Gauging the threat

How many great white sharks live at Dyer Island, South Africa?

A completely reasonable question in response to this research question would be – well, what does it matter? It matters for a number of reasons, but only one do I seem to remember – which is fortunate, because it’s an important reason.

Dyer Island (off the coast of Gansbaai, South Africa) is arguably the only place on earth where the presence of great white sharks is not seasonal. Yes, great white sharks are a migratory species and so the Dyer Island population cannot be considered a residential population in the strictist sense. But the animals do return to this location which is in no small part responsible for it being considered the great white shark capital of the world. Therefore, understanding how many individuals can be expected here is a critical step in determining the species’ global population status.

My involvement with the project was two-fold: process the data and create a short film to help promote the results.

Data analysis involved reviewing and logging fin photographs taken over several years. Each photograph was imported into a piece of software called Darwin, primarily developed for matching photographs of dolphins. I traced each fin which created a ‘shape’, which was then entered into the database to be automatically compared and sorted according to their similarity. In some cases this would give a match very quickly, whereas in others a great degree of manual reviewing was required.

We calculated a total of 532 individual sharks in the population. The model used to calculate this estimate was applied by Ryan Reisinger and I’m not going to do his work a disservice by attempting to explain it here – you’d be better off reading the publication itself.

In short, this is considerably less than informal estimates that had been made previously and my own take is that this is more likely to be an overestimation than underestimation. The reason for this is that as observed in the study itself, fins can change a lot over time and the quality of the photographs themselves can have a huge impact on their legibility. As a result, I think it’s more likely that fins weren’t matched that should have been, rather than the other way round.