Who's afraid of killer whales? — white sharks and prolonged absences

Is killer whale predation the sole driver of white shark long absence? Australian researchers used 12 years of acoustic telemetry and wildlife tourism sighting data to find out.

Around six killer whales (Orcinus orca) are observed attacking and killing a white shark (Carcharodon carcharias) about 20 metres from a cage-diving vessel at the Neptune Islands Group Marine Park in South Australia. During the incident, which occurred on 2 February 2015, three whales appeared to corral the shark, restricting its movement, while others rammed it with their heads; signs of a successful predation coming when the shark was pushed under the water and an oil slick appeared on the surface.

This presumed kill of a white shark was widely documented and shared publicly and was followed by an absence of white sharks from the area for around two months. Such disappearance was, at the time, widely attributed to killer whale predation and compared to similar events in South Africa. It has been thought that predation events by killer whales trigger dramatic changes in shark behaviour — causing abrupt departures from coastal aggregation sites — and can also cause cascading shifts in the wider marine ecosystem.

These absences can last from weeks to months or even result in long-term site abandonment, as studies in California, South Africa and Mexico have suggested. However, most of this previous research has relied largely on observational data rather than long-term tracking. And so, a team of researchers from across Australia set out to learn more, including whether extended absences from their aggregation sites are also part of the sharks’ natural behaviour.

White shark. Image credit: Andrew Fox/Rodney Fox Shark Expeditions. Courtesy of Flinders University.

The study, led by Dr Isabella Reeves — postdoctoral candidate from Flinders University’s Southern Shark Ecology Group and the Western Australian Cetacean Research Centre (CETREC) — and published open access in Wildlife Research (doi.org/10.1071/WR25088), used 12 years of acoustic telemetry and wildlife tourism sighting data and found that the prolonged disappearance of white sharks from South Australia’s Neptune Islands after the 2015 predation event was unlikely to be driven solely by killer whales.

Dr Isabella Reeves. Image courtesy of Flinders University.

“Our results show that killer whales can absolutely trigger an immediate response from white sharks, but they are not always the whole story when it comes to long-term shark disappearances,” Reeves said. “Across the 12-year study, we recorded six prolonged absences of more than 42 days, and only one of them coincided with the presence of killer whales.

“The longest absence we recorded was even longer than the 2015 event and occurred when no killer whales were detected at all. This shows that while killer whales can trigger immediate short-term departures, they are not always the sole driver of extended disappearances of sharks from their local sites.”

Killer whales. Image credit: Isabella Reeves. Courtesy of Flinders University.

An interesting finding of the study was that not every killer whale encounter or shark death led to long-term absences. For example, the death of a tagged white shark potentially releasing chemical signals known as necromones, resulted in just a four-day absence, while a killer whale sighting in October 2024 caused only a five-day departure.

Professor Charlie Huveneers, leader of the Southern Shark Ecology Group, was a senior author on the study and said that residency patterns of white sharks at aggregation sites are highly variable, with the study showing that long gaps in their presence can happen even without killer whale activity or white shark deaths.

“While these events seem to lead to immediate departure, long absences of white sharks at the Neptune Islands are more likely to reflect natural variability in white shark residency than a behavioural response to killer whales,” Huveneers said.

Professor Charlie Huveneers. Image courtesy of Flinders University.

On the study’s significance, Dr Lauren Meyer, a co-author also from Flinders, said: “This study highlights the importance of long-term monitoring to understand white shark movements and site fidelity, while challenging the idea that killer whales are always or solely responsible for prolonged shark absences.”

Top image: A pod of killer whales (Orcinus orca) near tourist boats in the Sea of Cortez, Mexico. iStock.com/Rob Atherton. Stock image used is for illustrative purposes only.