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 (IT Max), with average family IT Max values ranging from 23.3 to 25.0 °C. Fish weight and growth rate were not significantly related to IT Max , but a negative correlation (p < 0.01) was found between hepatosomatic index (HSI) and IT Max. In contrast, the critical thermal maximum (CT Max , 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 IT Max) did not differ (∼28.0 °C). A negative correlation (p < 0.01) between HSI and CT Max was also found. However, no relationship was evident between relative ventricular mass (RVM) and IT Max or CT Max. Our data show that IT Max is a more sensitive metric of a salmon's upper temperature tolerance than even an 'environmentally relevant/realistic' CT Max test, and suggest that it could be used in breeding programs that aim to enhance the salmon's tolerance to withstand rising ocean temperatures. • Twenty families of Atlantic salmon were given an incremental thermal maximum (IT Max) test (+ 0.2 °C day−1) or kept at 10 °C. • The IT Max of the families was significantly different and ranged from 23.3 to 25.0 °C. • The critical thermal maximum (CT Max) of the families with the 4 highest and lowest IT Max values did not differ (∼28 °C). • A significant negative correlation was found between hepatosomatic index (HSI) and both IT Max and CT Max. • IT Max is a more sensitive metric of an Atlantic salmon's upper thermal tolerance than CT Max. [ABSTRACT FROM AUTHOR]