Stress history affects heat tolerance in an aquatic ectotherm (Chinook salmon, Oncorhynchus tshawytscha)

The stress history of an ectotherm may be a pivotal predictor of how they cope with rapid spikes in environmental temperature. An understanding of how stressors in habitats and commercial operations affect ectotherm heat tolerance is urgently required so that management actions can be informed by thermal physiology. We hypothesised that brief exposure to mild stress would heighten tolerance to subsequent heat stress, indicative of a cross-tolerance interaction, whereas exposure to severe stress would reduce heat tolerance, reflecting a cross-susceptibility interaction. To test this hypothesis, we assessed how three acute stressors (salinity shock [10 or 33 ppt for 2 h]), air exposure (1 or 5 min) and crowding [95.6 kg m^-3 for 2 h]), commonly experienced by fish, affected the heat tolerance (measured as critical thermal maximum, CT_MAX) in juvenile Chinook salmon (Oncorhynchus tshawytscha). Fish were exposed to one of the three stressors and left for 24 h of recovery prior to measuring CT_MAX. Heat tolerance was improved by ∼0.6 °C in fish exposed to salinity shock (10 ppt) and air exposure (5 min) compared to unstressed controls, demonstrating cross-tolerance. However, the development of cross-tolerance was non-linear with stressor severity, and crowding stress had no effect on CT_MAX. Together these results show that some forms of stress can heighten acute heat tolerance in ectotherms, but the development of cross-tolerance is highly specific to both stressor type and stressor severity.