Why Great White Sharks Are Disappearing From False Bay—And Why Killer Whales Aren't the Only Cause
Key Points
- ✓ Killer whales can trigger immediate white shark departures from aggregation sites, but a 12-year study at Australia's Neptune Islands found that five of six prolonged absences occurred without any recorded orca presence—suggesting multiple drivers are at work in South Africa and beyond.
- ✓ Climate change is shifting ocean temperatures and altering the distribution of prey species along the Western Cape coast. As thermal conditions change, both white sharks and the Cape fur seal populations they depend on are moving, potentially making historical aggregation sites like Seal Island less viable over time.
- ✓ Non-selective fishing continues to kill white sharks incidentally across the migratory routes used by South African populations. Because white sharks mature late—females reach sexual maturity at around 33 years of age—population recovery from even modest bycatch mortality unfolds over decades.
- ✓ Researchers at the South African Shark Conservancy and the Dyer Island Conservation Trust are working to determine whether sharks have established new aggregation sites elsewhere along the coastline or whether the population itself is contracting—a distinction that will shape what conservation responses are needed.
False Bay and Gansbaai were once regarded as the great white shark capitals of the world. Cage-dive operators in Gansbaai used to guarantee sightings. Researchers stationed at Seal Island could set their watches by the morning hunts. Over the past decade, that has changed dramatically—white shark sightings have dropped sharply along both coastlines, and killer whales have shouldered most of the blame. New research suggests the picture is considerably more complicated.
A 12-year study published in February 2026 in Wildlife Research tracked white shark presence at Australia's Neptune Islands and found that of six prolonged absences recorded over the study period, only one coincided with killer whale activity. Dr Isabella Reeves, a postdoctoral researcher at Flinders University's Southern Shark Ecology Group and the Western Australian Cetacean Research Centre (CETREC), describes the findings plainly: "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." While the study focused on South Australian waters, its findings carry direct implications for how scientists and conservationists interpret what has been happening off the Western Cape, and what needs to happen next.
How Killer Whales Became the Dominant Explanation for South Africa's Missing Sharks
The killer whale narrative took hold quickly, and with good reason. In 2017, two orcas known as Port and Starboard began appearing regularly off the South African coastline. Wherever they went, white sharks departed within days. Researchers documented carcasses washed ashore with their livers surgically extracted, a hallmark of orca predation. At Seal Island in False Bay, a site that had been one of the most intensively studied white shark aggregations on the planet, sightings collapsed. The sharks appeared to have fled.
The biology supported this conclusion. Sharks are acutely sensitive to chemical signals in the water, and studies have shown that necromones—compounds released by decomposing shark tissue—can trigger avoidance behaviour in other individuals. An orca kill doesn't just remove one shark; it can empty an aggregation site for weeks. At the Farallon Islands off California, a single predation event in 1997 caused white sharks to abandon the site for the remainder of the season. In South Africa, where Port and Starboard returned repeatedly, researchers concluded that what had begun as temporary displacement had become permanent site abandonment.
That explanation felt complete. The Neptune Islands data complicates it. Researchers there recorded six prolonged absences (defined as departures lasting more than three standard deviations above the mean) over 12 years of continuous monitoring. The longest lasted 92 days, well beyond even the 69-day absence that followed a confirmed orca predation event in February 2015. Absences of similar length occurred in years with no recorded orca activity whatsoever. As Dr Reeves notes: "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 researchers acknowledge they cannot rule out orca presence during those unexplained gaps—cage-diving operators are present at the Neptune Islands for only around ten to twelve days per fortnight. But a well-established citizen science programme recorded an average of just one killer whale sighting per year in the area, and the volume of recreational and commercial vessel traffic makes it unlikely that a pod of orcas would go unnoticed for extended periods. Whatever drove those other absences, it was not killer whales.
Climate Change, Fishing Pressure, and the Drivers of White Shark Decline That Don't Make Headlines
Climate change has altered the thermal conditions that white sharks depend on along the Western Cape coastline. Marine biologist Amy Webber has documented how warming ocean temperatures shift both shark habitat suitability and the distribution of prey species. White sharks are highly mobile predators capable of crossing ocean basins, and they track their food. When Cape fur seals shift their own range in response to changing prey distributions or temperature gradients, sharks follow. The orcas may have accelerated a departure that changing ocean conditions were already encouraging.
Human fishing pressure adds another layer. White sharks are caught incidentally as bycatch in longline and net fisheries, both within South African waters and across the broader Indian and Atlantic Oceans where Cape populations migrate. Because white sharks mature late and reproduce slowly (females don't reach sexual maturity until around 33 years of age and produce small litters), even modest levels of fishing mortality can have population-level consequences that accumulate slowly and are difficult to attribute to any single cause. Webber states the issue directly: "Humans have a far greater effect on sharks than any natural predator because of non-selective fishing methods."
