How the decline of top predators is changing South Africa’s marine ecosystem


We all know the 1975 blockbuster Jaws, which inaccurately described great white sharks as “insane eating machines”. Although it is one of the top marine predators, there is a species of dolphin that has started preying on great white sharks. A pair of orcas, also known as ‘killer whales’, have 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 behavior on the marine ecosystem, and why are these predators suddenly feeding on great white sharks?

What’s going on in South Africa’s marine ecosystem?

A new studypublished 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 killer whale pair. Although killer whales have been recorded feeding on shark species, including great white sharks, there have been no direct sightings or predation of great white sharks locally – until now. Researchers found that dozens of great white sharks actively avoid parts of the coast of Gansbaai, a white shark aggregation site in South Africa’s Western Cape, when killer whales are present. They have used a combination of long-term observations and tagging data to find that tagged sharks sometimes disappeared for weeks or months at a time, giving up territory that historically had 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 use a large-scale avoidance strategy – a technique we also see used by wild dogs in the Serengeti in Tanzania.

Considering the precise way killer whales kill great white sharks, this might not be such a surprising answer. They are very intelligent mammals and this is reflected in the way they hunt great whites. First, they stun the shark by ramming it, then flip it over to disorient it, causing it to enter a trance state known as tonic immobility. Sharks essentially stop moving in this posture, allowing killer whales to drown the animal on the surface. The killer whales then selectively cut open the shark to extract its liver and, in some cases, other internal organs, which also helps the sharks maintain their buoyancy.

Predator-prey interactions between white sharks, other coastal sharks and killer whales are increasing. Further 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 broader impacts on shark populations and should be considered in future studies.

Killer whales

Killer whales (Orcinus orca) belong to the toothed whale suborder (called odontocetes) but they are the largest member of the dolphin family. Ancient sailors who saw a pod of orcas hunting larger species of whales 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 (octopoda and squid), seabirds and more . Having so many sources of prey most likely resulted in the niche specializations seen in killer whales today.

Various populations began eating different foods millions of years ago to avoid competing for the same prey. These groups, recognized as ecotypes, are now genetically distinct, in addition to their specific appearances and cultures. While ecotypes generally classify populations of a species based on their diet and habitat, morphotypes distinguish individuals of the same species based on morphological characteristics such as overall size and shape of fins and teeth .

The pair of South African killer whales are unique in that they are flat-toothed, shorter, and specialize in a shark-based diet. This rare type of killer whale resembles the Northern Hemisphere offshore ecotype in various ways, and it does not appear to have been previously observed in the Southern Hemisphere.

What causes this change in behavior?

What drives killer whales to adopt this new behavior is not fully known. Despite the fact that killer whales can be much larger than great white sharks, they rarely hunt them. The study researchers note that sea ​​temperature changes are known to influence the behavior of the great white shark. However, if the decline of the great whites along the coast of Gansbaai is another phenomenon that can be attributed to climate change or a pair of novelty-hungry orcas, yet to be determined.

How this change in behavior affects the marine ecosystem

According to the study, great white sharks flee the Gansbaai area due to their fear of killer whales. Although fear effects appear to be common in aquatic ecosystems, fear responses from animals closer to the top of the food chain can quickly induce changes at lower trophic levels.

With great white sharks increasingly absent from the waters near Gansbaai, the species typically eaten 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 one group of creatures – in this case great white sharks – could impact all other species. This phenomenon is called 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 the great white sharks. These sharks frequently serve as food for great white sharks, but since the disappearance of the great sharks, there have been many more sightings of bronze whale sharks. Not only that, but in the absence of great white sharks, Cape fur seals can feed uncontrollably on the endangered african penguin or compete for the small pelagic fish they consume. The balance of the marine ecosystem can be permanently impacted the more killer whales frequent these sites and the more the great whites stay away.

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