Abstract
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 ( f Hmax ) 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° C, f Hmax 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, f Hmax increased steadily between 0·5 and 5·5° C, with no significant difference between the CB and VA tests ( P > 0·05) and with an overall mean ± s.e. Q 10 of 2·4 ± 0·5. The first Arrhenius breakpoint temperature ( T AB ) for f Hmax was also statistically indistinguishable for the two sets of tests (mean ± s.e. 3·2 ± 0·3 and 3·6 ± 0·3° 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 5·5° C revealed a large variability among individuals in the upper thermal limits that triggered cardiac arrhythmia ( T arr ), ranging from 10·2 to 15·2° C with mean ± s.e. 12·4 ± 0·4° C ( n = 11) for the field study. A difference did exist between the CB and VA breakpoint temperatures when the Q 10 value decreased below 2 (the Q 10 breakpoint temperature; T QB ) at 8·0 and 5·5° 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.