Fish populations, and salmonids specifically, are known to match their physiology to the local thermal environment. Physiological variation between populations can be a result of acclimatization and local adaptation. Chinook salmon (Oncorhynchus tshawytscha) exist across a large latitudinal range, with individual populations experiencing a variety of temperature regimes. California Chinook salmon in the Central Valley live at the southern species range and experience temperatures warmer than northern populations. Therefore, they may exhibit a physiology suited for warmer waters. However, without controlled comparisons our understanding of the thermal capacity of Chinook salmon, and the potential for population-specific thermal adaptation remains limited. We hypothesized that across a latitudinal gradient, southern (California) populations would exhibit a more ‘warm-adapted’ physiology than Northern populations (Oregon and Washington).
Over the course of three years, we reared juvenile Chinook salmon from eight hatchery populations spanning the states of California, Oregon and Washington. Each population was reared at three acclimation temperatures (11, 16 and 20°C) to explore interpopulation variation in acclimation capacity and thermal physiology. We measured temperature dependent growth rate, acute thermal tolerance and aerobic metabolic scope in order to assess whether Chinook salmon populations exhibit intraspecific variation in thermal capacity and potentially local adaptation.
Results/Conclusions
Overall we found Chinook salmon across the sampled range to exhibit impressive thermal capacity, challenging the moniker of a ‘cold-water fish’. More specifically, our work found several patterns indicative of local adaptation among Chinook salmon populations. Within the Central Valley the Winter-run population, native to historically cold, high elevation rivers, exhibited thermal performance phenotypes suited for cooler waters with declining metabolic performance and growth when acclimated to warmer temperatures (16° & 20°C), however acute thermal tolerance remained high (~28-30°C). Our results also indicate that differences between populations are most apparent when populations are reared at warm (20°C) temperatures. While some evidence for a difference between populations along a latitudinal gradient exists, our results indicate that local environmental characteristics may serve as better predictors of population-specific thermal performance. This work has implications for how we manage and conserve at-risk salmonid populations in a warming climate.