Differences in Cardiorespiratory Performance and Thermal Tolerance among Sockeye Salmon Populations: Potential for Adaptation to Climate Change?

Climate change induced increases in summer water temperature have been repeatedly associated with elevated mortality of adult Fraser River sockeye salmon (Oncorhynchus nerka) during their upriver spawning migration. There are over 100 genetically and geographically distinct populations within the Fraser River watershed which encounter different migration distances (100-1,100 km), elevation gain (10-1,200 m), river temperature (9-22°C) and river flow (2,000-10,000 m³ s^-1). We hypothesized that sockeye salmon populations have locally adapted through natural selection to meet their unique migration challenges. Wild, migrating adult sockeye salmon were intercepted in the lower Fraser River, instrumented to measure cardiorespiratory performance and swum at single temperature (ranging from 8-26°C) in a Brett-type respirometer. All populations maintained swim performance, aerobic scope, cardiac scope and scope for heart rate across the entire range of temperatures typically encountered during their upriver migration. Chilko sockeye salmon emerged as the champions of high temperature tolerance among the populations tested, able to maintain maximal cardiorespiratory performance up to 21°C. In addition, maximum aerobic scope positively correlated with migration distance (populations traveling the furthest distance had a higher aerobic scope compared to coastal populations). Furthermore, significant differences in cardiac morphology among populations correlated with migration difficulty. Populations encountering more challenging migrations had larger hearts and better coronary supply (greater percent compact myocardium). Collectively, these results suggest population level adaptation of cardiorespiratory physiology to the river conditions.

Next, we sought a mechanistic explanation for the differences in high temperature tolerance observed among populations. Chilko sockeye salmon had a higher density of adrenaline-binding ventricular β-adrenoceptors compared to Nechako sockeye salmon, a population with a lower temperature tolerance. Remarkably, β-adrenoceptor density increased with high temperature acclimation in Chilko sockeye salmon, sharply contrasting previous results in the literature for rainbow trout. The elevated ventricular β-adrenoceptor density of Chilko sockeye salmon may provide enhanced cardiac capacity and protection, allowing them to maintain cardiorespiratory performance at temperature extremes, thereby expanding their thermal tolerance. As such, Chilko sockeye salmon may have a greater resilience to climate change. While this study suggests that there is potential for physiological adaptation to climate change, it is unknown whether Fraser River sockeye salmon will be able to adapt quickly enough to cope with the current warming trend. Supported by NSERC Canada and the BC Pacific Salmon Forum.