Acute critical thermal maximum does not predict chronic incremental thermal maximum in Atlantic salmon (Salmo salar)

Atlantic salmon is an important aquaculture species farmed in ocean net-pens and therefore subjected to changing environmental conditions, including rising temperatures. This creates a need for research on the thermal tolerance of this species for the future of sustainable aquaculture. We investigated the thermal tolerance of individually tagged Atlantic salmon post-smolts subjected sequentially to two common high-temperature challenges: critical thermal maximum (CTmax) followed by incremental thermal maximum (ITmax). Our goals were (1) to determine whether CTmax can predict ITmax for individual fish, and (2) to examine connections between various body size (mass, length, condition factor), cardiac (absolute and relative ventricle mass) and blood (hematocrit) metrics and thermal tolerance. We found no relationship between CTmax and ITmax. This is of concern because CTmax, which is a quick and easy test, is often used to predict upper lethal limits in fish despite not using real-world rates of temperature increase and not using death as the experimental endpoint (unlike ITmax). Also, some metrics which correlated in one direction with CTmax had the opposite correlation with ITmax. For instance, smaller fish or fish with smaller ventricles had a higher CTmax but a lower ITmax than larger fish or fish with larger ventricles. Taken together, these results highlight the need to take care when using acute thermal tolerance tests to predict real-world responses to rising temperatures.