Modelling of physical drivers of a large feeding aggregation of killer whales (Orcinus orca) in the western Great Australian Bight, Australia

Every year >100 killer whales accumulate on the continental slope near Bremer Bay in the western Great Australian Bight from late austral spring to early autumn (January–April), which can be regarded as a natural wonder. For unknown reasons, the whales tend to feed in the same confined region over a seafloor plateau near the head of the Hood Canyon, where the total water depth is ~800–1000 m. The hypothesis behind this work is that the killer-whale aggregation is the signature of a localized marine food web originating from flow-induced upwelling and concentration of benthic particulate organic matter (POM). To test this hypothesis a hydrodynamic model is applied to analyse the pathways of virtual suspended particles released near the seafloor in three large submarine canyons that exist in the region. Here we show that, due to its shape and positioning on the continental slope, the Hood Canyon operates to funnel significantly more benthic particles onto the ambient continental slope than the adjacent Bremer Canyon and the Whale Canyon. Moreover, findings suggest that smaller-scale topographic undulations within the Hood Canyon are the cause of this slope upwelling of POM. This transport mechanism of POM may explain the ecological uniqueness of the region.