Application of an ecologically relevant thermal tolerance metric reveals "heat weakening" in a salmonid

Research evaluating ectotherm thermal tolerance has primarily employed a metric designed for comparisons of thermal limits across individuals, populations, and species (i.e., critical thermal maximum; CT_max) which, as a relative index, has limited direct ecological application. Therefore, we sought to develop methodology and test the repeatability of a thermal tolerance metric (critical temperature; T_c) describing the temperature at which the instantaneous thermal injury and instantaneous recovery rates are equal in an individual, representing a threshold for physiological disruption. Over a four-month period, we conducted repeated thermal tolerance assays on Brook Trout Salvelinus fontinalis briefly conditioned to different temperatures (T₀: 12-20 °C). During these thermal tolerance assays, we recorded CT_max and used subsequent temperature profiles to estimate T_c following a balance equation. We then estimated the adjusted repeatability (R) of these metrics across individual fish. T_c was moderately repeatable (R = 0.19 ± 0.04) but less repeatable than CT_max (R = 0.54 ± 0.03), which was expected from the additional sensitivity of the estimation process. Models indicated that T_c and CT_max decreased with repeated assays, suggesting that repeated exposure to stressful temperatures may reduce seasonal-scale adaptive capacity within individual Brook Trout (i.e., "heat weakening"). Additionally, T_c increased with increasing conditioning temperatures, which could indicate the presence of acute physiological adaptive effects from short-term temperature exposures that could mitigate heat stress during heating events. Our study provides a methodology for estimating a thermal tolerance metric that is more closely tied with physiology and individual performance, which could prove useful for applying estimated thermal limits to ecological models and quantifying population effects from climate change.