthropogenic modifications made to the river’s structure and function through the construction of numerous dams.  However, further research has indicated that competition and predation by introduced nonnative fishes is also a primary factor contributing to their continued declines.  Since the hydrologic conditions of the Colorado River will likely never resemble the historical conditions these native species thrived under, and extirpating nonnatives from the systems is both physically and politically impossible, the Fish and Wildlife Service, the administrators of the ESA, mandated the creation of isolated, predator-free backwaters for these endangered species.  As the implementing agency for the Lower Colorado River Multi-Species Conservation Program (an ESA mitigation program), the Bureau of Reclamation is responsible for creating or restoring 146 hectares of backwater habitat along the lower Colorado River dedicated to native fish restoration.  Renovating existing backwaters involves improving the physical and/or chemical conditions through dredging or pumping, chemically removing all nonnative fishes, and restocking the backwater with native species.  The success of these restoration projects depends primarily on the assurance that the chemical renovation was successful at eliminating all life stages nonnative fishes and that those species can continue to be excluded.  In many of the isolated backwaters along the lower Colorado River ample surface water is required to maintain appropriate water levels and water quality during the extreme summer months.  To sustain adequate habitat conditions, while attempting to exclude all life stages of nonnative fishes, a screen system that provides sufficient surface inflows to the backwaters was installed at two restoration areas on the lower Colorado River.  Since 2005, the Bureau of Reclamation has been evaluating the effectiveness of various screen technologies that could possibly be used to meet the goals of their program.  This presentation will give an overview of the screen technologies tested and the challenges the Bureau of Reclamation has faced throughout the restoration process.  Details will be provided on two case studies conducted at the Havasu National Wildlife Refuge near Needles, California, and the Imperial National Wildlife Refuge near Yuma, Arizona.