The upper temperature and hypoxia limits of Atlantic salmon (Salmo salar) depend greatly on the method utilized

In this study, Atlantic salmon were: (i) implanted with heart rate (f(H)) data storage tags (DSTs), pharmacologically stimulated to maximum f(H), and warmed at 10°C h(−1) (i.e. tested using a ‘rapid screening protocol’); (ii) fitted with Doppler(®) flow probes, recovered in respirometers and given a critical thermal maximum (CT(max)) test at 2°C h(−1); and (iii) implanted with f(H) DSTs, recovered in a tank with conspecifics for 4 weeks, and had their CT(max) determined at 2°C h(−1). Fish in respirometers and those free-swimming were also exposed to a stepwise decrease in water oxygen level (100% to 30% air saturation) to determine the oxygen level at which bradycardia occurred. Resting f(H) was much lower in free-swimming fish than in those in respirometers (∼49 versus 69 beats min(−1)) and this was reflected in their scope for f(H) (∼104 versus 71 beats min(−1)) and CT(max) (27.7 versus 25.9°C). Further, the Arrhenius breakpoint temperature and temperature at peak f(H) for free-swimming fish were considerably greater than for those tested in the respirometers and given a rapid screening protocol (18.4, 18.1 and 14.6°C; and 26.5, 23.2 and 20.2°C, respectively). Finally, the oxygen level at which bradycardia occurred was significantly higher in free-swimming salmon than in those in respirometers (∼62% versus 53% air saturation). These results: highlight the limitations of some lab-based methods of determining f(H) parameters and thermal tolerance in fishes; and suggest that scope for f(H) may be a more reliable and predictive measure of a fish's upper thermal tolerance than their peak f(H).