Upper thermal limits of cardiac function for Arctic cod Boreogadus saida, a key food web fish species in the Arctic Ocean

The objective of this study was to determine the upper thermal limits of Arctic cod Boreogadus saida by measuring the response of maximum heart rate (fHmax) to acute warming. One set of fish were tested in a field laboratory in Cambridge Bay (CB), Nunavut (north of the Arctic Circle), and a second set were tested after air transport to and 6 month temperature acclimation at the Vancouver Aquarium (VA) laboratory. In both sets of tests, with B. saida acclimated to 0 degrees C, fHmax increased during acute warming up to temperatures considerably higher than the acclimation temperature and the near-freezing Arctic temperatures in which they are routinely found. Indeed, fHmax increased steadily between 0 center dot 5 and 5 center dot 5 degrees C, with no significant difference between the CB and VA tests (0 center dot 05) and with an overall mean +/- s.e. Q10 of 2 center dot 4 +/- 0 center dot 5. The first Arrhenius breakpoint temperature (TAB) for fHmax was also statistically indistinguishable for the two sets of tests (mean +/- s.e. 3 center dot 2 +/- 0 center dot 3 and 3 center dot 6 +/- 0 center dot 3 degrees C), suggesting that the temperature optimum for B. saida could be reliably measured after live transport to a more southerly laboratory location. Continued warming above 5 center dot 5 degrees C revealed a large variability among individuals in the upper thermal limits that triggered cardiac arrhythmia (Tarr), ranging from 10 center dot 2 to 15 center dot 2 degrees C with mean +/- s.e. 12 center dot 4 +/- 0 center dot 4 degrees C (n = 11) for the field study. A difference did exist between the CB and VA breakpoint temperatures when the Q10 value decreased below 2 (the Q10 breakpoint temperature; TQB) at 8 center dot 0 and 5 center dot 5 degrees C, respectively. These results suggest that factors, other than thermal tolerance and associated cardiac performance, may influence the realized distribution of B. saida within the Arctic Circle. (C) 2014 The Fisheries Society of the British Isles