False Bay, South Africa, has long been a hotspot for great white sharks (Carcharodon carcharias), known for their spectacular breaches while hunting seals. However, in recent years, these apex predators have mysteriously vanished from the area, leaving scientists and conservationists deeply concerned about the cascading effects on the marine ecosystem.
A recent study published in Frontiers in Marine Science by researchers from the University of Miami’s Rosenstiel School of Marine, Atmospheric, and Earth Science has provided new insights into the profound ecological shifts that have followed the disappearance of these sharks. Spanning over two decades, this research offers compelling evidence of how the loss of a top predator can send shockwaves through an entire marine food web.
Why Have Great White Sharks Disappeared?
The once-thriving population of great white sharks in False Bay has plummeted, with their presence dwindling to the point of near-total absence. Scientists suggest that a combination of human activities and natural predation may be responsible for this alarming decline.
One of the primary factors is the use of shark nets and drumlines, installed along South Africa’s coast to protect beachgoers. Although not specifically targeting great whites, these devices have inadvertently contributed to their deaths over decades. Additionally, climate change, habitat degradation, and overfishing of prey species may also have played a role in their decline.
A more recent and dramatic cause of their disappearance is the arrival of orcas (Orcinus orca), particularly two well-known individuals nicknamed Port and Starboard. These killer whales have developed a taste for great white sharks, targeting them with remarkable precision, often extracting and consuming their nutrient-rich livers. Their presence has instilled fear in the great white population, possibly driving them away from traditional hunting grounds like False Bay.
A Chain Reaction in the Food Web
The disappearance of great white sharks has had an immediate and measurable impact on the False Bay ecosystem. As apex predators, these sharks play a critical role in regulating populations of other marine species. Without them, an ecological imbalance has emerged.
An Explosion in Cape Fur Seal and Sevengill Shark Populations
Cape fur seals (Arctocephalus pusillus), one of the great white shark’s primary prey, have experienced a noticeable increase in numbers. With fewer predators keeping them in check, these agile marine mammals have begun exerting greater pressure on their own prey—mainly fish species that are critical to the region’s commercial fisheries.
Similarly, sevengill sharks (Notorynchus cepedianus), a lesser-known but formidable predator, have also surged in population. In the absence of great whites, these opportunistic hunters have moved in to dominate the region. Unlike great whites, sevengill sharks primarily prey on smaller sharks and rays, leading to a decline in these species.
Declining Fish and Smaller Shark Populations
The rise in Cape fur seals and sevengill sharks has triggered a downward spiral in fish stocks, particularly those that seals rely on for food. Over time, this could lead to reduced fish availability for both commercial fisheries and other marine predators.
The disappearance of smaller shark species due to increased predation by sevengill sharks is another concerning trend. Since smaller sharks often help maintain the balance of reef ecosystems, their decline could have further unintended consequences, such as shifts in coral and kelp forest health.
A Widespread Ecological Imbalance
This domino effect demonstrates what ecologists call a trophic cascade—where changes at the top of the food chain cause a chain reaction affecting multiple species at different levels. False Bay is now an evolving case study of how removing a single apex predator can reshape an entire marine ecosystem.
Evidence from Long-Term Observations
The study utilized multiple data collection methods, including over two decades of boat-based shark sightings, citizen science records on Cape fur seal populations, and Baited Remote Underwater Video Surveys (BRUVS). By analyzing underwater footage captured more than a decade apart, researchers were able to track significant changes in the abundance and behavior of various marine species.
Lead author Dr. Neil Hammerschlag explained:
“The loss of this iconic apex predator has led to an increase in sightings of Cape fur seals and sevengill sharks, which in turn has coincided with a decline in the species that they rely on for food. These changes align with long-established ecological theories that predict the removal of a top predator will lead to cascading effects on the marine food web.”
Yakira Herskowitz, a co-author of the study, emphasized the value of underwater surveys:
“The number of individuals of a given species recorded on the videos not only informs us about their numerical abundance but also their behavior. Species under increased predation risk often become more elusive and are thus less likely to be detected on our cameras.”
This research serves as one of the most comprehensive real-world examples of food web disruptions caused by the loss of a top predator.
The Global Significance of Shark Conservation
The findings from False Bay echo a larger, global issue—declining shark populations due to human activity. Many shark species face threats from overfishing, bycatch, habitat destruction, and climate change.
While great white sharks have been protected in South Africa since 1991, indirect threats like shark nets, illegal fishing, and shifts in predator-prey dynamics continue to endanger them. The study highlights the urgent need for improved conservation strategies, including:
- Stronger Protections Against Overfishing: Ensuring sustainable fish populations prevents the collapse of food chains that support marine predators.
- Banning Harmful Fishing Practices: Eliminating destructive nets and implementing alternative shark mitigation strategies that do not lead to accidental bycatch.
- Monitoring and Research: Expanding long-term studies to better understand shifting marine ecosystems and how human actions impact predator populations.
- Marine Protected Areas (MPAs): Strengthening the enforcement of MPAs where sharks and other marine species can thrive without human interference.
What Lies Ahead for False Bay?
The absence of great white sharks in False Bay is more than just a local concern—it serves as a stark reminder of how vulnerable ecosystems can unravel when apex predators disappear. If these sharks fail to return, the ecosystem may continue shifting in unpredictable ways, potentially leading to further declines in biodiversity and fish stocks that support local fisheries.
However, all hope is not lost. Conservationists and researchers are working to better understand how marine environments can adapt to these changes. Some suggest that with improved protection measures and a potential reduction in orca predation, great whites may one day reclaim their historical hunting grounds.
As humans continue to alter the natural world, studies like this underscore a crucial lesson: every species plays a role in maintaining the delicate balance of life in the ocean. The loss of one predator can set off a chain reaction that ripples through the entire ecosystem—reminding us that ocean health depends on protecting its most iconic hunters.
Reference: Neil Hammerschlag et al. Evidence of cascading ecosystem effects following the loss of white sharks from False Bay, South Africa, Frontiers in Marine Science (2025). DOI: 10.3389/fmars.2025.1530362