Maturation of an off-channel habitat concept to conserve native native fishes in the Lower Colorado River

Two endemic, “large river” fishes of the Colorado River basin of western North America, bonytail Gila
elegans and razorback sucker Xyrauchen texanus, are among several critically endangered species in the system. Wild populations of bonytail are gone, and there are no self-sustaining populations of razorback sucker anywhere; reproduction occurs but recruitment does not. Both species have been under intensive management in the Lower Colorado River since the 1980s. Today, with the single exception of Lake Mead, remaining populations are composed entirely of repatriated individuals that depend on stocking for their continued existence. In 2003, a conceptual off-channel habitat (OCH) management plan for these and other large river fishes of the system was published by the late W.L. Minckley and colleagues. The cornerstone of the approach was to move away from hatchery-based fish production and instead use isolated OCHs that were free of predatory and competitive nonnative fishes, where populations of native species could live, grow, reproduce, and recruit. Populations of adult fish also would live in open waters of the system, and through active management, individuals would be exchanged with those in OCHs to maintain genetic integrity and diversity of both species. Progress in the last 2 decades toward implementing the plan includes creation of new OCHs, studies of population dynamics and genetics of “wild” and captive populations, development of appropriate metrics to assess status of OCH populations, and refinement of the OCH concept itself. Our goals in this paper are to review management of bonytail and razorback sucker in the Lower Colorado River, present examples of species dynamics in OCHs, offer data-driven refinements to the OCH concept, and explore practical aspects including challenges and constraints to implementation of the concept. We conclude that bonytail and razorback sucker never can meet quantitative criteria required by current recovery plans, but long-term conservation of these species can be achieved if an OCH concept of management is successfully implemented and maintained.

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