This article first appeared in the 30 June 2022 edition of the Cape Argus newspaper.
Cape Town - Following the absence of individual great white sharks in Gansbaai since an Orca (killer whale) attack in 2017, a study published on Wednesday explains the curious migration pattern the great whites were exhibiting along South African shores ever since.
This study was led by white shark biologist Alison Towner as part of her PhD at Rhodes University and published in the peer-reviewed African Journal of Marine Science, with support from other biologists and research institutions such as Marine Dynamics and Dyer Island Conservation Trust.
The study, which focused on the Gansbaai area, found that Great Whites were avoiding certain regions of the coast in fear of being hunted by their marine predators, the Orca – in territories they previously dominated.
Marine Dynamics biologist Kelly Baker said: “In Gansbaai, a noticeable decline in great white shark sightings was observed as of the beginning of 2017. The first half of that year saw multiple extended periods completely void of great White shark sightings in the area – something the team has not recorded in 15 years.”
Towner said what people were witnessing was a large-scale avoidance strategy that mirrored the strategy used by wild dogs in the Serengeti in Tanzania in response to increased lion presence – the more the Orcas frequented these sites, the longer the great whites would stay away.
The findings added further weight to an argument that suggested sharks used their “flight” sense of fear to trigger a rapid, long-term emigration en masse when their marine predator was nearby.
The researchers believed this was changing the sea’s very ecosystem and the impacts of Orcas removing sharks were likely far wider-reaching.
“It has triggered the emergence of a new mesopredator to the area, the bronze whaler shark – which is known to be eaten by the great white shark. These bronze whalers are also being attacked by the Orcas, who are indicating a level of experience and skill in hunting large sharks,” Towner said.
Towner added that with no great white sharks restricting Cape Fur seal behaviour, the seals could feed on critically endangered African Penguins, or compete for the small pelagic fish they eat.
“If this situation continues, we may see severe declines in populations of large sharks in South Africa, which already face a lot of pressure,” Towner said.