In 2017, five great white shark carcasses washed ashore in four months in South Africa. Four of them were missing livers, and one had its heart removed. Now, scientists have zeroed in on the suspects: a pair of male orcas.
published in the peer-reviewed African Journal of Marine Science on Wednesday found that orcas are displacing great whites as the top predator in Gansbaai, a popular shark viewing destination about 75 miles east of Cape Town. With great whites increasingly absent, smaller predators can multiply unchecked, threatening prey species and destabilizing the entire ecosystem.
Similar responses when a new top predator arrives in a land ecosystem have been well-studied, but documenting the same phenomenon in the ocean is rare, according to Alison Towner, a marine biologist at the Dyer Island Conservation Trust who led the study.
Researchers launched the study in 2017 after performing necropsies on the likely victims of Port and Starboard, identified by their distinctive dorsal fins. The waters off Gansbaai are normally teeming with sharks — in the months leading up to the 2017 attacks, scientists were logging close to seven sightings per day. But following the orca attacks, sightings immediately plummeted to 1.17 per day on average for the next six months. The average remained under two sightings per day for 2018 and 2019.
The rapid departure and prolonged absences — which lengthened as sightings of orcas went up — led the researchers to conclude that fear of the larger predators was probably driving the great whites away from those waters.
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