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 degrees C per day from 12 degrees C) to mimic the rise in temperatures experienced at sea-cages in Atlantic Canada during the summer, or held at 10 degrees C. Post-smolt Atlantic salmon exposed to the incremental temperature increase up to 20 degrees C gained more weight than fish reared at 10 degrees 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 degrees 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 degrees C h(-1) from 10 degrees C) of the bottom and top four temperature tolerant families (as determined by ITMax) did not differ (similar to 28.0 degrees 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.