The applicability of macroecological rules to patterns in body size varies between taxa. One of the most examined is Bergmann's rule, which states that body size increases with decreasing temperature and increasing latitude, although the rule is not universal and the proposed mechanisms underpinning it are multifarious and lack congruence. This study considers the degree to which Bergmann's rule applies to the Southern Ocean mesopelagic fish community. We studied patterns in body size, temperature and latitude across a 12° latitudinal gradient within the Scotia-Weddell sector. Intra-specific Bergmann's rule was found to apply to 8 out of the 11 biomass-dominant species in the family Myctophidae. The rule was also apparent at an inter-specific level. Our study suggests that greater attainable body size in this community is a necessary attribute to reach colder regions further south. The adherence of these taxa to Bergmann's rule enables such species to act as sentinels for identifying the drivers and consequences of ocean warming on the Southern Ocean ecosystem.