Natural flow regimes, nonnative fishes, and native fish persistence in arid-land river systems

Escalating demands for water have led to substantial modi. cations of river systems in arid regions, which coupled with the widespread invasion of nonnative organisms, have increased the vulnerability of native aquatic species to extirpation. Whereas a number of studies have evaluated the role of modiC ed. ow regimes and nonnative species on native aquatic assemblages, few have been conducted where the compounding effects of modified. ow regimes and established nonnatives do not confound interpretations, particularly at spatial and temporal scales that are relevant to conservation of species at a range- wide level. By evaluating a 19-year data set across six sites in the relatively unaltered upper Gila River basin, New Mexico, USA, we tested how natural flow regimes and presence of nonnative species affected long-term stability of native. sh assemblages. Overall, we found that native. sh density was greatest during a wet period at the beginning of our study and declined during a dry period near the end of the study. Nonnative. shes, particularly predators, generally responded in opposite directions to these climatic cycles. Our data suggested that chronic presence of nonnative. shes, coupled with naturally low. ows reduced abundance of individual species and compromised persistence of native. sh assemblages. We also found that a natural. ow regime alone was unlikely to ensure persistence of native. sh assemblages. Rather, active management that maintains natural. ow regimes while concurrently suppressing or excluding nonnative. shes from remaining native. sh strongholds is critical to conservation of native. sh assemblages in a system, such as the upper Gila River drainage, with comparatively little anthropogenic modification.