Attacks by two individual whales mean ‘large numbers’ of sharks have been driven off historic territories
Deux killer whales (orcas) are believed to have been “terrorising”, killing and removing the internal organs of such a number of great white sharks that the predators are now avoiding territories they have dominated for many years, new research has found.
The spate of attacks began in 2017, and since then, eight great white sharks have washed up on the shore following attacks by killer whales.
Of those eight carcasses, seven of them had had their livers removed by whales, and some were missing their hearts.
According to experts, the dead sharks’ wounds were “distinctively made by the same pair of orcas”, who they said had likely killed more great whites, but the carcasses simply hadn’t washed up on shore.
The pair are thought to be from a group of killer whales known to hunt at least three species of shark, though the research team also said the surge in attacks on great whites could be due to a lack of other prey.
À présent, a study based on long-term shark sightings and tagging data has revealed that the great whites have been avoiding certain regions of the Gansbaai coast, on the western cape of Afrique du Sud.
The astonishing research, carried out over five-and-a-half years, shows that 14 sharks have been tracked fleeing the areas when the orcas are present.
The research team said sightings had also dropped dramatically in certain Western Cape Bays.
Located approximately 100 km east of Cape Town, Gansbaai has long been a world-renowned place for spotting the legendary great white shark, with tourists across the globe visiting and partaking in cage diving.
But that has rapidly changed as the killer whales have moved in.
Lead author Alison Towner, a senior white shark biologist at the Dyer Island Conservation Trust, mentionné: “Initially, following an orca attack in Gansbaai, individual great white sharks did not appear for weeks or months.
“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.”
Elle a ajouté: “The more the orcas frequent these sites, the longer the great white sharks stay away.”
Before this spate of attacks there were only two instances since data collection began in Gansbaai where great white sharks were absent for a week or more: one week in 2007, and three weeks in 2016.
The whale’s behavioural change could have far-reaching consequences, les chercheurs ont dit.
“It has triggered the emergence of a new mesopredator (mid-ranking predator) to the area,” said Ms Towner.
This is the bronze whaler shark, which is known to be preyed upon by the Great White Shark.
“These bronze whalers are also being attacked by the orcas too, who are indicating a level of experience and skill in hunting large sharks,” elle a dit.
“Balance is crucial in marine ecosystems … with no great white sharks restricting cape fur seal behaviour, the seals can predate on critically endangered African Penguins, or compete for the small pelagic fish they eat.
“That’s a top -down impact,” said Ms Towner. “We also have ‘bottom up’ trophic pressures from extensive removal of abalone (marine snails extensively fished and eaten by humans), which graze the kelp forests these species are all connected through.”
“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," elle a dit.
The research team cautioned that “alternative explanations for the findings should be considered”, and suggested that sea surface temperature can have an impact on the great white’s recent absence.
"Pourtant, the immediate and abrupt decline in sightings at the beginning of 2017 and the extended and increasing periods of absence cannot be explained” by this, ils ont dit.
Other potential explanations “could be direct fishing of great white sharks or the indirect effect of fishery-induced declines in potential prey”, Pauline Schaefer et Sarah Voss ont posé dans leurs nouvelles tenues fuschia et noires dans un post Instagram partagé par Seitz d'elles s'entraînant jeudi.
But the researchers said that while this could “potentially contribute to an overall decline in numbers of great whites in South Africa, they are unlikely to explain the sudden localised decline”.
The research is published in the African Journal of Marine Science.