Did orcas scare off False Bay’s cow sharks? And what turned of the nice whites? Marine biologist Leigh de Necker and the Shark Spotters analysis staff tried to unravel the thriller.
Dripping moist, we had barely settled again on to the boat, not to mention had an opportunity to ponder the killings and what may need prompted them, when Tammy Engelbrecht sounded the alarm.
“Look!” she exclaimed by a mouthful of a peanut butter sandwich, “A whale!”
Humpback, Bryde’s or southern proper whales have been recognized to go to boats, however this was one thing else.
“Orca!” Dr Alison Kock confirmed.
With most of my dive gear nonetheless on, I stumbled to the facet, punching my hand into the water to seize a fortunate shot with my GoPro.
The orca, also referred to as a killer whale (Orcinus orca), glided beneath us.
It was not even a minute later when a second orca surfaced a number of metres off the bow of our boat. We had our prime suspects – the chase was on.
This story begins in 2015 when members of the Shark Spotters analysis staff – Alison Kock, Tammy Engelbrecht, Dave van Beuningen and me – started getting reviews from leisure divers of useless broadnose sevengill, also referred to as cow sharks (Notorynchus cepedianus), at Miller’s Level in False Bay on the Cape Peninsula.
There’s an previous slipway which makes it simpler for divers to achieve the kelp forest from the shore. Granite rocks rise above the water. Under, sandy channels separate reefs, with caves and overhangs. Invertebrates brighten nature’s structure with bursts of color. Scuba and freedivers quickly discover themselves deep in a kelp wonderland.
It’s a posh habitat, residence to many fish and shark species discovered solely in temperate southern African waters. These embrace leopard (Poroderma pantherinum) and pyjama (Poroderma africanum) catsharks, spotted-gully sharks (Triakis megalopterus), puff adder (Haploblepharus edwardsii) and darkish (Haploblepharus pictus) shysharks.
Broadnose sevengill sharks are named for his or her blunt nostril, broad head and 7 gill slits, the place most shark species solely have 5. They’re among the many most primitive shark species, with adults reaching a most size of three metres. Behind a misleading, toothless smile, cover rows of razor-sharp, cusped enamel on the higher jaw, whereas the decrease jaw’s enamel are jagged and comb-shaped, permitting them to feed on quite a lot of prey.
They cross the summer time days, researchers suspect, sheltering within the kelp forest. At night time they go away to hunt in deeper waters. Between October and January, sevengill sharks saturate Miller’s Level. They cruise its sandy-bottomed highways by the kelp forest, whereas divers, awkwardly suspended in thick wetsuits, make approach for his or her prehistoric, armoured hosts.
It’s really a magical place, showcasing the variety of sharks discovered inside False Bay. Not less than that was the case till mid-November 2015, when the grisly discoveries started. Divers began discovering sevengill shark carcasses scattered among the many reefs, in what, opposite to expectation, had develop into an ominous, underwater graveyard.
Our analysis staff rushed to the location, launching the boat on an ideal early-summer morning. Van Beuningen and I kitted up and hopped into the water to seek for proof on the “crime scene”.
I’ve completed numerous dives at Miller’s Level and have been lucky to spend hours within the water observing these magnificent, docile dinosaurs. Discovering an animal I respect and recognize, useless, in its underwater residence, was heartbreaking.
Divers from Pisces Dive Centre in close by Simon’s City had collected three carcasses earlier than our investigatory dive, so once we discovered carcass quantity 4, we knew we needed to carry out full necropsies.
All of the carcasses shared the identical exterior and inside accidents. Most noticeably, a tear from the pectoral fins throughout the stomach, exposing the physique cavity, with solely the liver faraway from an in any other case intact carcass. It appeared like a lower, exactly executed, virtually as if completed with a knife.
At this level, we have been considering fishermen had caught the sharks. Sevengill shark meat has little business worth so they could have merely eliminated the big, oily livers to make use of as bait.
However Miller’s Level is in a marine protected space the place fishing is prohibited. It appeared unlikely that sharks have been being slaughtered with out the authorities noticing.
We, detective divers, have been barely again on the boat when Engelbrecht noticed the orca and we started the chase. For 2 exhilarating hours we tracked the pair, which later turned recognized, infamously, as Port and Starboard. They led us south for 6km till we overpassed them off Smitswinkel Bay.
We moored and went over to Pisces Dive Centre to do necropsies on the 4 carcasses. The killer whales, we found, had left behind a key little bit of proof. Stamped on the pectoral fins of every of the carcasses have been distinct tooth impressions. Responsible!
Quickly after we first noticed Port and Starboard, all sevengills fled the world. A number of months later, they started returning sporadically to Miller’s Level – however so did the shark hunters. The extra we noticed the 2 orcas in False Bay, the much less of the sharks we noticed. Finally, the sevengills deserted Miller’s Level.
Port and Starboard have been on the transfer too.
Quite a few accounts adopted of them being sighted about 200km east of False Bay, on the white shark (Carcharodon carcharias) hotspot of Gansbaai. Marine biologist Alison Towner and her staff from Marine Dynamics and the Dyer Island Conservation Trust investigated when white shark carcasses – minus their livers – washed up on the shores of Gansbaai in May 2017 and July 2020.
