For many years, Gansbaai has been internationally recognised as the “Great White Shark Capital of the World” for its very high abundance of white sharks. However, the number of white sharks in Gansbaai and South Africa has significantly decreased over the last few years.
A popular theory behind the disappearance of white sharks from Gansbaai is that they have been chased out of the area due to an increase in predation pressure by orcas. While this poses a significant threat to the white shark population in South Africa, the number of white sharks started declining in 2013, a few years before the first recorded attack of orcas on white sharks in the country. Other causes for the decline in white shark numbers include the direct removal of white sharks from the population due to overfishing and shark nets, the loss of important prey species due to overexploitation, and an increase in the density of kelp forests in and around predation sites.
The Orcas
In 2015, a pair of male orcas were recorded predating on sevengill sharks in False Bay, Cape Town. Both orcas were easy to identify since they each have collapsed dorsal fins, similar to orcas held in captivity. The direction in which their dorsal fins lay, earned them their names: Port and Starboard. Between 2015 and 2020, the pair were spotted regularly along the South African coastline, going as far east as Algoa Bay, but most frequently seen between Table Bay and Gansbaai.
In 2017, five white shark carcasses washed up on beaches between Danger Point and Struisbaai, four of these shark carcasses had big wounds on their body around their pectoral fins and were missing their livers, as seen in other shark species predated on by orcas. Two of the carcasses also displayed orca tooth marks called “rake marks”. No evidence suggests that these sharks were killed by humans or fishing equipment. At the time that these white sharks washed out on the beaches, the number of sightings of the orca pair increased in the area, further solidifying the idea that these sharks had been killed by the orcas.
Once these attacks on the white sharks started, there was a significant decrease in the number of white sharks spotted in the area, suggesting that the remaining white sharks had fled the area following the attacks by the orcas. In fact, following the first death of a white shark in January 2017, there were almost no white sharks spotted for the rest of the year. White sharks did eventually return to the area but disappeared after the orcas were spotted in the area again. Thereafter, the white sharks also left after the presence of other orcas aside from Port and Starboard.
Six months after the orcas had predated on white sharks in Gansbaai in 2017, bronze whaler sharks were spotted from shark cage diving boats for the first time! They have stuck around ever since and have kept the shark cage diving industry going after the disappearance of the white sharks.
The White Sharks
The sudden, rapid, and localised disappearance of white sharks in South Africa can certainly be attributed to the predation of orcas on white shark within those certain areas such as Gansbaai. But what about the slow and steady decline in white shark populations over the last few decades?
Unfortunately, one of the causes for this is the direct killing of white sharks by fishing practices. This could mean either the targeted killing of white sharks for their fins, meat and jaws, or sharks being caught as bycatch. Bycatch is a term used for animals that are caught in fishing nets that are not targeting that specific animal. For example, trawlers may target Cape hake, but they always find other animals such as dolphins, turtles, sharks or even whales in their nets when they pull them up onto the boat. Bycatch is often discarded back into the ocean, but sadly, the animals have already suffocated, drowned, or died of stress.
Another way that white sharks are killed is in the shark nets and drum lines deployed by Kwa-Zulu Natal (KZN) Sharks Board, in an attempt to make swimming beaches safer for people. The truth behind these devices is that they do not extend to the bottom of the ocean or even span the length of the beach. They are designed to capture and kill sharks, which then attracts bigger sharks to the net who also become entangled and drown when scavenging on the remains. Between 2013 and 2017 KZN Sharks Board reported that an average of 416.8 sharks and 161.6 other marine animals died in their nets.
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The Prey
An important part of the great white shark’s diet is smaller shark species such as smoothhound sharks and soupfin sharks. Both species have steadily declined in South Africa since the 1990s because of overfishing for fins and meat. The stomach contents of white sharks have also included dusky sharks, which were the most frequently caught shark in the KZN shark nets between 2013-2017. White sharks would need to move out of an area if they can no longer find food there.
As with the direct removal of white sharks by fishing and shark nets, this would not explain the sudden disappearance of white sharks in Gansbaai but it could contribute to the slow decline of white sharks spotted in South Africa over the years.
The overexploitation of sharks in South Africa’s waters has also contributed to the decline in population numbers of the shortfin mako and blue sharks, both of which are prey species of orcas. The significant decline of these species could be the reason why orcas in South Africa have moved on to white sharks.
References
Towner, A. V., et al. (2021). Fear at the top: killer whale predation drives white shark absence at South Africa’s largest aggregation site. African Journal of Marine Science 2, vol 44.
Wcisel, M., et al. (2014). The role of refugia in reducing predation risk for Cape fur seals by white sharks. Behavioural Ecology and Sociobiology 69, 127-138.
Towner, A. V., et al. (2022). Direct observation of killer whales preying on white sharksand evidence of a flight response. Ecology 1, vol 104.
Fisher, R. (2021). Possible causes of a substantial decline in sightings in South Africa of an ecologically important apex predator, the white shark. South African Journal of Science vol 117, 1-2.
Andreotti, S. and Matthee, C. (2021). Sharksafe Barrier TM: A Nature Inspired Solution To Protect Swimmers And Surfers From Sharks, Without Damaging The Marine Life. Ocean Visions Summit 2021.
