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Friday 22 September 2017

Hunter becomes the hunted as orcas prey upon great white sharks

Killer whales can grow as long as 30ft, dwarfing great white sharks
Killer whales can grow as long as 30ft, dwarfing great white sharks

Wounds found on three great white shark carcasses which washed up on South African beaches this month indicate that the formidable ocean predator is itself hunted by killer whales.

Examinations conducted on the carcasses have shown how the mammals, also known as 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 and pollution.

In contrast, killer whales, the biggest member of the dolphin family, have a softer public image due to their intelligence and presence at water parks, as well as their portrayal in the 1993 movie Free Willy.

Bite marks on the shark carcasses in South Africa were inflicted by orcas seen recently near Cape Town, conservationists said. The livers of all three sharks were removed, suggesting the orcas targeted the nutrient-rich oil and fat in the sharks' organs and discarded the rest.

Each of the sharks had a big wound on its underside, a sign of the dexterity of the killer whale, whose biggest males can be well over 30ft long - considerably larger than great white sharks, which rarely grow larger than 18ft.

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: "It's just a classic sort of scenario of what orcas are capable of doing."

Ms Towner participated in examinations of the shark carcasses, providing rare confirmation of orca killings of the big fish off South Africa.

George Burgess, director of the Florida Programme for Shark Research, said that even if they have an advantage, orcas are unlikely to hunt great white sharks on a regular basis.

However, 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.

Two of the sharks in South Africa were recovered in Gansbaai and another in Struisbaai. Closer to Cape Town, several cow shark carcasses were discovered with their livers removed in False Bay after orca sightings, South Africa's department of environmental affairs said.

The department 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."

It said 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 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," Mr Worm said.

"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."

Orcas have been seen attacking cow sharks off Cape Town, New Zealand and South America. Additionally, orcas were filmed attacking great white sharks around the Farallon islands off California and Neptune islands off Australia.

South African cage-diving operators reported a drop in great white shark sightings around the time that the three sharks were killed, a sign that other sharks had left the area, at least temporarily, because of the orcas.

Researchers are debating the size of South Africa's great white shark population and say more inquiry is needed. Human threats far outweigh any threat from orcas, they say.

"We, as shark biologists, recognise that the sharks are going to get eaten sometimes by predators, just like they eat things themselves," said Mr Burgess, the Florida expert.

"It's all part of the give-and-take of the natural world."

Press Association

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