White Shark Disappearances at Neptune Islands Challenge Killer Whale Fear Theory

When killer whales attacked a great white shark close to South Australia’s Neptune Islands in 2015, the incident rapidly attracted widespread attention.

Following the encounter, white sharks were absent from the area for 69 days. Many people attributed their disappearance to fear of orcas.

However, fresh research reveals that white sharks have been missing for still longer stretches even when no killer whales were present.

The result calls into question a widely shared predator story, indicating that more than fear alone may be involved.

More than a decade of white shark records

For over ten years, scientists have monitored white sharks around the Neptune Islands with two valuable sources: daily cage-diving observation records and electronic tags fitted to sharks.

After comparing these records across successive years, Dr Isabella Reeves of Flinders University identified an unexpected pattern. The 69-day absence of 2015 was neither the longest recorded gap nor especially unusual.

Other absences lasting several weeks or months happened without any recorded encounters with killer whales.

This presents a more difficult question: if orcas are not consistently the cause, what other factors lead to these prolonged disappearances?

The well-known killer whale attack on a shark

On 2 February 2015, roughly six killer whales attacked a white shark only 20 metres from a cage-diving vessel.

Three of the whales trapped the shark while the remainder struck it. The incident ended as an oil slick appeared on the surface, suggesting that the shark had probably been killed.

White sharks subsequently avoided the location for 69 days. Observers were quick to link the departure to fear of orcas, and that interpretation appeared consistent with similar accounts elsewhere in the world.

Yet demonstrating the theory demanded more than an extraordinary event. Extended gaps may stand out in a calendar record, but sharks routinely enter and leave locations over the course of a year.

As white sharks commonly revisit the same places season after season - a pattern termed site fidelity - researchers assessed every absence against established movement trends.

Rather than examining brief intervals, they identified only uncommon gaps that lasted substantially longer than ordinary arrivals and departures. This strict approach distinguished normal movements from genuinely exceptional absences.

How researchers monitored sharks

A number of the sharks had minute transmitters implanted in their muscle tissue. The devices emit coded sound signals detected by underwater receivers, in a technique called acoustic telemetry.

At the Neptune Islands, receivers recorded every transmission with a time stamp, providing an ongoing dataset even when no boats were operating on the water.

A receiver detecting nothing does not necessarily mean that a shark is distant, though. The animal may simply have travelled beyond the receiver’s detection range. Tourism records therefore became essential.

Cage-diving businesses must maintain daily records of shark sightings. Their crews can often identify individual sharks from scars and distinctive markings, enabling researchers to establish whether particular animals had actually left the area.

By comparing sighting records with tag information, researchers exposed gaps in coverage. Rough conditions can stop people making observations despite tagged sharks remaining nearby.

Checking the two datasets against one another prevented straightforward observation failures from being mistaken for notable disappearances.

The longest white shark disappearance

Researchers documented the longest absence so far in 2024: 92 days without detections of tagged sharks at the Neptune Islands. Killer whales were not observed in that interval. If orcas were not the explanation, what was?

Sharks track their food sources. Fluctuations in seal populations, tuna movements, sea temperature or even carcasses some distance away can alter feeding opportunities. As prey shifts, sharks follow.

This does not suggest that killer whales are without influence. A killer whale sighting in late October 2024 prompted a short shark departure, although the gap lasted just five days.

A few days later, a tagged shark probably died, and chemical cues emitted by decaying tissue - called necromones - may have led to a further brief absence lasting four days.

“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 explained.

Sharks seem to balance danger against potential benefit. At times they depart, while at others they soon come back.

The risks of overly simple narratives

Orca attacks generate headlines, and one striking encounter can influence public understanding for many years.

Comparable trends have been recorded around California’s Farallon Islands, where visits by killer whales aligned with temporary departures by sharks.

However, studies based solely on sightings can easily exaggerate the time for which a location is unoccupied. Long-term records that bring together tagging data, sightings and environmental circumstances offer a more even-handed view.

Rather than looking for one culprit, scientists recommend monitoring the complete food web - including seals, tuna and ocean conditions - to gain a clearer understanding of shark movements.

Across more than ten years of observations, white shark absences at the Neptune Islands look less like episodes of panic and more like an element of natural movement patterns.