When asked what the ocean’s greatest predator is, many people would answer the great white shark (Carcharadon carcharias). The subject of many a thriller film, the great white shark is a formidable species, growing to an average body length of 11-13ft for males and 15 to 16ft for females. Living in much of the world’s oceans, they are responsible for the most recorded shark bites on humans. However, rare interactions between these sharks and orca (Orcinus orca) have shown that they are not always the ocean’s top predator.
While great white ‘pups’ have several predators, adult great whites are not reported to have any known natural predators. This is why their interactions with orca are so interesting. First documented in 1997, predation by orcas on great whites is rare. Orcas use several different techniques when attacking the sharks: ramming, biting off the tail fin and flipping the shark by biting close to its dorsal fin, which induces a state of paralysis caused by tonic immobility. The target seems to be the shark’s liver. Orcas have also been seen targeting the livers of other elasmobranch species as well, including broad nose sevengill sharks (Notorynchus cepedianus), bronze whalers (Carcharhinus brachyurus), and tiger sharks (Galeocerdo cuvier).
The arrival of the orca in certain areas such as the Farallon Islands, off the coast of San Francisco, California, and the Cape of Good Hope, South Africa, sparked a dramatic ecosystem shift. Great whites, once common in these areas, began to disappear, moving away to different habitats. According to Shark Spotters in the Cape of Hope, great white shark sightings have dropped dramatically from an average of 205 a year between 2005-2015 to not a single sighting in 2020. The disappearance of these apex predators leads to direct and indirect impacts on their ecosystems. A 2019 study found that displacement of great whites disrupts their feeding behaviour within the area, reducing the number of pinnipeds being predated upon. This can cause a trophic cascade, as the increased pinniped populations may lead to decreases in the populations of their prey, such as the African penguin (Spheniscus demersus). Additionally, sevengill sharks have moved into some areas where great whites were displaced; the effects of this shift in apex predator on the ecosystem are currently unknown.
It’s not known for certain why orcas target great whites. It could be opportunistic feeding behaviour, an attempt to eliminate competition for similar prey, or it may be a result of anthropogenic activities. The overfishing of species that orca prey on may be causing the orca to look for a different food source.
Reference
Jorgensen, S. J., Anderson, S., Ferretti, F., et al., 2019. Killer whales redistribute white shark foraging pressure on seals. Scientific Reports, 9(1), 6153
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