When three great white sharks mysteriously washed ashore along the coast in Gansbaai, researchers weren’t sure what to think. But after doing necropsies (animal autopsies) on all three sharks, the researchers identified a possible culprit.
All of the sharks were missing their livers, and one was missing its heart. It’s likely that an orca, also known as a killer whale, attacked the sharks and ate their organs.
The great white shark is one of the ocean’s most powerful predators, yet it is sometimes prey for orcas that killed several of the formidable creatures off South Africa’s coast this month.
The rare necropsies conducted on the three carcasses that washed ashore show how orcas dominate the marine food chain at the expense of great white sharks, whose ferocious reputation (thanks partly to the 1975 movie “Jaws”) belies their vulnerability to shark nets, fishing, poaching, pollution—and the biggest member of the dolphin family.
Orcas, which also face human threats, have a softer image (the 1993 film “Free Willy” helped) due to their intelligence and accessibility at water parks.
Ingrid Visser, who has studied killer whales for nearly 20 years, said orcas used their powerful tails to create strong underwater currents that forced the sharks to the surface, then stunned them.
“Once the shark is at the surface, the killer whale pivots and lifts its tail out of the water and comes down on top of it like a karate chop,” she said. It is the buoyant livers that then cause the shark’s body to rise to the surface.
What really shocked scientists was that the livers of all three sharks were removed, with surgical precision suggesting the orcas, also known as killer whales, targeted the nutrient-rich oil and fat in the organs and discarded the rest of their prey. Sharks have enormous livers (can weigh more than 60kg) filled with fats that give the shark buoyancy.
“Bony fishes have a swim bladder that they can fill with gases, and those gases provide buoyancy for the fish,” said Andrew Nosal, an assistant professor of biological sciences at Saint Katherine College in San Marcos, California, and a visiting assistant researcher at Scripps Institution of Oceanography in San Diego. (Nosal was not involved in the recent shark analyses.)
“Sharks do not have a swim bladder. Instead, they have a very large liver. In addition to being filled with fat, shark livers are “very energy- and nutrient-rich,” Nosal said. It’s unclear why orcas would go after the sharks’ livers, but Nosal said he imagines “they have a high metabolism, and they have a lot of nutrient needs, and so those fats might really help them out.”
Alison Towner, a marine biologist with the Dyer Island Conservation Trust, which seeks to protect the fragile ecosystem in waters around the southern tip of Africa said: “They work together and are very co-ordinated. Two orcas take a pectoral fin in their mouth and pull open a shark together to extract the liver. Nature can be so cruel and the dexterity these enormous animals are capable of is mind blowing, almost surgical precision as they remove the squalene-rich liver of the white sharks and dump their carcass.”
Each of the sharks had a big wound on its underside, a sign of the dexterity of the orca, whose biggest males can be well over nine meters (30 feet) long—considerably larger than great white sharks.
Even if they have an advantage, orcas are unlikely to hunt great white sharks on a regular basis, said George Burgess, director of the Florida Program for Shark Research. “But smaller sharks can be a routine part of an orca diet, and studies have shown that the rough texture of shark flesh grinds down the enamel on orca teeth over time,” he said.
Killer whales are apex predators and while we are accustomed to viewing great white sharks as occupying the top of the food chain in our waters, orcas are much more specialised hunters and consider almost anything in the ocean as potential prey,” according to the South Africa’s Department of Environmental Affairs.
“The mammals have highly developed social groups as well as a large brain, the capacity to learn and the ability to locate with the help of sound waves that bounce off objects.”
“Orcas are known to have culture, much like humans do, and different orca cultures specialise on different prey and different hunting strategies,” said Dr Boris Worm, a marine research ecologist at Dalhousie University in Nova Scotia, Canada.
Offshore orcas are rare and very mobile, so it is very conceivable that they show up in an area where they were previously not seen and exploit sharks in that region,” Worm wrote in an email to The Associated Press. “Of course, a white shark would be a difficult prey to tackle due to its power and size, but orcas (again like humans) use group hunting strategies that can outsmart almost any prey.”
“We, as shark biologists, recognise that the sharks are going to get eaten sometimes by predators, just like they eat things themselves,” said Burgess, the Florida expert. “It’s all part of the give-and-take of the natural world.”
Article by Dyer Island