Dozens of gutted sharks have washed up on a South African beach, shining a spotlight on a pair of shark-hunting killer whales whose behavior has fascinated scientists and wildlife enthusiasts.
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 (93 miles) southeast of Cape Town.
“Dead sharks are torn in the pelvic girdle, have orca teeth marks known as rake marks on their pectoral fins and their livers are missing,” said Alison Towner, 37, a shark scientist at the Dyer Island Conservation Trust.
All evidence points to ‘Port’ and ‘Starboard’, a notorious pair of killer whales spotted off Gansbaai just three days earlier.
Recognized by their twisted dorsal fins, the animals are well known to locals, who have developed a penchant for sharks.
“We found a total of 20 sharks,” said Ralph Watson, 33, a marine biologist with local conservation and diving group Marine Dynamics.
The victims included 19 sevengill broadbills and one spotted shark, he added.
Towner said the carnage was noticeable as it was the first time Port and Starboard had hunted these species in the area and “so many of them washed up after one visit”.
However, it was not the most daring hunt of the orcas.
Experts credit the duo with causing great white sharks, one of the world’s largest marine predators, to disappear from some of the waters near Cape Town.
Last year, Starboard and four other orcas were caught on camera chasing and killing a great white in Mossel Bay, a southern port city.
The unusual behavior had never before been shown in detail.
Orcas, the top predators of the ocean, commonly hunt dolphins in these areas and are known to prey on smaller shark species. But evidence of attacks on great whites was previously limited.
Port and Starboard were first spotted near Cape Town in 2015.
“They probably came from somewhere else. West Africa, East Africa, the Southern Ocean, we don’t know,” said Simon Elwen, 45, head of Sea Search, a scientific collective.
Unlike other killer whales, the pair like to hunt close to shore – which has made their strange fins a common sight in the area.
“In southern Africa, Port and Starboard have been seen from as far west as Namibia to as far east as Port Elizabeth,” Elwen said.
The technique used to kill the marine mammals is “surgical”, Watson added, explaining that the pair are targeting the sharks’ liver, “a very nutritious organ, full of oils”.
“They open up the chest area of the pectoral girdle … then the liver falls out,” Watson said.
The 2022 video showing Starboard in action has alarmed biologists, as it suggests the practice is spreading as studies have proven that black and white animals have the ability to teach hunting techniques.
Some Antarctic orcas are using the cunning tactic of hunting in packs and creating waves to flush seals off the floating ice, according to researchers.
In Antarctica, two populations of orcas—not subspecies, but different groups that overlap at the margins—used very different hunting techniques, taught from generation to generation.
Such behavior is not hard, but learned – one of the arguments suggesting that whales have a “culture”.
In the video, the other four killer whales shown were not known to have attacked white sharks before.
“This is now an additional threat to shark populations in coastal South Africa,” Towner said.
Elwen said it was “exciting and disappointing” to see “a rare, endangered animal kill another endangered species”.
However, the overall risk Port and Starboard posed to the South African shark population remained very limited.
Hundreds of thousands of sharks are fished from the sea each year, Watson said.
“Two killer whales are not going to wipe out a species,” Elwen said.
© 2023 AFP
Reference: ‘Surgical’ shark-killing orcas fascinate 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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