Assessing freshwater life-stage vulnerability of an endangered Chinook salmon population to climate change influences on stream habitat

We linked a set of climate, hydrology, landscape, and fish population models to estimate the relative influence of freshwater habitat variables on the abundance of a population of endangered stream-type Chinook salmon Oncorhynchus tshawytscha responding to a warming climate. The hydrology models estimated that increases in annual air temperature and winter precipitation would lead to increases in water temperature and changes in discharge, including higher flows during the egg-incubation period and lower flows during the summer rearing period. The spatially explicit population model estimated a resulting decline of 0 to 7% in the number of spawners, with 3 of 4 global climate models estimating a decline of 4 to 7%. Increased water temperature during the summer spawning period was the most limiting among habitat variables modeled, but our modeling suggested that aggressive habitat restoration (increasing forested area and reducing impervious area) could mitigate some spawner abundance reductions. Better knowledge of the links between climate changes and habitat response, including increased streambed scour due to the larger and more frequent winter high-discharge events predicted by our hydrology models, would improve our ability to estimate climate effects on populations. Future limitation by elevated summer water temperature, and potentially egg-pocket scour, would further stress an endangered population currently limited by the percentage of fine sediment around egg pockets. Identifying such changes demonstrates the utility of models that consider climate and integrate life-stage-specific habitat influences over a species' life cycle, thereby indicating restoration actions with the potential to benefit sensitive life stages.