Six years ago, I was lucky enough to see South Africa’s majestic great whites while cage-diving off the coast of Gansbaai. This small town is at the heart of the country’s lucrative shark-diving industry. The visibility was poor and the water was freezing, but nothing could dampen the thrill of watching those torpedo-shaped sharks materializing out of the gloom just a few feet from the cage bars.
South African great whites are in trouble
Now, a study led by Stellenbosch University researcher Sara Andreotti reports that South African great whites may be facing extinction in these waters. Published in the journal Marine Ecology Progress Series, the results of Andreotti’s six-year project show that the South African great-white population has declined to between 353 and 522 individuals. To reach this conclusion, Andreotti and her research partner Mike Rutzen spent two years studying sharks in the Gansbaai area. They spent another four years following individuals in a sailboat along the east coast.
The team identified individual sharks by using mark-recapture techniques to organize nearly 5,000 dorsal fin photos into a comprehensive database. Each dorsal fin has unique markings. These allow scientists to distinguish one shark from the next, so Andreotti was able to determine a population estimate with 95 percent confidence. Unfortunately, the results showed a 52 percent decline when compared with previous mark-recapture studies. This led Andreotti to conclude that “the numbers in South Africa are extremely low. If the situation stays the same, South Africa’s great white sharks are heading for possible extinction.”
What’s to blame?
Andreotti corroborated the results of the mark-recapture study with genetic data gleaned from biopsy samples. She thinks a number of factors are behind the species’ rapid decline. These include the shark nets and baited hooks used to protect bathers and surfers along South Africa’s eastern seaboard. Between 1978 and 2008, these culling mechanisms killed 1,063 great white sharks. Shark nets in KwaZulu-Natal killed an average of 20 white sharks per year between 2010 and 2014. A reduced death toll in recent years is further proof of the sharks’ decline.
Poaching for trophies like shark jaws is another issue, the study says. This is despite the fact that in 1991, South Africa became the first country to make great-white shark fishing illegal. More insidious factors are also exacerbating the problem, including overfishing. In 2011, the World Wildlife Fund declared 50 percent of South Africa’s marine resources fully exploited. As fish stocks continue to dwindle, it is possible that there are fewer Cape fur seals. These make up the bulk of the sharks’ diet. A key reason for the decline of South African great whites in particular, however, is their limited genetic diversity.
Is there anything to be done?
Results from the analysis of Andreotti’s biopsy samples show that South African great whites have the lowest genetic diversity of any white shark population. Just 333 individuals contribute to their overall gene pool. Diversity is key, as it increases the likelihood that individuals can adapt in the face of disease or environmental changes. This could also explain why populations elsewhere remain relatively healthy. According to Andreotti, “previous research on other species indicates that a minimum of 500 breeding individuals is required to prevent [the problems caused by] inbreeding.”
If this proves true for great whites, Andreotti believes that “their numbers might already be too low to ensure their survival.” As key apex predators, great whites are essential to South Africa’s marine ecosystem. If they disappear, the ecological ramifications would be disastrous. Andreotti believes that urgent management measures are the South African population’s only hope — and the only hope that future generations have of experiencing their magic as I did.