There are half as many great white sharks as previously thought and the number of sharks is not enough to ensure a healthy population with the number of breeding animals numbering only 333 around the southern African coast.
This is according to new research by Sara Andreotti of Stellenbosch University.
“This is not what I was expecting,” she said. “This is supposed to be the white shark capital of the world.”
Dr Andreotti spent six years to gather this information, working closely with shark behaviour specialist Michael Rutzen of Shark Diving Unlimited in Gansbaai, sometimes living at sea for up to two months at a time.
Initially, between 2009 and 2011, they concentrated in the area around Gansbaai, with Dr Andreotti relying on Mr Rutzen’s expertise in getting close enough to the sharks so she could photograph their dorsal fins. The fins have notches and each is unique, so they operate as a kind of a shark “fingerprint” enabling identification.
There have only been two studies on how big the shark population is southern Africa is. The first, said Dr Andreotti, was in 1999 by the KwaZulu-Natal Sharks Board which estimated 1000 sharks. The other, published in 2013, was off Gansbaai which showed the same number.
This was why, when her count got to around 400 and didn’t get any higher, Dr Andreotti did a double-take.
So she decided to look into how the sharks had been counted in Gansbaai and, as a supplementary paper feeding into her research, found that the software that was used, comparing photos of dorsal fins, was developed for dolphins and would identify the same sharks as different sharks.
Her method of identification was more specific for the species.
“I identify them manually with photos, double checking them one by one in my database to see if it’s a new individual,” said Dr Andreotti. At the point where she had 400 sharks on her database, she found that she was photographing the fins of the same sharks over and over again – there were no more sharks. Human error was ruled out by mathemeticians on the team who programmed an ad hoc algorithm to check the data directly from the data folders of each shark.
Was this because that there were only 400 sharks in Gansbaai, or were these the same sharks that were roaming the southern African coastline? To make sure, Dr Andreotti and Mr Rutzen spent another four years sailing along the coast, between Port Nolloth and Algoa Bay, collecting biopsy samples from the sharks, concentrating on five known sites where sharks were found: Gansbaai, False Bay, Struisbaai, Mossel Bay and Algoa Bay. Sailing close to the sharks, they would poke the sharks with a small sterilised biopsy sampler at the end of a pole to get a small genetic sample. The sharks rarely notice this, she said.
One hundred and fifteen field trips out to sea led to 302 tissue biopsies, found to belong to 233 unique white sharks (duplicates were confirmed using both photo-identification and genetic fingerprint techniques).
It turned out that the sharks seen at Gansbaai were also seen along the coast, establishing that there is only one shark population.
The bad news is that the genetic count showed that there are too few sharks to ensure a robust population. The “effective population size”, that is the number of sharks who contribute to offspring in the next generation – the number of breeding individuals – is only 333.
Dr Andreotti said that although they don’t know numbers needed for sharks to have a healthy breeding population, research on other species indicated that a minimum of 500 were needed. This to ensure sufficient genetic diversity so that if there is a sudden change, for example in the environment or a disease, that all sharks will not react in the same way and that there will be some survivors.
“We have come to the conclusion that South Africa’s white sharks faced a rapid decline in the last generation and that their numbers might already be too low to ensure their survival,” said Dr Andreotti. “The chances for their survival are even worse than what we previously thought.
“Given the low population numbers of white sharks along the South African coastline, we predict a negative effect on the ecological stability of the marine environment in this region.”
Alison Kock, research manager of the Shark Spotters programme, and Johan van der Merwe, the City’s mayoral committee member for energy, environmental and spatial planning, said the research would not result in any changes to the Shark Spotting Programme at this time.
“False Bay and Cape Town waters remain a high natural aggregation area for white sharks and their presence in our waters have been relatively constant over the last 12 years,” said Mr Van der Merwe. He said the outcome of the study would not change their approach to integrated beach safety, but did highlight the importance of the Shark Spotter programme in that it was involved in long-term data collection, extensive research tagging, tracking and monitoring sharks and keeping sharks in the public eye.
Dr Kock said the research highlighted the need for ongoing monitoring of shark sightings from different places and sources.
“Counting sharks is difficult, especially for the whole South African coastline. Different methods can yield different results, which is why there are currently conflicting studies on the actual number of individuals.
“However, regardless of the actual number, it is important to know what the trend in the population is: is it decreasing, increasing or stable?
“The recent research does not provide any information in this regard and further research is needed to determine this.
“Last year we recorded 104 different sightings at our beaches, which resulted in 75 temporary beach closures.
“This year so far we have had 36 individual sightings. We typically experience low to no shark sightings over winter months, but expect sharks to start visiting our beaches again in spring as they have done in previous years.”
* Dr Andreotti is the lead author of “An integrated mark-recapture and genetic approach to estimate the population size of white sharks in South Africa”, with Shark Diving Unlimited’s Michael Rutzen, university colleagues from the applied maths and evolutionary genomics group Stéfan van der Walt, Sophie von der Heyden, Romina Henriques, Herman Oosthuizen, Conrad Matthee as well as Department of Environmental Affairs’s Mike Meyer.
It was published in the latest Marine Ecology Progress Series.
