We are all familiar with the 1975 blockbuster Jaws, which inaccurately depicted great white sharks as “mindless eating machines”. Despite being one of the top marine predators, there is a species of dolphin that has started to prey on great white sharks. A pair of orcas, also known as “killer whales”, has been documented to have killed at least eight great white sharks off the coast of South Africa since 2017. What are the effects of this unusual behaviour on the marine ecosystem, and why are these predators suddenly feeding on great white sharks?
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What Is Going On In South Africa’s Marine Ecosystem?
A new study, published in the African Journal of Marine Science, documented the emigration of great white sharks from Mossel Bay, South Africa (Carcharodon carcharias) in response to the presence of the orca pair. While orcas have been recorded preying on shark species, including great whites, there had been no direct observation or predation on great white sharks locally – until now. The researchers discovered that dozens of great white sharks have been actively avoiding parts of the Gansbaai coast, a white shark aggregation site in the Western Cape in South Africa, when the orcas are around. They used a combination of long-term sightings and tagging data to find that tagged sharks sometimes disappeared for weeks or months at a time, abandoning territory that, historically, has been dominated by these animals. Alison Towner, lead author of the study and senior biologist studying great white sharks in South Africa, claimed that the sharks employ a large-scale avoidance strategy – a technique that we also see used by wild dogs in Tanzania’s Serengeti.
Given the precise manner in which orcas kill great white sharks, this might not be such a surprising response. They are highly intelligent mammals and this is reflected in the way they hunt great whites. First, they stun the shark by ramming it, then turn it upside down to disorient it, causing it to enter a trance-like state known as tonic immobility. Sharks essentially stop moving in this posture, allowing the orcas to drown the animal at the surface. The orcas then selectively cut open the shark to extract its liver and in some cases other internal organs, which also helps sharks to maintain their buoyancy.
Predator-prey interactions between white sharks, other coastal sharks, and killer whales are on the rise. More research is needed to determine how these predation events affect the long-term ecological balance of these complex coastal seascapes. These long-term shark migrations and unusual predation events will have wider reaching impacts on shark populations and will need to be considered in future studies.
Orcas
Orcas (Orcinus orca) belong to the suborder of toothed whales (known as odontocetes) but they are the largest member of the dolphin family. Ancient sailors who saw a pod of orcas hunting larger whale species originally called them “whale killers” but this epithet was later reversed to “killer whale”. Orcas are generalist eaters, consuming fish, seals and sea lions, dolphins and porpoises, sharks and rays, large whales, cephalopods (octopods and squids), seabirds and more. Having such numerous prey sources most likely resulted in the niche specialisations seen in orcas today.
Various populations began eating different foods millions of years ago to avoid competing for the same preys. These groups, recognised as ecotypes, are now genetically distinct, in addition to their specific looks and cultures. Whereas ecotypes generally classify populations of a species based on diet and habitat, morphotypes distinguish individuals of the same species based on morphological characteristics such as overall size and fin and tooth shape.
The pair of South African orcas are unique in that they are a flat-toothed morphotype, shorter in length and specialise in a shark-based diet. This rare type of orca resembles the offshore ecotype from the Northern Hemisphere in various ways, and it does not appear to have been observed previously in the Southern Hemisphere.
What Is Causing This Behavioural Change?
What is causing the orcas to adopt this new behaviour is not entirely known. Despite the fact that orcas can be much larger than great white sharks, they rarely prey on them. The researchers of the study note that changes in sea temperature are known to influence great white shark behaviour. However, whether the decline of great whites along the coast of Gansbaai is yet another phenomenon that may be attributed to climate change or to a pair of orcas hungry for something new, has yet to be determined.
How This Behavioural Change Impacts The Marine Ecosystem
According to the study, great white sharks are fleeing the Gansbaai area due to their fear of the orcas. Although fear effects appear to be common in aquatic ecosystems, fear responses by animals closer to the top of the food chain can rapidly induce changes at lower trophic levels.
With great white sharks increasingly absent from the waters near Gansbaai, the species typically consumed by these sharks are increasing in number. In a healthy marine ecosystem, the number of predators is kept to a minimum by the control they exert over their prey. All ecosystems typically exhibit such a meticulous balance. However, the sudden absence of a group of creatures – in this case of great white sharks – might have an impact on all other species. This phenomenon is referred to as a trophic cascade.
The ecological balance has already been impacted by these long-term shark migrations. The bronze whaler shark has occupied the former habitat of great white sharks. These sharks frequently serve as food for great whites, but since the big guys disappeared, there have been a lot more sightings of bronze whaler sharks. Not only that, but in the absence of great white sharks, cape fur seals can feed uncontrolled on the endangered African penguin or compete for the small pelagic fish they consume. The balance of the marine ecosystem may be permanently impacted the more the orcas frequent these sites and the longer the great whites stay away.
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