Hunters of the seas: Orcas prey on marine life such as seals, sharks, fish and even sea birds. Photo: Francois Gohier/Getty Images
Many people think the great white shark is the ocean’s apex predator. But it isn’t. The erroneously named killer whale owns that title. And orcas aren’t known to have predators — except humans.
Over the years, sightings of great white sharks have dipped, leaving researchers pondering why this is happening. Winter is their peak hunting season but, from about 2017, the number of sightings in places like Gansbaai, False Bay and Mossel Bay went down.
Enter Port and Starboard, a pair of killer whales, or orcas, who hunt various kinds of sharks and have been spotted along the coast from Namibia to Gqeberha.
Their appearance was first recorded by scientists Tamlyn Engelbrecht, Alison Kock and Justin O’Riain. They observed the orcas in False Bay preying on broadnose sevengill sharks. They found that the orcas had a new feeding technique where the liver of the sharks were taken out.
Shark liver — which is heavy, making up about a quarter of a shark’s weight — is rich in nutrients and oils.
The researchers wrote how the orcas attacks caused the sharks to leave the area.
In another paper Alison Towner, the lead great white shark biologist for Marine Dynamics, and other researchers documented how the presence of the orcas led to great white sharks leaving Gansbaai. The findings suggest there is a link between the orcas’ arrival and the great white sharks’ disappearance.
Last week, about 20 sharks, mostly broadnose sevengill sharks, washed up in False Bay, all disembowelled.
Orcas, which are part of the dolphin family, feed on anything from seals and fish to sharks and whales.
They develop specific styles of killing, which are passed on down through the generations, according to the Natural History Museum. They may herd fish and then stun them with strikes of their tail or together create waves to knock prey off floating ice, according to the BBC Wildlife Magazine. In Patagonia they beach themselves to catch seals.
Scientists were quoted in an article in Down to Earth as saying Port and Starboard learnt how to hunt larger sharks from other orcas and, in turn, may have taught others how to do so.
Last year, footage emerged of one such attack in Mossel Bay. Here, more than one orca can be seen ripping into a great white shark.
The orcas grip the shark’s pectoral fins and flip it over. They then rip open the shark and pull out the liver.
Teeth marks found on the fins of the shark carcasses by Towner and a team of researchers led them to believe that this is Port and Starboard’s modus operandi.
Towner said great white shark carcasses don’t usually wash up on shore, because the attack usually takes place deep in the ocean. So being able to see the sharks’ carcasses has provided rich pickings for researchers.
The fact that orcas are smart hunters and teach their ways to other orcas has biologists concerned, because the loss of great white sharks can have ripple effects on the food chain. In Towner’s research, and separately in Engelbrecht’s, scientists had concerns about cascading effects on other marine species in the areas from which the sharks disappeared.
For example, Towner says their absence means the populations of other species such as seals could grow because fewer sharks are hunting them. This may in turn affect fish numbers because seals eat fish, as do African penguins. African penguin numbers are already in decline and a further loss of food could be detrimental to the species.
But, cautioned Towner, how the food chain could be affected by the loss of great white sharks requires more investigation.
You may wonder why the two orcas are named Port and Starboard. It’s after nautical terms; Port’s fin falls to the left and Starboard’s to the right.
Floppy fins are unusual for orcas when they are in the wild.
Writing for Shark Spotters, a shark safety and research organisation, Simon Elwen, the director of Sea Search and a research associate in the department of botany and zoology at Stellenbosch University, explained what is unique about the fins of Port and Starboard.
He said male orcas have large, upright dorsal fins that develop at sexual maturity and grow to nearly 2m. The dorsal fins of orcas in captivity stick out of the water and without the support of water the fins are likely to bend.
Wild orcas rarely have bent dorsal fins. Those that do have collapsed fins probably experienced some sort of injury such as an entanglement with a net. Elwen said another possible reason was Port and Starboard are eating sharks and may not be getting enough calcium and other minerals to make the dorsal fin strong — but more studies need to be done to determine this.
Ozayr Patel is the Mail & Guardian’s environment editor.