Photo identification uses the flank region just behind the fifth gill slit, where the spot pattern is dense and consistently visible. The matching software adapts the Groth algorithm, developed for the Hubble Space Telescope to match patterns of stars, treating each spot as a point and searching for triangles of points with the same geometry between images.
The approach underpins a global photo library to which divers and tour operators contribute, and it has produced multi-decade resight histories for individual animals. Because it needs only a clear photograph of one flank, it costs nothing beyond a camera and does not require tagging or handling.
What photo identification cannot do is measure a population directly. It gives sighting histories at aggregation sites, which are heavily biased toward juvenile males in shallow water; adult females and deep-water behaviour remain badly undersampled. Where whale sharks mate and give birth is still largely unknown.