Assessing Vulnerability of Pacific Salmon to Climate-Driven Changes in Riverine Thermal Heterogeneity

Climate adaptation strategies for freshwater biota have typically focused on temporal changes in hydrology and temperature, but characterizing of the occurrence, size, and distribution of cold-water refuges is also essential for evaluating vulnerability of salmon to future climate and identifying and protecting climate change refugia. Using high-resolution remotely sensed water temperature data for 2^nd-7^th-order rivers in 127 sub-basins throughout the Pacific Northwest and California, we calculated metrics describing thermal heterogeneity at multiple spatial scales. Specifically, we quantified the total amount, size, and spacing of cold patches at higher spatial resolutions than can be obtained from broad-scale stream temperature models. We also quantified the amount, size, and spacing of locally cooler habitat within areas otherwise considered to be too warm for salmon. We found that in a warmer climate, there may be more thermally intolerable habitat and fewer and more distantly spaced cold-water patches. However, the sizes of remaining cold-water patches remained similar, and cold patches were spaced closely enough to remain accessible to salmon in many rivers. We also found that predictions about vulnerability of salmon to changes in thermal heterogeneity are sensitive to the spatial resolution of temperature data.