The shark measured 4.1m in total length,” says the team behind shark-diving operator Marine Dynamics, which is part of the collaborative group working to document these predations. “The carcass may be a few days old but it seems relatively fresh and bled out massively.”
The orcas thought to be responsible for the shark’s death – a pair of males known as “port” and “starboard” – were seen off Danger Point at the southern tip of Walker Bay, near the fishing town of Gansbaai. Find out more about these cases in the original article below!
Image: Marine Dynamics
Here the orcas move from an inland aggregation area to one near Dyer Island. Image: Marine Dynamics
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A surgical drama is unfolding in South African waters, and scientists are just starting to figure out what’s going on.
The gash you see in the image below wasn’t made by a scalpel – it was inflicted by the four-inch tooth of a monochrome behemoth. Welcome to “Greyscale Anatomy”.
A necropsy with Marine Dynamics shows wounds consistent with an orca attack. Images: Michelle Jewell/Facebook
The carcasses of three great whites turned up recently in Gansbaai, a white shark hotspot in the country’s Western Cape. Even here, events like this are rare, and a collaborative necropsy (animal autopsy) on the shark trio points to killer whales as the culprits.
The story has been covered widely online, but contrary to some media reports, this kind of predatory behaviour is not unheard of: experts have known that orcas hunt white sharks for at least two decades. How these hunts play out, however, has remained a bit of a mystery – and the Gansbaai carcasses might just help researchers unlock a piece of the puzzle.
During a 1997 encounter off the Farrallon Islands, a group of whale watchers witnessed a killer whale flipping over a white shark, and then holding it in place for 15 minutes. In this position, the sharks enter a catatonic state known as “tonic immobility”, and without water flowing over the gills, they eventually drown.
We’ve long thought that killer whales use this to their advantage when attempting to immobilize great whites, and that particular group proceeded to snack on the shark’s fatty liver, which was (at some point) “separated from the body”.
Oil oozes from the liver of a ragged-tooth shark. Image: Earth Touch
In the recent Gansbaai incident, two of the sharks washed up with gaping holes just between the pectoral fins. Based on what’s known about orcas’ affinity for liver, researchers on the scene suspect that puncturing the sharks’ skin and muscles in this way allows the two-tone predators easy access to the organ.
Predator-prey ecologist Michelle Jewell, who is currently in South Africa as part of a white shark research expedition with MBARI and the Dyer Island Conservation Trust, takes the liver hunch one step further.
“If you make a medium-sized hole between the pectoral fins of a shark and then flip it over, the extremely buoyant liver will slip out and you can munch it,” she postulates.
While our own livers are all too often rich in booze, shark livers are rich in oil. This makes them a great energy store for the highly migratory fish, and also keeps them buoyant.
In other words, killer whales could be floating the sharks’ oily organs like delicious weather balloons. And sure enough, a deeper dive into the animals’ abdomens revealed the liver was missing in all three of the Gansbaai great whites.
“Imagine that kind of death!” says Jewell.
This shark, a 4.4-metre male, was also missing his liver. Image: Michelle Jewell, Marine Dynamics, Facebook
It takes a village, a military vehicle and a Land Rover to move a 2,500-pound white shark. Image: Michelle Jewell, Marine Dynamics/Facebook.
Necropsy shows a missing liver in one of the animals. Image: Michelle Jewell, Marine Dynamics/Facebook
Of course, until we observe such predatory behaviour in action, this remains informed speculation. That said, orcas have been known to pluck the livers from the bodies of cow sharks, blue sharks and mako sharks caught on longlines in South African waters, as well as the brains from billfish.
“The dexterity these enormous animals are capable of is mind-blowing,” Alison Towner, a white shark biologist for the Dyer Island Conservation Trust, said in a blog post with Marine Dynamics. “[They use] almost surgical precision as they remove the squalene-rich liver of the white sharks and dump their carcasses.”
The whales employ equally creative strategies when taking down other prey species. Off the coast of New Zealand, orcas force smaller sharks to the surface by creating powerful vortexes with their tails. Once topside, the whales pummel their prey into a stupor. Ray-hunting orcas team up with a partner: one to restrain the tail (and its painful barb!) and one to deliver the fatal bite. And then there’s the far-flinging punts of mammal-hunting pods, which can eviscerate prey upon gravity-powered reunion with the water.
The Gansbaai carcasses represent the first documented case of orca predations on white sharks in the area, and there’s still a lot to unpack. While it’s never easy to see an animal wash up dead, the necropsies present a unique opportunity to delve into the lives of these animals.
These interactions are extremely difficult to study because “liver-less” sharks sink extremely quickly. It’s possible that more victims are lying splayed on the seafloor, and we simply haven’t found them. While white sharks in these waters aren’t “dying out” as was widely reported last year, these predations by orcas could be problematic for the threatened population.
On the other hand, there’s a chance these attacks could be a positive sign about the local marine ecosystem. In order for apex predators to move into a new area, they need a stable food source to keep them going. There appear to be two orca pods that regularly cruise these waters, but if the influx in white shark predations is being caused by newcomers, that may be indicative of a healthy, well-managed ecosystem.
“This is a difficult yet fascinating time, something rarely documented in marine top predator behaviour in South Africa,” says Towner. “We are very grateful for everyone’s help and patience, especially the local community members of Gansbaai who have been incredible.”
A partially digested seal skull recovered from one of the shark’s stomachs. image: Michelle Jewell, Marine Dynamics/Facebook
Locals gather to catch a glimpse of the ocean giants. Image: Michelle Jewell, Marine Dynamics/Facebook
This collaboration involved the teams of Marine Dynamics, Shark Spotters, Dyer Island Conservation Trust, and International Marine Volunteers. Samples taken from the carcasses can teach us a lot about what these sharks are eating, their age and overall health, and even where they’ve been traveling. To support future response and retrieval efforts, head to the DICT page here.
