One size does not fit all: variation in thermal eco-physiology among Pacific salmonids

Pacific salmonids, cold-water fishes native to the northern hemisphere, span a massive geographic range (~ 33° latitude) and are exposed to a wide variety of environmental conditions regionally and temporally. California is home to the greatest concentration of at-risk anadromous salmonids and warming river temperatures pose both current and future threats to numerous populations. Thermal standards for management of California populations are currently based on guidelines for multiple salmonid species and from populations across the Pacific Coast. However, a growing body of literature suggests that salmonid populations exhibit population-specific thermal requirements. Furthermore, in California, salmonid populations regularly encounter temperatures that exceed current thermal standards based upon performance of outside populations. This review focuses on Chinook salmon (Oncorhynchus tshawytscha), providing evidence for interpopulation variation in thermal performance across life stages, and explores the drivers of variation. To describe the formation of interpopulation variation, we define fundamental and ecological thermal physiologies. Fundamental thermal physiology is the composite of intrinsic physiological traits and abiotic factors that define a species’ thermal window. Ecological and environmental interactions constrain this fundamental thermal physiology, yielding an ecological thermal physiology. Thermal physiology, viewed through this lens, provides researchers and managers avenues for salmonid research and conservation at the population scale. A more nuanced approach to west-coast salmonid conservation will be required to protect the most at-risk and vulnerable populations. Successful salmonid management must incorporate population-specific traits and present and future watershed conditions.