Weddell seals (Leptonychotes weddellii) and killer whales (Orcinus orca), specifically Type C killer whales (TCKW), have been identified as the top predators most likely to be directly affected by the removal of Antarctic toothfish (Dissostichus mawsoni; ‘toothfish’) in the Ross Sea region. However, extreme paucity of information about the diet of TCKW, and uncertainty regarding the degree of reliance on toothfish as a critical prey item, have limited our ability to (a) assess the risk posed by the fishery to TCKW and other marine mammals and (b) mitigate potential risk by implementation of management strategies such as closures to protect key foraging habitats. During late January 2014, feeding by TCKW was recorded during 5 of 8 helicopter flights. For 3 of these observations, the prey was identifiable as toothfish parts; no consumption of non-fish prey by TCKW was detected. We also observed a large number of TCKW with small and presumably suckling calves. Given that lactation represents a significant energy cost, we calculated energy expenditures of adult TCKW during maintenance, late-stage gestation, and lactation. A comparison of energy expenditure with energy densities of fish prey indicates that available fish other than toothfish are insufficient to support the increased energetic costs of lactation. This conclusion holds even assuming unlimited access to prey and without considering the energy cost of foraging (CPUE). Marine mammals and birds contain more energy than fish prey, but re-orientation of TCKW towards non-fish prey would represent a major behavioural modification of unknown probability. A dependence of lactating TCKW on toothfish to support the additional cost of milk production implies a strong trophic dependency even if limited to a relatively brief period of the year. We conclude that there is a probable risk to TCKW if there was a reduction in the availability of toothfish during the lactation period. Further information on this dependency, including to what extent it has clear spatial or seasonal demarcation, is urgently required.