Study uses dorsal fins for great white shark population estimate

The following is a press release from Dyer Island Conservation Trust (DICT)

Fin recognition image
Fin recognition software was used to identify individual white sharks.
(Photo courtesy of Dyer Island Shark Trust)

Pioneering study in South Africa shows population could be 50% lower than previously thought.

Gansbaai, Western Cape, South Africa – The global population of great white sharks  – generally estimated at 3000-5000 – may have been significantly overestimated.  As a result the great white shark – already listed as a species “vulnerable to extinction in the wild” by the International Union for the Conservation of Nature (IUCN) – may be in greater danger than has been previously recognised.

This is the implication of a pioneering study conducted by marine biologists from the Dyer Island Conservation Trust (DICT) working with the shark cage diving company Marine Dynamics shark tours, in the world’s densest population of great white sharks at Gansbaai, near the southern tip of Africa.  The study is published in PLOS ONE today.

The Trust’s marine biologists conducted a non-invasive study by collecting more than 20,000 photographs of great white shark dorsal fins between 2007-2012 from Marine Dynamics shark tours vessel, Slashfin.  As each dorsal fin is unique to each shark, the researchers adapted a computerised fin recognition programme previously used on dolphins to accurately identify individual sharks.  This analysis took more than three years to complete.

Only 532 individual sharks were identified over the five year collection period.  As individual great white sharks are not resident in Gansbaai, DICT marine biologists used open population statistics to extrapolate their findings to estimate the number of great white sharks in this densely populated area is most likely less than 1000 individuals.

“These results came as a surprise to everyone as previously unpublished but widely accepted estimates based on non-computerised photographic studies predicted the population was twice this number,” a member of the team, Alison Towner, commented.

As global population estimates of the great white shark have been calculated without accurate data provided by such a dorsal fin recognition programme, the DICT’s results are very worrying as they suggest that current estimates of great white sharks numbers may be significantly exaggerated.

Implications for conservation and protection of Great White shark

The species is already classified as “vulnerable” in the Red List issued by the IUCN, but now the Trust believes that South Africa could – and should – take the lead in urgent, scientific re-evaluation of the threat to the great white shark.

The founder and chair of the Trustees of the DICT, Wilfred Chivell said:  “Since 1991 when South Africa became the first country to protect Great White sharks, South Africa has been at the forefront of the study and conservation of the species.  Now, for the first time, we have scientific evidence that the threat is greater than was previously perceived.   At the Trust, we are contributing to the research urgently needed to allow effective, evidence-based conservation policies and interventions.  This requires the active support of government at home – who are mandated to protect this species – and form them to urgently develop their leadership in international conservation forums.  It is time for South Africa to take the initiative, because time is clearly not on the side of the great white shark.”

The Trust is continuing to collect data which is being contributed to the national white shark dorsal fin database being analyzed by NMMU Ph.D. student, Rabi’a Ryklief.  This project will be the first national estimate of white sharks based on dorsal fin identification.  Additionally, the Trust is sending their results to international conservation organizations such as the World Wildlife Fund through WWF SA and the IUCN and CITES in order to inform their efforts to protect white sharks globally.

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