White sharks have disappeared from False Bay and Gansbaai, two sites off South Africa where they have historically been commonly sighted.Scientists have a number of theories about this, including predation of sharks by orcas, and fishing activity that targets species that juvenile sharks feed on.Scientists say it’s important to look at the big picture — while sharks have gone from some areas, they’ve increased in others — but data covering South Africa’s whole coastline is still patchy.The COVID-19 lockdown is also hampering data-gathering efforts, with scientists not yet permitted to go out to sea, potentially leading to a gap in the long-term data. CAPE TOWN — South Africa’s great white shark population has been the subject of international scrutiny since 2017, when cage-diving operators reported a sudden, sharp decline in sightings around False Bay and Gansbaai. From 2010 to 2016, white sharks (Carcharodon carcharias) were sighted in False Bay an average of 205 times each year, according to conservation and research organization Shark Spotters. In 2018, the sharks were spotted just 50 times; and in 2019, nothing. In January 2020, the first white shark in 20 months was seen in False Bay. “The reality is that we have way more theories than we have facts to support them at the moment,” says marine biologist Alison Kock, who currently works for South African National Parks. She has been researching white sharks in South Africa since 1998. “There are three or four possible reasons. Each one may be contributing in its own way.” It’s unclear how many white sharks there are around South Africa — estimates have ranged from around 500 to 900. Sara Andreotti of Stellenbosch University studies the genetics of white sharks around the South African coast. Her research found the sharks to be a single population, moving from site to site and breeding with each other. In a study from 2009 to 2011, she estimated there were around 300 breeders in the population — but the minimum to avoid inbreeding, she says, is thought to be around 500. “So our population was in real trouble already,” she says. But what explains the sudden disappearance of white sharks from False Bay and Gansbaai in 2017? Orca off British Columbia. In 2017, five white shark carcasses washed up on the shores around South Africa’s Gansbaai: their livers had been removed, and teeth marks clearly pointed to orcas as the predators. Image by Jellybeanz via Flickr (CC BY-NC-2.0) Orcas The answer most white shark scientists point to is the presence of orcas in the area — two in particular. Port and Starboard, as they have been named, were first spotted in False Bay in 2015 (though orcas had become increasingly present since 2009). At this time, several carcasses of broadnose sevengill sharks (Notorynchus cepedianus) were found in the bay — and the predator appeared to be the orcas. These were the first records of orcas predating on sharks in South Africa. Kock published a paper on “the first documentation of a novel feeding technique”: The killer whales were using force to break the shark’s pectoral girdle to enable them to bite out the liver, discarding the rest of the carcass. Following the attacks, sevengill sharks vanished from the bay — one of the largest known aggregation sites for this species anywhere in the world — for up to a month. Then, in 2017, five white shark carcasses washed up on the shores around Gansbaai. Like the sevengill sharks, only their livers had been removed. Teeth marks clearly pointed to orcas as the predators. Sharks’ livers are rich in fat and make up a third of their total weight, so it’s no surprise these incredibly intelligent predators target this nutritious organ. Kock theorized that the disappearance of white sharks from False Bay and Gansbaai could be due to the presence of the orcas. In 2017, there was an increase of white shark sightings further along the coast, in Mossel Bay, where they may have relocated to evade the orcas. Orca predation on white sharks has also been documented in California. Salvador Jorgensen showed that white sharks disappeared from an area when orcas were present; in 2009, 17 tagged white sharks suddenly vanished from the area around the Farallon Islands, which he linked to the presence of orcas in a 2019 paper. Jorgensen found that white sharks might disappear for up to year when orcas passed through their hunting grounds. A common theme in the study of white sharks is uncertainty. Orcas have been observed with white sharks in the past; Andreotti says she recalls sampling white sharks in Gansbaai in 2012, while orcas were also in the bay. “I had almost 20 sharks around,” she says. Kock says while there is evidence that the overnight disappearance of white sharks is related to orcas, it’s not clear-cut. “It doesn’t seem to be all orca pods that have the effect at this stage. So, sometimes we’ve got orca pods that come into an aggregation site, and there’s no change in white shark sightings.” Until last year, Port and Starboard were the only orcas thought to be having an impact on the shark population off South Africa, says Kock. “Then at the end of last year, a totally different pod that didn’t have Port and Starboard in it came into Mossel Bay. There’s a video of one of the orcas [showing] an interest in one of the white sharks around a cage diving boat. And overnight, they went from having seven to 10 different white sharks to having nothing.” Kock has been working on a paper led by marine biologist Alison Towner, which she hopes will be published later this year, revealing the extent of the orcas’ impact on the white shark population. One theory is that the shark-eating orcas are part of a different ecotype, drawn to coastal waters from deeper, pelagic waters for a variety of reasons, including changing water temperatures due to climate change and overfishing.
