UPDATE: SA mystery of the killer orcas continues as 4th great white found dead

2017-06-26 09:30
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Cape Town - The mysterious orca predation of endangered great white sharks is on again, according Marine Dynamics 

On Saturday, 24 June the shark Cage diving company confirmed that another great white shark was found dead with wounds indicative of an orca attack.

This comes after a suspected orca whale killing spree of great white sharks gripped the coast of South Africa in May 2017, resulting in the first ever dissection of a white shark following an attack of this nature.

Biologist Alison Towner has confirmed that another 4.2m male great white shark was found dead with the same injuries as previous sharks - livers neatly removed but little else eaten - indicating orca predation. 


SEE: South Africa's great white shark mystery: The full story

 

Towner told Traveller24 that the attacks are largely being attributed to a pair of orca, nicknamed port and starboard due to the slanting of their dorsal fins, also see at Dangerpoint over the weekend. 

She says the two orcas are known to be shark hunters, as they have apparently killed other shark in species in the area.

Towner says these two animals were also spotted in the bay, when the previous attacks took place. 

But what's most concerning about this rather unusual behaviour is the lack of Great White Shark activity - with Marine Dynamics posting to its blog about little to no sightings in the area.

As a result there are is no cage diving or boating activity. 

Towner says that winter in South Africa is prime hunting season in SA, when you would typically expect to see the highest number of great white sharks - but unfortunately that’s not the case as the is no white shark activity around the island.  

Great White sharks would instinctively avoid the area, because of the orca threat, says Towner who says a positive is that both species are highly transient and migratory and "don’t specifically live in this area and would just be moving through".

But at present, the long term patterns or effects of the orca predation remain unclear. 

"It is worrying and we don’t know what the long term impact of these attacks will be," says Towner.   

Timeline of great white killings

Following the dissections in May, the Department of Environmental Affairs (DEA), confirmed that orca whales were responsible for three shark killings, and that they were targeting the large nutrient-rich livers of the sharks. Various small pods of orcas were also spotted around Gansbaai during the time the killings occurred.

In all three previous cases Townsend confirmed the livers of the sharks in all three cases were neatly removed, with almost surgical precision, but little else was eaten. Even the hearts of the two of the sharks were left intact. 

"We have never seen anything like this," said Towner at the time, who contributed to the dissection of the sharks. In all three cases, "there was a large gaping hole between her pectoral fins where they were torn apart to reveal her body cavity … and that their large livers were completely missing. This information, combined with the recent sightings of orca and disappearance of white sharks in the area, provides convincing evidence that the orcas are responsible for the shark's death."

“This is an extremely rare occurrence,” she said in an email with Traveller24, “This is also the first time worldwide the carcass of a white shark was recovered post orca predation, let alone three carcasses within one week!”

 

The first dead shark—a massive one-ton great white—washed up on the shore in Gansbaai on 3 May. The first people at the scene were initially perplexed as to how such a large shark could have died, and it wasn't until a dissection, authorised by the Department of Environmental Affairs (DEA) in conjunction with the White Shark Research Group, Marine Dynamics and the Dyer Island Conservation Trust, revealed that the shark’s liver was missing.

The next day, another 3,5 meter shark washed up on the shore at Franskraal beach, and a later dissection revealed that this one’s liver was also missing.  “This is a difficult yet fascinating time,” said Towner on the Marine Dynamics blog. “It is something rarely documented in marine top predator behaviour in South Africa,”

On 7 May, a third dead shark washed up, this time in Struisbaai, matching the injuries of the previous two specimens, and cementing the suspicion that these were not just random isolated incidents. “Obviously this is a very sad time for us all,” said Towner, who was on hand to assist again. “Nature can be so cruel and the dexterity these enormous animals are capable of is mind blowing, almost surgical precision as they remove the liver of the white sharks and dump their carcass."

How and Why are Orcas targeting Great White Sharks?

Known as the “Wolves of the Sea”, orcas are the true apex predators of the ocean, and the only known predator of the great white shark. 

They are extremely intelligent, specialised hunters, feeding above sharks on the overall oceanic food chain. They hunt in organised social groups, using echo-location, strategy, and teamwork to kill their prey, which can be anything in the ocean, from seals, to dolphins, dugongs, otters, turtles, birds, squid, and sometimes even land mammals.

According to the DEA statement, the orcas were targeting the squalene rich livers that assists sharks with their buoyancy. This substance is also highly nutritious pound for pound, compared with the muscle tissue. Although this type of selective feeding on livers is extremely rare in orca whales, seals have been known to predate sea birds where they often remove and consume only the stomach/abdominal content and not the rest of the carcass.   

Orcas are also suspected to be responsible for a decline in Cape Town’s cow shark population, and have been known to predate these sharks. Alison Kock, a Marine Scientist for Shark Spotters, reported recovering several cow shark carcasses in a similar condition with their livers removed, in False Bay subsequent to a series of orca sightings.

How the orcas are able to extract the livers so neatly from the sharks is a bit of a mystery. But footage off the coast of California showed team-work between the orcas pushing a white shark to the surface, belly up, biting into its flesh, before letting buoyant, oil-rich liver as it float out of the cavity. 

What does this mean for Great White Sharks in South Africa?

Orca whales are widely distributed in the ocean, extending from the Arctic to the Antarctic, into the tropics, and are present in both coastal and oceanic waters.

According to figures from the DEA, orca whales are fairly common along the coast of South Africa, and some 785 sightings have been recorded ranging from the Western Cape all the way to Northern KwaZulu-Natal. “The sightings of Orca pods appears to be increasing in South Africa,” they said in a recent statement, but these sightings could also be attributed to more people on the ocean with eyes on the water.

The incidents have already affected the number of great white sightings, as the sharks leave the bay to avoid the orcas. The killings are a blow to the already struggling population of great white sharks in South Africa, a local population which some scientists say is facing extinction.

The DEA, along with various shark scientists and marine mammalogists, is currently collating all the scientific information about the incidents, and they are urging the public to be aware that this is a natural phenomenon, and might have to do with changes in seasons or temperatures and prey regimes of the orcas.

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