Though orcas have been by no means recorded killing white sharks in False Bay, white-shark sightings have decreased dramatically in each False Bay and Gansbaai.
White sharks and sevengills share lots of the identical prey, together with seals, different sharks, rays and bony fish. Nonetheless, they seem to hunt in several areas or at completely different occasions.
Analysis by Engelbrecht and Kock discovered that sevengills are nocturnal hunters. By day, they could relaxation within the shallow kelp forests, venturing into deeper, open waters at night time, the place they’re much less conspicuous and fewer more likely to fall prey to white sharks, False Bay’s charismatic apex predator.
In distinction to the lesser-studied sevengill, white-shark actions in False Bay had been comparatively nicely documented and located to be slightly predictable … till lately. Dr Kock has tagged and tracked the actions of white sharks in False Bay for a few years. Her research has discovered that white sharks would sometimes spend the winter feeding on younger seal pups at Seal Island, then, in summer time, they might transfer to the inshore areas to make the most of migratory fish and different shark and ray species.
Nonetheless, because the first go to of Port and Starboard to False Bay in 2015 (and their sevengill shark-liver feast), native fishermen and Shark Spotters, a shark security and analysis organisation, reported fewer white-shark sightings alongside the inshore areas throughout summer time too. Cage-diving operators turned involved on the dwindling numbers at Seal Island, within the traditionally peak winter season.
With sevengills absent from Miller’s Level and white sharks now not enthused by the buffet on provide at Seal Island, divers and cage-diving operators have been, by 2018, turning into very despondent. No sharks, no enterprise. Researchers and conservationists turned involved too. No prime predatory sharks, no balanced ecosystem.
Simply because the final glimmer of hope was fading, there was an surprising flip of occasions for Seal Island shark ecotourism. Sevengills started tugging on bait strains and investigating awkwardly caged vacationers across the diving boats.
My grasp’s analysis revealed that seals type an necessary a part of the sevengill shark’s food plan. Actually, there seems to be the next proportion of seal within the food plan of sevengills than in that of white sharks. That is in all probability because of sevengills consuming seals all yr spherical, whereas white sharks feed on seals seasonally.
Sevengill sharks gave the impression to be profiting from the white sharks’ rare visits to Seal Island, as they may exploit an plentiful prey supply – Cape fur seals (Arctocephalus pusillus pusillus) – with out the potential risk or competitors.
Though cage divers wait in hope to see the majestic nice white, they’ve been having fun with visits from the common-or-garden sevengill shark, with their pleasant grin and faces resembling a unclean oven mitt.
Seal Island will, nevertheless, at all times be the territory of the legendary white shark, and it’s anyone’s guess as to if and when they’ll return to their kingdom.
The ocean and marine life are extremely complicated and dynamic. By no means static, by no means predictable, at all times fascinating. Paradoxically, what was anticipated to be a refuge space for sevengills, turned out to be a spot the place many have been killed.
So as to add to the irony, sevengill sharks now occupied the territory of white sharks (the predators they have been in all probability in search of refuge from within the first place). Though shark-specialist orcas are nicely documented elsewhere on the earth, we (and maybe the sharks themselves) by no means anticipated any animal to scare off an important white.
There aren’t any conclusive solutions as to why the interactions and dynamics between prime predatory sharks have modified through the years. Neither is it clear to what diploma the elevated presence of the superpredatory orcas could have performed an element.
It will be ignorant to count on a easy reply and to imagine people haven’t any position to play on this. As a lot as we can’t low cost the orcas’ affect, a mixture of things are in all probability influencing the presence of predatory sharks and orcas within the bay.
One concept is that persistent offshore business long-line fishing is depleting a lot of the orca’s offshore shark prey, whereas inshore shark fishing might be depleting a number of the key prey species for white sharks. The inshore shark fishery in South Africa targets predominantly soupfin/tope (Galeorhinus galeus) and smoothhound (Mustelus mustelus) sharks, with the meat bought as “flake” and chips in Australia.
Most shoppers are unaware they’re consuming shark, and there’s no authorized requirement that or not it’s bought below its precise title. The fishing of tope sharks is now unlawful in Australian waters, the place shares have been depleted.
The ocean has no fences, no partitions, no boundaries. If a specific habitat ceases to be beneficial for no matter cause – be it a scarcity of prey or threats from predators or folks – animals transfer … they adapt or die.
I desperately hope to see white and sevengill sharks return to False Bay quickly. Not just for the sake of the ecotourism that depends on their presence, however for the crucial position they play in sustaining the world’s ecology. DM
Leigh de Necker is a marine biologist, aquarist and business diver on the Two Oceans Aquarium. She has accomplished her Grasp of Science (MSc) diploma, the place she researched the feeding habits of broadnose sevengill and nice white sharks in False Bay, South Africa.
De Necker was one among seven winners in a current writing competitors on sharks and rays run by Roving Reporters. The competitors was supported by WildOceans, a programme of the WildTrust, which facilitated entry to conservation-minded youth eager to share their ardour and develop writing abilities with mentorship from Roving Reporters. The opinions and views expressed on this Ocean Watch collection are usually not essentially these of the WildTrust.