Whale sharks are ocean wanderers. They swim about 25 kilometres each day; around ten thousand kilometres per year. That’s the distance from London to Cape Town. Some of them, though, just swim around in circles. Mafia Island, 20km off the Tanzanian coast, is home to what may be the world’s laziest whale sharks. Or possibly the smartest. Potentially both. This “Mafia” has no particular association with organised crime. Rather, the island’s name is thought to be derived from a Swahili phrase that translates, roughly, as “healthy dwelling place”. Mafia Island, slightly under 50km in length, is smaller, less populated, and feels a world away from the bustling tourism industry of its big brother, Zanzibar, to the north.

Mafia is a quiet place. The small airport welcomes arrivals from Tanzania’s largest city, Dar es Salaam, with a statue of a whale shark installed on the island’s one and only roundabout. These sharks, known as “papa potwe” on Mafia, are an iconic species. They are the world’s largest cold-blooded animal, growing to around 20 metres in length, and the largest fish that has ever lived. Despite their enormous size, whale sharks are remarkably inoffensive. Their diet consists of zooplankton, tiny animals that get swept around by ocean currents, and small fishes.

Though whale sharks are true sharks, they are so placid that almost anyone, regardless of previous snorkelling experience, can safely enter the water and swim with them. Their gentle nature has made the whale shark a popular focus of marine wildlife tourism. Sadly, these same qualities, coupled with valuable fins and meat, have resulted in people hunting whale sharks in all sorts of inventive ways, with major fisheries having occurred in mainland China, India, the Philippines, and Taiwan. Around half of the world’s whale shark population has been killed since the 1980s. They were classified as a globally endangered species in 2016.

Mafia Island, though, still has whale sharks. All the time. My work at Mafia began in 2012. Dr Chris Rohner and I, both from the Marine Megafauna Foundation, won a research contract advertised by WWF Tanzania. Our assigned objectives were to find out what the whale sharks were doing off Mafia Island, and to identify the factors that influence their abundance.

We teamed up with Liberatus Mokoki, owner of The Whale Safari, who has been taking tourists out to see the whale sharks since 2004. “Whale shark season” at Mafia is from around October to February. Over this period, the sharks are routinely seen feeding at the surface, heads protruding and dorsal fins carving through the water as they’re propelled by wide sweeps of their enormous tails. They’re very easy to find. We can sometimes watch them swimming by from our breakfast table, onshore.

For the sharks, this swimming requires an enormous cost in energy. Whale sharks are, necessarily, masters of conserving effort. The stereotypical tropical ocean – warm, blue and clear – is a biological desert. Warmer water carries a lower oxygen content, and tropical waters are typically low in vital nutrients, which means less phytoplankton, the tiny ocean plants. Without an abundance of phytoplankton at the base of the food web, fewer large animals can be supported.

A giant shark needs a lot of food. That’s why whale sharks migrate. Whale sharks seem to be less concerned about what they’re specifically eating, and more focused on how much. They will travel vast distances to take advantage of short-lived explosions of productivity, whether that be plankton blooms, fish spawning events, or schooling baitfish.