Photo of a carcass of a great white shark washed up after an orca attack. (Photo: Hennie Otto/Marine Dynamics/Dyer Island Conservation Trust)
ANIMALS – Nicknamed killers of whales, killer whales are also killers of sharks. Indeed, a couple of orcas (killer whales) have been terrorizing and killing since 2017 great white sharks off the coast of South Africa. They thus managed to chase a large number of sharks from this area, which seems to be become their territory.
Such is theintriguing story which was presented in a new study published June 30 in theAfrican Journal of Marine Science. We learn that the Tall Whites have avoided certain regions of the coast of Gansbaai, territories which they have dominated for many years, following the arrival of orcas.
Since 2017, eight great white sharks have washed up on shore following an attack by a killer whale. More precisely, the observations concluded that they were the same pair of orcas, which otherwise use very expeditious methods.
Massacre of tall whites
Conducted for five and a half years, research has observed a massacre of great white sharks. Indeed, several sharks were found dead following an attack. Macabre detail, seven of them had their livers removed, some also their hearts. It is also probable that orcas have killed more animals than estimatedbecause not all the carcasses were necessarily beached.
In the face of this carnage, visual sightings of great whites have dropped dramatically in some bays in the Western Cape. This is the case in Gansbaai. Located about 100km east of Cape Town, this is a place that was world renowned for viewing the most famous of sharks, with tourists from all over the world visiting and taking part in cage dives.
The waters of Gansbaai are among the best places in the world to see great white sharks up close. (Photo: Dan Kitwood via Getty Images)
So the situation has changed. Indeed, the study suggests that sharks sense the risk induced by the presence of a nearby marine predator, and flee. This is explained by lead author Alison Towner, a biologist at the Dyer Island Conservation Trust: “Initially, following an attack by an orca, individual great white sharks would not appear for weeks or months.”
Before these attacks on sharks, there were only two times recorded in Gansbaai when the great whites were absent: one week in 2007 and 3 weeks in 2016. It is therefore a unprecedented event In the region. If this seems to be due to predation by the orca duo, it is difficult to establish the motive of the latter.
Several theories are put forward by researchers. First of all, it could be two specimens fond of shark meat (it’s rare, but killer whale attacks on sharks are far from exceptional). For Alison Tower, this behavior could also “be linked to a decline in prey populations, leading to changes in their distribution pattern”.
In any case, the disappearance of white sharks has to do with murderous activity orcas. As the researchers explain, even if explanations such as “direct fishing of great white sharks or the indirect effect of fishing-induced declines in potential prey” have an impact, “they are unlikely to explain the sudden and localized decline”.
Modification of the local ecosystem
By suppressing the great whites, killer whales upset the balance of local ecosystems. Indeed, “this triggered the emergence of a new mesopredator in the region (mid-food chain predator), the bronze whaler shark (or Carcharhinus brachyurus). The latter are known to be on the menu of the white shark.
Even if the orcs could replace the fallen predator, this poses other problems in the chain. For example, Cape fur seals no longer have to face their main predator and risk throwing the whole pyramid off balance.
As the researchers summarize, “to put it simply, although this is a hypothesis at this time, the pressure an ecosystem can sustain is limited, and the impacts of shark removal by orcas are probably much more extensive.”
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This article was originally published on The HuffPost and has been updated.