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Objective: Conversion of lotic to lentic habitat after river impoundment can lead to habitat loss and threats from introduced species to native aquatic organisms. Some native species can persist in reservoirs despite these alterations. Some of the largest reservoirs in North America were constructed in the Colorado River basin and are used by endangered Razorback Sucker Xyrauchen texanus. This species successfully spawns in several reservoirs and recruits to adulthood in Lake Mead, whereas recruitment in other reservoirs and altered river habitats is limited or nonexistent.Hypothesized mechanisms for the general lack of recruitment include flow and habitat alteration as well as competition and predation from nonnative fishes. Methods:We analyzed 16 years of netting data from Lake Mead to assess spatial and temporal patterns in fish assemblage structure and the size structure of Razorback Sucker across four sites.Result: There was a substantial shift in assemblage structure across all sites after the establishment of Gizzard Shad Dorosoma cepedianum in 2010-2011 and an apparent decline in Threadfin Shad D. petenense. Captures of juvenile Razorback Sucker (<450 mm total length in Lake Mead) were higher at sites where overall fish abundance was also higher, suggesting that all fishes might be responding to similar spatial and temporal environmental variation. We identified differences in Razorback Sucker size structure that varied over time and among sites, largely driven by the presence of juvenile Razorback Sucker at some sites in some years. Conclusion:Razorback Sucker was among the top-10 most abundant species at all sites sampled, but assemblages were dominated by abundant nonnative fishes.Razorback Sucker continue to recruit in Lake Mead in the face of abundant and diverse nonnative fishes, but the adult population size remains relatively small (<1000). Further investigation of limiting factors will be necessary to understand which actions might be successful at improving recruitment conditions.

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The purpose of this Circular is to provide a synthesis of published information and a summary of technical findings and associated implications that may affect natural resource management of Lake Mead National Recreation Area (LMNRA). Synthesized information and summarized findings should lead to a better public understanding of the natural resources of Lakes Mead and Mohave, and the issues related to maintaining their resources into the future.