Nonlinear survival of imperiled fish informs managed flows in a highly modified river

Water is a fundamental resource in freshwater ecosystems, and streamflow plays pivotal role driving riverine ecology biodiversity. Ecologically functional flows, managed hydrographs that are meant to reproduce the primary components of natural hydrograph, touted as potential way forward restore ecological functions highly modified rivers, while also balancing human water needs. A major challenge implementing flows will be establishing shape hydrograph so optimize improvements ecosystem given limited resources. Identifying flow–biology relationship thus critical for determining environmental consequences flow regulation. In California's Central Valley, studies have found increased can improve survival imperiled juvenile salmon populations during their oceanward migration. These not explored nonlinearities between survival, giving managers difficult task designing intended help without clear guidance on targets. We used an information theoretic approach analyze migration data from 2436 acoustic-tagged Chinook spanning differing years (2013–2019) extract actionable flow–survival relationship. This was best described by step function, with three thresholds we defined minimum (4259 cfs), historic mean (10,712 high (22,872 cfs). Survival varied threshold: 3.0% below minimum, 18.9% mean, 50.8% high, 35.3% above high. these design alternative over same included important component flows: spring pulse flows. compared predicted cohort actual hydrographs. Managed targeted were increase annual 55–132% any additions budget 79–330% modest addition budget. quantitative estimates biological different provide regimes benefit constrained management arena.