The Atlantic salmon's (Salmo salar) incremental thermal maximum is a more relevant and sensitive indicator of family-based differences in upper temperature tolerance than its critical thermal maximum

Rising sea surface temperatures and heat waves are a challenge to salmon aquaculture, and the industry must endeavour to mitigate their impacts. To investigate genetic-based differences in upper thermal tolerance, 20 salmon families were exposed to an incremental temperature increase (+0.2 °C per day from 12 °C) to mimic the rise in temperatures experienced at sea-cages in Atlantic Canada during the summer, or held at 10 °C. Post-smolt Atlantic salmon exposed to the incremental temperature increase up to 20 °C gained more weight than fish reared at 10 °C over the same period, and there were family-specific differences (p < 0.05) in growth and body morphometrics. Significant differences were also detected between the families' incremental thermal maximum (ITMax), with average family ITMax values ranging from 23.3 to 25.0 °C. Fish weight and growth rate were not significantly related to ITMax, but a negative correlation (p < 0.01) was found between hepatosomatic index (HSI) and ITMax. In contrast, the critical thermal maximum (CTMax, measured using a temperature increase of 2 °C h−1 from 10 °C) of the bottom and top four temperature tolerant families (as determined by ITMax) did not differ (∼28.0 °C). A negative correlation (p < 0.01) between HSI and CTMax was also found. However, no relationship was evident between relative ventricular mass (RVM) and ITMax or CTMax. Our data show that ITMax is a more sensitive metric of a salmon's upper temperature tolerance than even an ‘environmentally relevant/realistic’ CTMax test, and suggest that it could be used in breeding programs that aim to enhance the salmon's tolerance to withstand rising ocean temperatures.