Range‐wide life history diversity and climate exposure in adult Chinook salmon

Abstract Climate change is threatening ecologically and culturally important species. For species with broad ranges and complex life cycles, such as migratory Pacific salmon ( Oncorhynchus spp.), climate exposure may vary across space and diverse life history strategies. Here, we quantify climate exposure and adaptive capacity across the North American range and remarkable phenological diversity of Chinook salmon ( O. tshawytscha ). We compiled data on adult freshwater migration timing and its environmental covariates for 295 populations of Chinook salmon spanning 29 degrees of latitude from California to Alaska. We linked this migration timing data to recent (1990s) and future (2040s) water temperatures to quantify thermal exposure during each population's timing of entrance to freshwater. At northern latitudes, Chinook migration timing was compressed to 3 months during the summer, while at southern latitudes, migration occurred across months before and after stressful peak summer water temperatures. Earlier migration timing was associated with longer migration distances and greater elevations gained. Thermal exposure was controlled by latitude and run‐timing, with lower latitude and summer and fall runs being most exposed to potentially harmful temperatures both now and into the future. However, potentially harmful climate exposure was predicted to increase the most in mid‐latitude populations (~45° to 55°) that have not yet adapted to migrate before and after peak summer temperatures. If Chinook salmon phenology were to keep pace with projected climate warming by the 2040s, the majority (75%) of populations would need to shift their migration timing earlier in the year, pulling their migration farther apart from their fall spawning phenology. Collectively, these findings showcase how latitude and life history diversity influence climate change risk and indicate the importance of preserving existing migration timing diversity and adaptive capacity across the broad range of a migratory species.