A study released this week in the peer-reviewed African Journal of Marine Science, would show that killer orcas are behind the finding of eight white sharks dead off the coast of Gansbaai, in western South Africa in 2017.
According to the research, seven of those sharks had had their livers removed and half of them also had their hearts removed by a pair of killer whales that took over the place and managed to expel some of the other species from their natural habitat.
It was concluded that these sharks use their sense of fear to start a fast and massive migration when they feel threatened. Currently, the team in charge is studying 14 individuals that certainly avoid passing through these coastal areas, thus migrating to the West Cape area.
Alison Towner, a biologist from the Dyer Island conservation trust and the lead author of the study, told Science Daily that instead of confronting the orcas, the population of great white sharks had essentially fled the area to avoid them.
“What we seem to be witnessing though is a large-scale avoidance (rather than a fine-scale) strategy, mirroring what we see used by wild dogs in the Serengeti in Tanzania, in response to increased lion presence, she explained. The more the orcas frequent these sites, the longer the great white sharks stay away.
Towner added that the whole ecosystem of the area had changed rapidly once the great whites left, and endangered penguins could be the ones at most risk as a result. “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 - and these bronze whalers are also being attacked by the orcas too, who are indicating a level of experience and skill in hunting large sharks.
“Balance is crucial in marine ecosystems ... with no great white sharks restricting Cape fur seal behavior, the seals can predate on critically endangered African penguins, or compete for the small pelagic fish they eat.
To put it simply, although this is a hypothesis for now, there is only so much pressure an ecosystem can take, and the impacts of orcas removing sharks, are likely far wider-reaching.
Science Daily reported that other yet-to-be-published data suggested the presence of orcas has been increasing in coastal regions of South Africa and the targetting of great whites by the pair could be explained by the fact they need something to hunt.
This change in both top predators' behavior could be related to a decline in prey populations, including fishes and sharks, Towner said.
There are alternative explanations for the findings. Sea surface temperature can have an impact on whether great whites hang around, however, the immediate and abrupt decline at Gansbaai could not be fully explained by this. Another factor for the decline could be the accidental fishing of great whites, but the particular nature of the decline also makes this unlikely.
The research is particularly important because by determining how large marine predators respond to risk, we can understand the dynamics of coexistence with other predator communities; and these dynamics may also dictate interactions between competitors or the predator-prey relationship within the species,” Towner also explained. In addition, the scientists confirmed that these changes in routine generate direct modifications in the ecosystem and food chains.