Predicted fish species vulnerability to stream drying in the western U.S.A.

The frequency, magnitude and extent of stream drying is increasing due to climate change and human water demand. Fish vulnerability to increased stream drying is a combination of sensitivity (innate tolerance to low streamflow) and exposure to stream drying. To understand fish tolerance to low flow and susceptibility to decline under changing streamflow conditions, we estimated species-specific measures of sensitivity to low streamflow, determined relationships to species traits and evaluated vulnerability to future reductions in streamflow for 60 species. We found that sensitivity varied across species, and some variation was explained by life history strategy, spawning strategy and body size. Periodic life history strategy, pelagic spawning and larger size corresponded to an increased sensitivity to stream drying. Under future projections of August streamflow, 90% of sites were predicted to decrease in flow magnitude. Vulnerability to changes in streamflow, the combination of sensitivity and exposure, varied slightly across the study species, with the percent of inhospitable sites under future climate scenarios increasing for 87% of the species. Despite being relatively insensitive to low streamflow, vulnerability was high for multiple species dominant in mountainous areas, driven by high levels of exposure to stream drying. Our results illustrate the potential for species traits to predict sensitivity to low streamflow and demonstrate that exposure may play a large role when defining species vulnerability to stream drying. The ability to predict species tolerances and susceptibility to decline will become increasingly important in prioritising conservation efforts under changing environmental conditions.