South Africa’s great white shark may be under threat, and while that may be music to the ears of anyone terrified by the movie Jaws, this actually may have dire consequences for the balance of the local ecosystem.
More than 90% of South Africa’s great white sharks share the same genetic sequence, threatening their genetic diversity and the future of the unique type of great white found in South Africa’s waters, according to research out of Stellenbosch University.
Great white sharks are “important for the balance in the ecosystem”, says Dr Sara Andreotti, who undertook the research as part of her PhD. “They get rid of sick individuals from other species and they maintain a balance in the population numbers.”
“If the top predator [in the ecosystem] is healthy, the rest of the food chain is healthy to sustain the top predator,” she explains.
However, the lack of genetic diversity is cause for concern. Genetic diversity is considered important because it indicates a population’s resilience: if there is a change in environmental conditions or the sharks are exposed to diseases, greater genetic diversity means it is more likely that some members of the population will survive.
Andreotti’s research, which has been published in the Journal of Biogeography, found that “we have one population of white sharks around South Africa. There is no such thing as False Bay or Gaansbaai sharks. Just South African [great white] sharks, we see the same sharks [in all these places].”
Within this shark population, there are four maternal lineages, with very low genetic diversity, she says.
Asked why, she says it could be an historic extinction and then recolonisation or a recent occurrence.
“South Africa is a very strange place: Africa is a very strange place: during glaciation [during an ice age], ice from Antarctica touched the base of the continent. Many of our species went extinct or had to migrate. It could also be a recent bottleneck that wiped out the diversity,” she says.
However, “one maternal lineage is unique. It does not link with anything else in the world…. It is either an ancient high diversity or [from sharks in] a place that hasn’t been sampled yet.”
For four years, Andreotti – with shark expert Michael Ratzen – has sailed the country’s coastline, initially taking more than 5,000 pictures of the sharks and then later genetically analysing them.
With the photos, Andreotti and Ratzen developed a novel method of identifying sharks from pictures: thenotches on their dorsal fins, which Andreotti describe as the shark-equivalent of a “fingerprint”.
Once they had identified about 240 great white sharks that roam the waters around South Africa’s coast, they decided to sample them for DNA. From a boat, “you have a 2.5m-long pole, and you jab them”, she explains. On the end of the pole, there is a “screw the size of a pinkie” which takes a tissue sample from the shark.
From her research, “it is obvious that current conservation measures should take the low levels of genetic diversity into account, otherwise one of these days we will not have any white sharks left to worry about”, she says.