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

ABSTRACT

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.

Abstract

Summary: Heart rate responses to environmental challenges in free-swimming salmon are different from those measured using more invasive techniques. This has important implications for predicting the susceptibility of fishes to climate change.