Mixed species assemblages: What do Burrunan dolphins, common dolphins and Australian fur seals have in common?

Mixed species assemblages (MSAs), noted as two or more species interacting and forming one interspecific group, can vary between closely related species to those from differing Orders. Marine mammal MSAs have been documented... [ view full abstract ]

Mixed species assemblages (MSAs), noted as two or more species interacting and forming one interspecific group, can vary between closely related species to those from differing Orders. Marine mammal MSAs have been documented to provide foraging, social and reproductive advantages and can assist in protection against predators. MSAs are not necessarily mutually beneficial with aggressive and harassing behaviour also noted. Port Phillip Bay, Victoria, hosts a small resident population of Burrunan dolphins (Tursiops australis), a small community of common dolphins (Delphinus delphis) and provides numerous haul out sites for the Australian fur seals (Arctocephalus pusillus doriferus (AFS)). For the first time we report on delphinid and ottorid heterospecific interactions in Port Phillip Bay. We assessed sightings data collected from 39 surveys across 2006-2008. Twenty of the 77 sightings recorded were MSAs; 30% of those were between all three species, the remainder being between two species (10 Burrunan/AFS; 3 Burrunan/common). In assessing the behaviours observed during MSAs, 70% were related to foraging activities, which may be indicative of local enhancement and/or a foraging advantage. Seals were often observed surface feeding and/or ‘tracking’ Burrunan foraging at greater depths. Aggressive behaviour was only noted between Burrunan and common dolphin MSAs. Further investigation is required to assess the correlation of MSAs with environmental variables, prey choice and other predator interactions known to utilise local enhancement techniques when foraging, such as Australasian gannets. Assessing MSAs will assist in identifying areas of significance for Port Phillip Bay’s iconic marine mammal species.