Great White Sharks Are Being Hunted - And Not By Humans

So far, five great whites have been found washed up since May this year, all with their livers removed by orca. © Marine Synamics/Dyer Island Conservation Trust

With their massive size, rows of razor sharp teeth, and emotionless eyes, many consider great white sharks to be the ultimate predator. But even these incredible animals have to watch their backs, as earlier this year more and more great white shark carcasses were being washed up on South African beaches.

This year, five carcasses of the formidable hunters have been washed up, with the remains ranging in size from 2.7 to 4.9 meters (9 to 16 feet). All of them had a similar brutal wound inflicted on them, with a hole running down between their pectoral fins and the liver. As it turned out, the sharks were being hunted by a team of killer whales, recently spotted in the area when the sharks were found dead.

It is not unusual for orca to target sharks as prey. Different populations of the marine mammals are known to specialize in different foods, with those off the north-western coast of North America focusing on fish, while some subpopulations in Antarctica specialize in minke whales, for example. Orca off the South African coast have been known to target sharks and rays before, plucking out the liver while leaving the rest of the carcass to drift away.

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It is thought that they target the liver of elasmobranchs (as sharks and rays are technically known), due to the organ’s high-energy content. Unlike most fish, sharks don’t have a swim bladder to help with buoyancy. Instead, they've evolved an oil-rich liver, which has the dual role of helping them move up and down the water column, while providing them with energy. It seems, however, that the orca have also figured this out.

What is more impressive is that the killer whales are managing to take down an almost 5-meter (16-foot) great white shark. It is thought, based on observational evidence of the mammals hunting, that the whales capitalize on the strange physiological quirk seen in sharks known as tonic immobility.

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This is where, when turned upside down, sharks and rays go into a kind of trance. Long known about by humans studying sharks, who exploit this natural state of paralysis in order to measure and tag the predators, the killer whales apparently know about it too. One observation off the west coast of the US recounts how a pod of killer whales rammed into the side of a great white, knocked it over, before holding it upside down. If some shark species stop swimming, they cannot breathe, and so the whales may in effect drown the predator, before devouring the carcass.

Why these carcasses have suddenly started appearing this year is not known, but researchers are sure to keep an eye out for more.  

[H/T: The Guardian]

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