Amy M. Regish Lizabeth Bowen Ashley E. Stanek Shannon C. Waters-Dynes Michael P. Carey Christian E. Zimmerman Jonathon Gerken Daniel Rinella Stephen D. McCormick Vanessa R. von Biela 2023 <p class="chapter-para">Rapid and accelerating warming of salmon habitat has the potential to lower productivity of Pacific salmon (<i>Oncorhynchus</i><span>&nbsp;</span>species) populations. Heat stress biomarkers can indicate where warming is most likely affecting fish populations; however, we often lack clear classifications that separate individuals with and without heat stress needed to make these tools operational. We conducted a heat exposure experiment with trials lasting 12 or 36&nbsp;h using juvenile Chinook salmon (<i>Oncorhynchus tshawytscha</i>) and coho salmon (<i>Oncorhynchus kisutch</i>) to validate heat stress biomarkers in white muscle. Following habituation to 13°C, individuals were exposed to water temperatures that increased to 15°C, 17°C, 19°C, 21°C or 23°C. Heat shock protein 70 abundance (HSP70 measured by ELISA) and transcription of 13 genes (mRNA measured by qPCR) including three heat shock protein genes (<i>hsp70, hsp90, hsp27</i>) were measured. A distinct heat stress response was apparent by 21°C in juvenile Chinook salmon and 23°C in juvenile coho salmon using HSP70. A threshold for heat stress classification in Chinook salmon of &gt; 2&nbsp;ng HSP70 mg^.1<span>&nbsp;</span>total protein identified heat stress in 100% of 21 and 23°C treated individuals compared to 4% in cooler treatments. For coho salmon, &gt; 3&nbsp;ng HSP70 mg^.1<span>&nbsp;</span>total protein identified heat stress in 100% of 23°C treated individuals compared to 4% in cooler treatments. Transcription from a panel of genes separated individuals between cooler and stressful temperature experiences (≥21°C for Chinook salmon and ≥23°C for coho salmon) with ~ 85% correct classification. Our findings indicate that juvenile Chinook salmon were more temperature-sensitive than juvenile coho salmon and support the use of a HSP70 threshold sampled from muscle for assessing heat stress in individual wild Pacific salmon with an option for non-lethal biopsies for spawning adults.</p> application/pdf 10.1093/conphys/coad092 en Oxford Academic Differential heat shock protein responses in two species of Pacific salmon and their utility in identifying heat stress article