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Marine animals - 28 January 2021

Confusion Underwater: Great White Sharks Killed

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Emma Barbier

@emmabarbier

If you’ve already been swimming in the ocean, chances are that this particular thought has crossed your mind at some point:

 

‘…Are there sharks in these waters?’

In today’s society, when someone mentions great white sharks (Carcharodon carcharias), we automatically think of them as aggressive predators with hundreds of teeth ready to tear anything apart; including yourself.

We can’t deny these are indeed one of the top scariest predators of the oceanic food chains, but they are not the apex predator of the ocean. Scientist have recently found more compelling evidence that there is a mammal out there which is having the audacity to take down these sharks. And it’s succeeding. This is the Orcinus orca, commonly known as the killer whale.

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Great White Shark (Carchadrodon Carcharias) swimming at bait – Photo by E. Barbier

5 CARCHARODON CARCHARIAS WASHED-UP DEAD ON SOUTH AFRICAN BEACHES

Gansbaai is the shark-cage diving capital of the world, and up to recently you were guaranteed to see a shark on nearly every boat excursion you went on.

 

However, in the beginning of 2017 there was a turning point when two orcas started roaming around the bay and 5 great white shark carcasses washed up on Pearly, Franskraal and Struisbaai beaches. All had consistent bite wounds and were missing their livers. These wounds matched those of Notorynchus cepedianus, another species of shark that was documented being a victim of orca predation in False Bay, 70km west of Gansbaai.

 

After the authorised necropsies, Marine Dynamics concluded that two previously spotted orcas were behind the attacks. Port and Starboard – named because of their collapsed dorsal fins – made their apparition the same day as some of the carcasses were found.

 

They were spotted on the 8th of February 2017, a carcass washed up on the 9th of February 2017. Then they were later spotted on the 19th of April and the remaining sharks were found in May.

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Carcass of dead Great White Shark

KILLER WHALES LIKE LIVERS

Eating almost nothing but the shark liver could sound surprising at first, but it really is not.

To access the liver, orcas have to flip the shark over 180° which causes ‘tonic immobility’, a state where the shark is paralysed and drowns because water flow through the gills is non-existent.

After this step has been achieved, orcas have easy access to the liver which is a very fat and oil dense organ that weighs as much as an average human. A shark liver is made from n3-polyunsaturated fatty acids, which cannot be synthesised by mammals and thus must be consumed.

REPERCUSSIONS OF CHANGING TOP PREDATORS

A community is a stable environment, but since it is dynamic it can be easily affected by changes in the environment and be source of abnormal behaviours.

After the orcas made their apparitions and dead sharks washed up on the shore, the Carcharodon carcharias of Van Dyks Bay vanished and shark-sightings were rare. We cannot yet explain how they sensed the danger, but I think it is a fascinating area that needs to be studied further.

As the great white shark made themselves rare for the next few months, a new species, the Carcharhinus brachyurus, slowly took over the bay. These bronze whaler sharks were temporarily the new apex predators of the food chain, and they have a very different diet than the great white sharks. Contrarily to great whites, cape fur seals that live on Geyser Rock Island are not their main prey, so were rarely hunted when the bronze whalers ruled on the bay.

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The Killer Whales, Port and Starboard

ONGOING MYSTERY

Months have passed since killer whales have been spotted in the bay. Great white sharks are starting to return to their normal feeding grounds and can be spotted much more often on boat excursions.

Nevertheless, scientists are still puzzled about why orcas started eating great white sharks out of the blue, and numerous other questions remain unanswered. Why did the orcas stop feeding in Van Dyks Bay? Where did the great whites disappear too?

And more recently, 8 bronzer shark carcasses washed up near Gansbaai.

Was it the orcas striking again?

We don’t know.

But I do know that next time you go for a swim and that irrational fear of sharks creeps up, you should question it.

Is it really be sharks you are worrying about?

 

Reference:

https://sharkwatchsa.com/en/blog/