The Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) is currently considering plausible models of the Southern Ocean ecosystem within the Working Group on Ecosystem Monitoring and Management (WG-EMM). This paper describes properties for two groups of marine mammals that may be included in future ecosystem models of the Southern Ocean: cetaceans and pack-ice breeding seals. Important ecological differences exist among and between these groups, which should be considered in the development such models.