Winter is coming

The first clue that something was amiss for the white sharks at Gansbaai was the carcass of a 2.7-metre (9-foot) female that washed ashore on 9 February 2017. Researcher Alison Towner and a team from the Dyer Island Conservation Trust (DICT) found nothing conclusive. The animal was completely intact, with just a couple of scratches around its head, and there were no obvious signs of what might have killed it. Even more extraordinary was the fact that every other shark in the area left the bay and did not return for more than three weeks. South African officials refused to grant permission for a more in-depth necropsy and for the next three months the death of the small female and the departure of the others from their kingdom remained a mystery.

Then, at 8 am on 3 May 2017, Towner and the DICT team responded to a call about a large white shark that had washed up dead during high tide. When they reached the shore they were saddened by the sight of a massive female shark that had visited their research boat on multiple occasions and been named ‘Khaleesi’, after the Game of Thrones character. She was almost five metres (16 feet) long and weighed more than a ton – a very dominant female shark. Shark scientists travelled from around the country and the next day the shark was autopsied in front of the public. As external measurements were being taken, nothing seemed out of the ordinary with Khaleesi, but when they rolled her mammoth body onto its back, a huge gaping wound was revealed. They opened her stomach cavity and waited for her liver to slide out – but the liver wasn’t there.

Castle Rock is a protected area and Kock and her team were concerned about the possibility of humans killing sharks, so on 13 April 2016 they went to investigate. When they found the carcass it was very decomposed. On examination, they realised that both of the shark’s pectoral fins showed tooth impressions and its liver was missing. All its other organs were intact. Over the next few weeks more sevengills were discovered and all had lost their livers. The other sharks fled and Kock concluded that they were being predated on by orcas. ‘It looks like at least two orcas had to have worked together. We saw the bite marks on the pectoral fins and then the shark was torn open to the pectoral girdle. Based on that examination, it seems the orcas are biting on the pectorals, ripping the shark apart and then taking its liver out,’ she explains.

The necropsy was not the only clue that pointed to killer whales. About four months earlier, Kock and her team had been diving at Castle Rock to collect some data from a receiver. As they returned to the boat, someone shouted ‘Whale!’ Bryde’s and southern right whales are common in the bay, but this was a different kind – and there were two of them. ‘We had two killer whales that must have been swimming around us in the kelp forest without us knowing. They were right on the boat and they both had floppy dorsal fins,’ exclaims Kock.

Two years later, Port and Starboard have become the prime suspects in the case of the ‘eliverated’ sharks. Recorded sightings of the pair correlate exactly with the discovery of the shark carcasses at Gansbaai, starting with the small female that washed up in February. Researchers suspect she beached herself while being chased by the orcas. Because of their distinctive fins, Port and Starboard are easy to recognise and have been seen at numerous points along the southern tip of Africa, from Walvis Bay in Namibia to Langebaan on South Africa’s West Coast and to Gansbaai in the east – a total distance of more than 1,400 kilometres (870 miles).

Although the recovery of shark carcasses without livers is a world first, incidents of orcas predating on sharks have been reported in California, Australia, New Zealand and Canada, and for some time experts have suspected that South Africa could also be home to shark-eating orcas. Dr John Ford has been studying orca populations in British Columbia since the late 1970s. Several years ago he was asked to review a paper by Dr Peter Best, South Africa’s top cetacean researcher at the time. ‘The report mentioned one or two stranded animals that showed profound wear of their teeth. In the manuscript he commented that this must be a sign of old age,’ Ford remembers. ‘I wrote back immediately and said, ”Don’t jump to that conclusion because we have a whole population on the West Coast of Canada (from California to Alaska) that have profound tooth wear of the same kind and for many years we hypothesised that it was due to something they were eating,”’ he continues.

Ford and his colleagues speculated about this for 20 years and then, once in 2008 and again in 2009, they witnessed events that confirmed their suspicions. They were running surveys in very deep water (200–350 metres, or 650–1,150 feet) when they came across a titanic deepwater battle between two great predators. The first incident involved five offshore orcas, the second a group of about 100. The animals were diving in unison for up to 10 minutes at a time. The researchers could see from their behaviour that they were feeding, but they had no way of telling what was going on at that incredible depth. Then, as the orcas surfaced, large pieces of soft, oily tissue floated to the surface with them. The researchers watched as the animals picked up the chunks and shared them with one another just beneath the surface – a cultural practice that helps to maintain harmony within orca pods. Ford and his team collected some of the floating tissue and tests later revealed that it was the liver of Pacific sleeper sharks and that orcas had killed at least 11 sharks during the first event and seven during the second.

