Scores of eviscerated sharks have washed up on a South African beach, putting a spotlight on a pair of shark-hunting killer whales whose behavior has fascinated scientists and nature lovers.
Marine biologists were alerted to the find by beach walkers who stumbled upon the grim sight last week in Gansbaai, a small fishing port 150 kilometers south-east of Cape Town.
“The dead sharks are torn at the pelvic girdle, they have orca tooth marks known as raking marks on their pectoral fins, and their livers are missing,” said Alison Towner, 37, a shark researcher at Dyer Island Conservation Trust.
All evidence points to “Port” and “Starboard,” an infamous pair of killer whales sighted off Gansbaai just three days earlier.
Recognizable by their curved dorsal fins, the animals are well known to locals who have developed a fondness for sharks.
“We found 20 sharks in all,” said Ralph Watson, 33, a marine biologist with local conservation and diving group Marine Dynamics.
Casualties included 19 broad-nosed sevengill sharks and a spotted gully shark, he added.
Towner said the carnage was striking as it was the first time Port and Starboard had hunted these species in the area and “so many of them were washed out after one visit”.
Still, it wasn’t the orca’s most daring hunt.
Experts credited the duo with ensuring that great white sharks, one of the world’s largest marine predators, disappeared from some waters near Cape Town.
Last year, Starboard and four other orcas were caught on camera chasing and killing a great white off Mossel Bay, a southern port town.
The unusual behavior had never been observed in detail before.
Orcas, the ocean’s apex predator, typically prey on dolphins in these parts and have been known to prey on smaller species of sharks. But evidence of attacks on great whites was previously limited.
Port and Starboard were first sighted near Cape Town in 2015.
“They probably came from somewhere else. West Africa, East Africa, Southern Ocean, we don’t know,” said 45-year-old Simon Elwen, who heads the scientific collective Sea Search.
Unlike other killer whales, the pair likes to hunt close to shore – something that has made its distinctive fins a common sight in the region.
“Within southern Africa, port and starboard were seen from as far west as Namibia to as far east as Port Elizabeth,” Elwen said.
The marine mammal killing technique is “surgical,” Watson added, explaining that the pair are targeting the shark’s liver, “a very nutritious organ full of oils.”
“You tear open the chest area of the chest belt…then the liver falls out,” Watson said.
The 2022 video showing starboard in action has biologists worried as it suggests the practice is spreading as studies have found the black-and-white animals have the ability to teach hunting techniques.
Some orcas in Antarctica use the cunning tactic of hunting in packs and making waves to wash seals off drift ice, researchers say.
In Antarctica, two orca populations — not subspecies but distinct groups that overlap at the edges — used very different hunting techniques that were taught through generations.
Such behavior is not hardwired but learned – one of the arguments for whales having a “culture”.
In the clip, the other four orcas were not known to have previously attacked great white sharks.
“This is now an additional threat to shark populations off the coast of South Africa,” Towner said.
Elwen said it was “fascinating and frustrating to see a rare, endangered animal killing another endangered species”.
Nonetheless, the overall threat posed to the South African shark population by port and starboard remained very limited.
Hundreds of thousands of sharks are fished out of the sea every year, Watson said.
“Two killer whales will not wipe out any species,” Elwen said.
© 2023 AFP
Citation: ‘Surgical’ shark-killing orcas fascinating off South Africa (2023, March 4) retrieved March 4, 2023 from https://phys.org/news/2023-03-surgical-shark-killing-orcas-fascinate- south.html
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