Tough places and safe spaces: can refuges save salmon from a warming climate?
The importance of thermal refuges in a rapidly warming world are particularly evident for migratory species, where individuals encounter a wide range of conditions throughout their lives. Thermal refuges are believed to be particularly valuable for species such as Pacific salmon, where adults have been observed to frequently use patches of cold water in their upstream reproductive migrations. The fitness benefits of such thermoregulatory behaviors can be constrained by individual variability in migration timing, spawning timing, and adaptive capacity to behaviorally thermoregulate. Here, we use a spatially-explicit individual-based simulation model to evaluate the buffering potential of behavioral thermoregulation for anadromous salmon and trout (Oncorhynchus spp.) migrating upstream in the Columbia River from Bonneville Dam to the Snake River confluence (distance = 288 km). For fall-run Chinook Salmon (O. tshawytscha) and summer-run steelhead (O. mykiss) we were able to evaluate the energetic consequences of upstream migration through a heterogeneous thermalscape for varied migration phenotypes with differing migration phenology and propensity for behavioral thermoregulation. We considered two emergent direct indicators of fitness: survival, and percent of available energy used, both of which influence the completion of upstream migration and spawning. We found that energy use was influenced by the temperature of the mainstem Columbia River more than by the use of coldwater refuges. However, we found that loss of coldwater refuges led to lower diversity of energy-conserving migratory phenotypes. We also considered one indirect emergent indicator of fitness, the cumulative sub-lethal exposure to stressful temperatures, and found that coldwater refuges can effectively decrease accumulated thermal units. Our simulation results suggest that, while coldwater refuges do not entirely mitigate for stressful mainstem Columbia River temperatures, a heterogeneous thermal riverscape is important for maintaining a diversity of migration phenotypes. Furthermore, under predicted future climate conditions, conserving portfolios of migration behaviors could be an important part of conserving metapopulation-level stability in salmonid returns.