Mafia Island in Tanzania is one of those whale shark hotspots where divers are almost guaranteed to bump into one of the charismatic giants when sinking below the surface. The world’s largest fish aggregate here to feed, as Dr Chris Rohner, principal scientist with MMF, an organisation created in 2009 to research, protect, and conserve the populations of threatened marine megafauna around the world, points out: “We see mostly large juveniles at Mafia Island, ranging from 2.5 to 10m long. Most of them are somewhere between 5 and 8m. Whale sharks are born at ~60cm, so we don’t see the small ones, and we also don’t see the large, mature sharks in the area. This is similar to most other coastal whale shark aggregations around the world. At that life stage, they are mostly interested in eating lots of plankton and growing up as fast as they can. At Mafia Island, there are dense patches of sergestid shrimps, a type of large zooplankton, which the whale sharks target. There seems to be food available for whale sharks here throughout the year, which is probably why many of them stick around.”

The unusually resident whale shark population off the island allows the MMF to apply robust capture-mark-recapture modelling techniques to estimate its population trend; an important field of research that seeks to find out more about the relationship between the rare mortalities of the group that seem to nevertheless keep the population from recovering. Rohner says: “Mafia Island was the perfect place for this investigation. We see many of the same whale sharks year after year and have gotten to know them really well.” MMF senior scientist Dr Steph Venables adds: “These high re-sighting rates also mean that the models estimate abundance with a high level of precision.”