Sleeper sharks can grow to more than four metres (13 feet) long and are found at depths of up to 2,000 metres (6,560 feet). Their flesh is toxic, but this does not appear to be the case with their livers and somehow the orcas know that. ‘Of course, shark livers are wonderfully rich and killer whales love lipids. They are really driven by high lipid content – lots of oil or fat – and that is why they selectively remove the tongue and the skin as well as the blubber of large whales that they kill because these parts have the highest energetic punch. Some sharks can be up to 30% liver and of that liver, up to 80% can be lipids,’ explains Ford. So far, the research suggests that more than 93% of the offshores’ diet is shark and includes blue sharks, sleeper sharks and spiny dogfishes. Ford has also discovered that wear on the teeth starts very early in the animals’ lives, but does not appear to affect their general lifespan.

But why would the top predators in the ocean be so picky about their prey and does it serve them to have such limited diets? ‘I think it’s basically because by being specialists, they can actually out-compete generalists. If they get really good at a particular foraging tactic, and there is sufficient prey abundance over time, they can become sort of culturally specialised on that particular food resource,’ explains Ford. ‘It’s not necessarily a good thing, because as we have seen over the past few decades, when Chinook salmon abundance fails, the mortality rate in resident orca populations shoots right up and the numbers decline. Once they attain these cultural blinkers that have them focusing on a particular prey, they don’t seem to be able to switch quickly to an alternative.’

After the grisly spate of killings, the great whites were not seen again in Gansbaai for almost three months, despite the tantalising smell of a deceased sperm whale. In the meantime, shark researchers further east reported record numbers of sightings. Alison Kock is not surprised that the sharks fled after the death of their kin, but what fascinates her is why they were gone for so long. ‘I think there are a lot of really interesting theories, but you can imagine that when two orcas start predating on such a small aggregation, all the other sharks will hear and see the commotion, which could cause an immediate flight response. And if dead sharks are sinking to the bottom and being left there to decompose, the chemicals released by the decaying shark could also create a deterrent,’ she speculates. The animals have since trickled back and cage-diving operations have resumed, but it is impossible to predict how long peace will reign in the sharkdom of Gansbaai – or when the orcas might return. As Dave Hurwitz says, ‘They don’t follow a set path like other whales or cetaceans. They are such good predators because they are so unpredictable.’

A new study funded by the Save Our Seas Foundation will use sophisticated population models to combine data collected from all of South Africa’s white shark aggregation sites over the past two decades (including previously unpublished data) to assess how trends have shifted during that time. Currently, we do not have an accurate number for South Africa’s white shark population. Between 2007 and 2013, Towner observed about 1,000 individual sharks at Gansbaai and over a similar time period, Kock estimated that there were just over 700 in False Bay. Her sightings data show a steady decline in great white sightings from 2005 to now. While Port and Starboard are almost certainly responsible for the sudden disappearance of sharks at Gansbaai, the reason behind the overall decline in great whites is harder to pinpoint and for Kock that is the greater mystery. ‘We work in a dynamic environment. Sometimes it might be orcas. Sometimes it might be extreme water temperatures. These animals are adapted to move to where conditions are favourable. They can leave an area and move somewhere that is better for a short while and then they might come back. It’s really hard to say what is going to happen because these interactions are so complex. We just don’t know enough and that is the bottom line,’ she concludes.

A few months later I am swimming through a cave in the kelp forest when I hear what sounds like a conversation between two underwater sirens. I fly up to the surface to breathe and the sound disappears. I descend again and hover in the cave. The submarine chamber seems to amplify the noise. I hold my breath, suspended in awe. There are many whale species in False Bay and I don’t know enough about them to identify their voices, but I try to feel out into the ocean around me and picture what the sound is coming from. My mind stretches across the bay to Seal Island and I think of the sharks and wonder what they are picking up on and whether it fills them with fear.