Kenneth W Zillig ^1,2 Heather N Bell ¹ Alyssa M Fitzgerald ^3,4 Nann A Fangue ^1*

• ¹ University of California, Davis, Davis, United States
• ² University of Minnesota Twin Cities, St. Paul, Minnesota, United States
• ³ Fisheries Ecology Division, Southwest Fisheries Science Center (NOAA), Santa Cruz, California, United States
• ⁴ Institute of Marine Sciences, Division of Physical & Biological Sciences, University of California, Santa Cruz, Santa Cruz, California, United States

The capacity for a species or population to respond to rapid environmental change will be pivotal to its resilience in the coming decades. We assessed the heat hardening response and acclimation capacity of juvenile Chinook salmon (Oncorhynchus tshawytscha) from seven hatchery populations distributed across a range of ecoregions of the West Coast of the United States to address questions of thermal plasticity, physiological trade-offs, and associations with environmental characteristics. Progeny from each population were acclimated to one of three temperatures (11, 16 or 20°C) and then put through two acute thermal maximum (CTMAX) trials 24 hours apart. We quantified each fish's heat hardening response (ΔCTM) as the difference between its second and first CTMAX trial.We found that fish acclimated to 16 or 20°C exhibited heat hardening (positive ΔCTM), while fish acclimated to 11°C exhibited 'heat weakening' (negative ΔCTM), a rare occurrence in the literature. We then calculated a population's acclimation capacity as the maximum difference between a trial-naive 11°C acclimated fish and a 20°C acclimated fish during their second CTMAX, thereby capturing a maximum difference in acclimatory scope. We used Bayesian models to investigate the thermal trade-off hypothesis which posits that taxa, populations, or individuals with higher thermal tolerance exhibit reduced capacity to acclimate to future change. We investigated trade-offs at two organizational scales and found that while individual fish within a population did adhere to the expectations of the thermal trade-off hypothesis, the performance of discrete populations did not. We additionally investigated whether ΔCTM and acclimation capacity was associated with aspects of the source population's natural rearing environment. We found that acclimation capacity was positively associated with a population's latitude as well as the temperature of its rearing environment. Furthermore, a fish's ΔCTM performance, relative to its conspecifics, was improved when a fish was acclimated to temperatures closer to that of its natural rearing environment, a result which supports hypotheses of local adaptation among Chinook salmon populations. Our results further the understanding of intraspecific differences in thermal physiology and are relevant to the conservation and management of species facing rapid environmental change.