Just water over the dam? Reservoir fishery disruption following drought‐mediated reoperation for environmental flows

Environmental flow (e‐flow) management is a growing driver of dam reoperation in which impounded water is allocated to rivers for ecosystem benefits. However, the effects of reoperation on reservoir ecosystems remain largely unstudied, despite their global importance in providing critical social‐ecological services (e.g., fisheries, food and water security). This deficiency has limited our ability to conduct inclusive assessments of the multiuse tradeoffs associated with reoperation. Here, we compiled fisheries survey data spanning five decades to evaluate potential responses to the e‐flow‐driven reoperation of a large montane reservoir in the arid southwestern United States. Following reoperation, seasonal water release patterns were modified, and late‐season reservoir drawdowns diminished by 74% (from 13.6 to 3.5 m on average). Reoperation coincided with the population expansion of an introduced apex predator, destabilization of a recreational fishery, and adaptation by fisheries managers to mitigate. The degree and timing of late‐season water‐level stabilization was sufficient to facilitate predator reproduction. An exponential growth modeling framework, grounded in principles of invasion ecology, demonstrated that a two‐stage (vs. one‐stage) population productivity process was required to explain patterns in survey data and to generate biologically relevant parameter estimates. This outcome indicated that predator population expansion was not strictly inherent. Processes (e.g., system productivity) other than changes to seasonal water‐level fluctuations could not explain the expansion. Collectively, results lend evidence that reoperation was a plausible contributing factor to predator population expansion, which disrupted reservoir services. Importantly, however, e‐flow operations were mediated by periodic drought. Late‐season drawdowns were reduced during relatively wet years, partially reflecting modifications to seasonal releases to help facilitate e‐flows, but also during drier years to mitigate storage loss. Thus, these processes were not independent and operated in tandem. More comprehensive understanding of the reciprocal effects of drought‐mediated reoperation may improve our ability to manage impoundments to better meet multiple conservation and societal objectives while adapting to change across a broad spectrum of drivers and social‐ecological contexts. Hydroclimate may ultimately be an overriding process regulating dam operations, reservoir water levels, and delivery of effective e‐flows given expected increases in drought frequency and severity in arid and semi‐arid regions.