California steelhead populations were regionally buffered and individually resistant to a severe multi-year drought

Weather extremes, such as drought, are predicted to be a strong determinant of species persistence under climate change. Yet predictions often fail to consider that variation in streamflow responses, variation in population dynamics, or adaptations to drought could buffer species against extremes. In this study we examined the responses of eight California (USA) steelhead populations to a severe drought from 2012 to 2016. We observed that streamflows were highly synchronous across the region in all seasons and did not appear to buffer drought impacts. Population dynamics were variable across the region and did appear to buffer the region from drought impacts. Some populations had very low productivity for four years associated with the drought, while others had slightly below-average productivity for only two years. Population synchrony was associated with spring-smolt flow, temperature and drought over time, but was not associated with winter-spawner or summer-juvenile conditions, suggesting populations may be adapted to drought. Our results highlight how regional buffering and adaptation can be important mechanisms against climate extremes both now and into the future.