Numbers don’t add up: About 12 years ago more than 400 great white sharks were seen off the coast of Gansbaai. . The population may have moved eastwards but the shark counts there don’t reflect this.(Photo by: Education Images/Universal Images Group via Getty Images)
A group of marine biologists has challenged the findings of a recent study claiming South Africa’s white shark population has not decreased, but has moved eastwards to flee shark-eating orcas.
In September 2023 the study, Decline or Shifting Distribution?, was published in the journal Ecological Indicators. It was followed by an article in Nature, Orcas Blamed For Missing Great White Sharks, and another in The Conversation, South Africa’s Great White Sharks are Changing Locations.
But, in the rebuttal article by the group of marine biologists, titled Uncertainty Remains For White Sharks in South Africa and also published in Ecological Indicators, the authors highlight several problems with the methods and inferences made in the September study. They argue that the data cannot support the claims that the white shark population is stable and has merely moved from the west to the east.
The authors raised these issues given how it could change the way South Africa manages the population of the apex predators, said the article’s lead author, Enrico Gennari, of the Oceans Research Institute and the department of ichthyology and fisheries science at Rhodes University.
“If the declines in white shark sightings seen in former hotspots are actually representative of the population, then conservation action is urgently needed.
“If we didn’t do the rebuttal, that article which was published last year would have been used by the department of forestry, fisheries and the environment … [but we cannot say], ‘The white sharks are fine, we don’t need to do anything. [It’s] mostly about the orcas and they’re just moving east,’” Genari said.
“We don’t necessarily say the population is not stable. However, if we look at the disappearance of white sharks from the Western Cape, if we look at the decline in white sharks in the nets and drumline catches [in KwaZulu-Natal], if we look at the decrease in average mean size of females caught in the nets, these are all signs …. of a population that is not in good health and is mostly likely declining,” he warned.
One of two objections the authors cited concern the claim that South Africa’s white shark population has migrated eastwards. The first has to do with the two datasets in the September 2023 study.
“One cannot infer that an increase in the number of shark sightings in one spot — in this case Algoa Bay — is directly comparable to a reduction in shark sightings in another spot such as False Bay,” Gennari said. “Putting it simply, a decrease of, let’s say, 80% from 100 individuals at location A cannot be the same as a 80% increase from 10 individuals at location B.”
This is because a significant increase in shark sightings in one spot could be caused by a variety of reasons, including improved technologies such as the use of aerial drones, the authors said.
Other potential factors that could have contributed to an increase in the number of white sharks in Algoa Bay include the establishment of Marine Protected Areas since 2004 and the completion of a new port in 2006.
According to the rebuttal, during the period of increased sightings in Algoa Bay (2017 to 2019), anglers in Algoa Bay reported a maximum of 59 white sharks in a single year (some of which could have even been of the same individuals). In contrast, the Western Cape experienced a decrease from an average presence of several hundred individual white sharks to less than 10 sightings a year.
Kogel Bay near Gordon’s Bay Western Cape South Africa, Shark spotter lookout point on a clifftop. (Photo by: Education Images/Universal Images Group via Getty Images)
The authors cited a previous study in Mossel Bay, which identified 261 unique individuals from 2008 to 2010; two population studies in Gansbaai identified 532 individuals from 2007 to 2011 and 426 individuals from 2009 to 2011 and, finally, 303 white shark individuals were identified in False Bay from 2004 to 2012.
“If the entire white shark population was indeed regionally stable and those sharks previously observed in the West had redistributed toward the Eastern Cape, one would have expected the numbers of white sharks in Algoa Bay to be about tenfold higher,” read the rebuttal.
The rapid decline of white shark presence in the Western Cape has had “recorded ecological consequences” and a negative effect on the white shark cage-diving industry and the livelihoods depending on this industry.
On whether the apparent white shark redistribution eastward was driven by shark-eating orcas, the authors point out that the onset of white shark declines in False Bay (2012-13), Gansbaai (2013-14) and Mossel Bay (2015) predates the first appearance of orcas in False Bay and in Gansbaai in 2015, and in Mossel Bay only in 2017. In other words, the alleged cause cannot appear two years later than its effect.
“While we agree that orcas have likely influenced white shark numbers and behaviours, and at least temporarily displaced many from their historical aggregation sites, the data as currently presented do not suggest that orcas are the primary driver of the declines in white shark observed in the Western Cape.”
Sara Andreotti, a marine biologist in the department of botany and zoology at Stellenbosch University and one of the co-authors of the rebuttal, said they are worried about the effect of the shift-of-white-sharks-to-the-east narrative on conservation efforts.
“Our concern is that unsupported claims of population stability could jeopardise conservation actions urgently needed for white sharks.
“There is no evidence of the hundreds of white sharks counted in False Bay, Gansbaai and Mossel Bay 10 years ago to be aggregating now somewhere else along the South African coastline. ”
Andreotti said that sightings differed from photo-identification of dorsal fins, as her work had shown. “When you base your work on your sightings, you can double-count the same individuals multiple times. So one shark, seen by three or four people, immediately becomes the shark sightings for the day and people think there are four sharks.
“With photo-identification you can distinguish one [shark from] another. The ones that I counted in Gansbaai, they were individually different sharks and there were 426 of them 12 years ago. We no longer see them in False Bay, we no longer see them in Gansbaai, there are few in Mossel Bay. We should have at least 400 bunched up in the east and we’re not seeing 400, we’re not even seeing 50 at the same time, not even 20. The numbers are always five or six, not even 10,” Andreotti argued.
“While people mention sharks in Plettenberg Bay to argue that we still have sharks, we say, yes, but how many? That’s what we needed to point out with this work.”
In 1991, South Africa became the first country in the world to protect white sharks, based on a precautionary approach that recognised the species’ ecological and socio-economic importance.
“We are not saying that the population is definitely declining,” Gennari stressed. “We’re just saying that a precautionary approach might be needed, which is exactly what South Africa did in 1991 when they said: ‘We don’t know what is happening but because this fish is so important for the marine environment and for the economy, we decided to protect it.”
But after 1991, the KwaZulu-Natal Sharks Board was allowed to continue killing white sharks, at an average of about 30 a year, while a conservative estimate with the coastal shark longline fishery is that 40 to 50 white sharks are being killed.
“So, we’re talking a number close to 100 white sharks [being killed],” said Gennari.
The authors are urging authorities such as the forestry, fisheries and the environment department to take a precautionary approach in light of the declines in white shark sightings from their historical hotspots.
This approach must also factor in the consequent negative effect on the country’s eco-tourism economy and the reduction in sightings of large mature white sharks, both in the Western Cape and Eastern Cape.
Furthermore, the conservation approach adopted should consider the very low genetic diversity of this white shark population; the historical and current unsustainable levels of white shark deaths from the lethal shark control programme of the KwaZulu-Natal Sharks Board and the white shark deaths caused by the fishery industry.