Wild adult and reared larval Boreogadus saida were acclimated to 3?5??C before testing their cardiac response to acute warming. Heart rate transition temperatures during warming were similar for adult and larval hearts, except that the maximum temperature for heart rate was 3??C warmer for adults. Thus, in a rapidly warming Arctic Ocean, the upper temperature limit for larval rather than adult B. saida appears more likely to dictate the southern range of the species.