Killer whales that attack great white sharks are changing a marine ecosystem

A pair of South African killer whales have begun changing their entire ecosystem after targeting great white sharks.

The presence of the orca pair, known as Port and Starboard by scientists, has been linked with eight great white shark carcasses washing up on the country's shoreline since 2017. Since the attacks began, great whites appear to be avoiding the area around Gansbaai, which has been a tourist hotspot for seeing the predatory fish.

PhD student Alison Towner, who led a new study investigating the impacts of the attacks, says, 'Following an initial orca attack in Gansbaai, individual great white sharks did not appear for weeks or months.'

'What we seem to be witnessing though is a large-scale avoidance strategy, mirroring what we see used by wild dogs in the Serengeti in Tanzania, in response to increased lion presence. The more the orcas frequent these sites, the longer the great white sharks stay away.

'By determining how large marine predators respond to risk, we can understand the dynamics of coexistence with other predator communities; and these dynamics may also dictate the interactions between competitors or intra-guild predator/prey relationship.'

The findings of the study were published in the African Journal of Marine Science. 

How do great white sharks and killer whales interact?

Great white sharks and killer whales are both apex predators, sitting at the top of their respective food chains. As they compete for similar prey, including seals and whales, these two species can come into conflict when they cross paths.

In these instances, the marine predators will attack each other, and there have been multiple observations of unusual behaviours used by killer whales to attack great whites.  

In the 1990s, a killer whale was observed ramming a great white near Southeast Farallon Island in the USA. After bringing it to the surface upside down, it then swam around with the shark at the surface for around 15 minutes before beginning to eat it. 

It is suggested that after stunning the shark by ramming it, turning it upside down disoriented the animal further, causing it to enter a state known as tonic immobility. In this trance-like state, sharks relax and stop moving, which allowed the killer whale to suffocate it at the surface.

The killer whale was then observed eating the shark's liver, which is rich in fats and oils that help the great white to maintain its buoyancy. The same is also true of the recent attacks in South Africa, where the killer whales selectively removed the liver, and sometimes the heart, of their prey.

Another similarity between both incidents was that in the time afterwards, the number of sharks dropped significantly. 

Both Gansbaai and Southeast Farallon Island are home to significant seal populations which cause great whites to gather annually. However, when the killer whales were present just before or during these shark gatherings, the number of great whites dropped to as little as a seventh of its expected amount.

Follow-up analysis using trackers on the Farallon sharks revealed that following killer whales arriving in the area and eating seals, the great whites deserted the area, with some staying away for up to a year. Similar study of the Gansbaai sharks has shown a similar pattern.

While consuming the sharks provides a valuable new source of nutrition for the killer whales, the mammals are also responsible for much wider changes in South Africa's marine ecosystem. 

How does the loss of great white sharks affect an ecosystem?

With great white sharks increasingly absent from the waters around Gansbaai, the species that are normally eaten by these sharks are increasing in number.

This has concerned scientists, who believe that the killer whales have unbalanced the food chain. In a healthy ecosystem, predators control prey numbers through consuming them. At the same time, the abundance of prey, which are generally herbivores, controls plant growth and limits how many predators there can be.

Stopping some of these controls by removing a group of animals from the ecosystem can have knock-on effects for the other groups in a phenomenon known as a trophic cascade.

'It has triggered the emergence of a new mesopredator (a predator in the middle of a food chain) known as the bronze whaler shark to the area which are known to be eaten by the great white shark,' Alison explains. 'These bronze whalers are also being attacked by the orcas too, who are indicating a level of experience and skill in hunting large sharks.'

'However, balance is crucial in marine ecosystems. With no great white sharks restricting cape fur seal behaviour, the seals can predate on Endangered African penguins or compete for the small pelagic fish they eat.'

There is only so much pressure an ecosystem can take, and the impacts of these orca removing sharks are likely far reaching.'

As well as impacts on the natural world, the killer whales are also having an impact on the human one. With tourists flocking from around the world to try and see great whites at Gansbaai, a decline in their number will have knock on effects for those who rely on the income from the industry.

The scientists note that other causes, such as changes in sea temperature, could be responsible for the decline in great whites. However, they believe that the decline in shark abundance in 2017 is too sudden for temperature alone to explain, and that the killer whales are a more likely cause.

If the pressure on the sharks is kept up, it has the potential to significantly decrease their population. As they take a long time to mature, and their populations are already declining from fisheries and bather protection nets, it could contribute to the trophic changes becoming more pronounced over time.

For now, the researchers hope to continue studying the Gansbaai great white shark population, as well as the killer whales, to discover more about how the ecosystem may change in the years to come.