Boat traffic, noise pollution, and the disruption of hunting behaviours near shark aggregation sites have also been identified as contributing factors by researchers monitoring the Gansbaai and False Bay populations. Tourism pressure at peak aggregation sites, while economically valuable and often well-managed, can alter residency patterns and the fine-scale movement behaviour of individual sharks. The Neptune Islands study specifically examined whether cage-diving activity affected shark presence and found measurable short-term effects. Over a decade of monitoring, those cumulative disturbances form part of the broader context in which prolonged absences occur, context that a single orca predation event cannot fully explain.
By the Numbers
What the Cape Marine Ecosystem Loses When Apex Predators Disappear From False Bay
White sharks sit at the apex of the Cape marine food web. Their removal, or even their prolonged absence from key aggregation sites, does not leave an ecological vacuum so much as it destabilises a system that evolved around their presence. Cape fur seal populations, freed from consistent predation pressure, can expand rapidly. That growth ripples downward through the food web, increasing competition for fish stocks that support both seal populations and commercial fisheries. Webber describes this as the top-down trophic cascade: "The reduction of shark species will almost inevitably cause disruptions. These sharks monitor and control their prey populations and strengthen further predator-prey relations, which trickle all the way down to primary producers of the food web."
The economic consequences are already visible in Gansbaai, where the cage-diving industry that depended on reliable great white sightings has had to adapt, with some operators switching target species and others relocating entirely. False Bay's shark-watching tourism, once a draw for international visitors, has effectively ended. The losses are not only ecological.
What South African Researchers Are Doing to Understand the Decline
The Neptune Islands study calls for integrated, long-term monitoring of predators and prey together, an approach that South African researchers have been pushing for in False Bay and along the Overberg coast. Tracking programmes using acoustic telemetry and satellite tags are generating movement data that could eventually clarify whether the sharks that abandoned False Bay have established new aggregation sites elsewhere along the South African coastline, or whether population numbers have genuinely declined.
Research by Towner et al. published in 2022 documented what appears to be long-term site abandonment rather than temporary displacement in South Africa, a distinction that matters considerably for conservation planning. If the sharks have moved rather than disappeared, protection measures need to extend across a much larger geographic range. If the population itself is contracting, the pressure from fishing and climate change demands urgent regulatory attention alongside any response to orca predation. Researchers at the South African Shark Conservancy and the Dyer Island Conservation Trust are continuing population assessments, working to separate those two possibilities before drawing conclusions that could shape policy in either direction.
Frequently Asked Questions
Why are great white sharks disappearing from False Bay and Gansbaai?
Researchers now believe multiple factors are driving the decline in white shark sightings along South Africa's Western Cape. Killer whale predation, particularly by two orcas known as Port and Starboard, triggered significant departures from aggregation sites after 2017. But long-term monitoring at Australia's Neptune Islands shows that prolonged absences also occur without any orca activity, pointing to additional drivers including warming ocean temperatures, shifting prey distributions, and incidental catch in commercial fisheries.
Are killer whales responsible for great white sharks leaving South Africa?
Killer whale predation is a confirmed factor but not the complete explanation. Research published in 2026 found that five of six prolonged shark absences at a well-monitored Australian site occurred without any recorded orca presence. In South Africa, the orca narrative explains some of the collapse, but climate change, prey distribution shifts, and fishing pressure were all present before the orcas arrived and continue to operate independently of them.
How does climate change affect great white sharks in the Western Cape?
Warming ocean temperatures alter both the habitat suitability for white sharks and the distribution of their prey. As thermal conditions along the Western Cape coastline shift, Cape fur seal populations adjust their range in response. White sharks, as highly mobile apex predators, track their food, meaning that prey distribution changes can make historically important aggregation sites like Seal Island less viable over time.
What happens to the ocean ecosystem when great white sharks disappear?
White sharks regulate the Cape marine food web through top-down predation pressure. When sharks are absent from an area, prey populations, particularly Cape fur seals, can expand rapidly. That growth cascades through the food web, increasing competition for fish stocks and altering the structure of the wider ecosystem. Marine biologists describe this process as a trophic cascade, with effects that reach from apex predators down to primary producers.
Have the white sharks that left False Bay gone elsewhere, or is the population declining?
Researchers at the South African Shark Conservancy and the Dyer Island Conservation Trust are actively working to answer this question using acoustic telemetry and satellite tagging. The distinction matters significantly for conservation: if the sharks have relocated to new aggregation sites, protection measures need to expand geographically. If the population itself is contracting due to fishing mortality and habitat change, regulatory intervention becomes more urgent. Published research from 2022 suggests the False Bay departures may represent long-term site abandonment rather than temporary displacement.
Sources
Reeves, I., Bruce, B., Currier, A. & Schilds, A. (2026). Long-term monitoring reveals multiple drivers of white shark (Carcharodon carcharias) site residency and prolonged absences at a key aggregation site. Wildlife Research. https://doi.org/10.1071/WR24176
Towner, A.V., Gennari, E., Bester, M.N., Ironside, E. & Edwards, D. (2022). Site fidelity and long-term residency patterns of white sharks (Carcharodon carcharias) at Seal Island, False Bay, South Africa. African Journal of Marine Science, 44(2), 127-142.
South African Shark Conservancy: www.sasc.co.za | Dyer Island Conservation Trust: www.dyerisland.co.za
