Whale sharks swim about 25 kilometres each day. Around ten thousand kilometres per year. That’s the distance from London to Cape Town.
But some of them just swim around in circles.
Mafia Island, 20 km off the East African coast, is home to what may be the world’s laziest whale sharks… or possibly the smartest. Potentially both.
This ‘Mafia’ isn’t associated with organised crime. Rather, the island’s name is derived from a Swahili phrase that translates, roughly, as ‘healthy dwelling place’.
Mafia is quiet. The island feels a world away from the bustling tourism industry of its big brother, Zanzibar, to the north. A small airport in Kilindoni, the main town, welcomes arrivals from Dar es Salaam with a statue of a whale shark installed on the island’s one and only roundabout.
Whale sharks, known as ‘papa potwe’ on Mafia, are an iconic species. Growing to around 20 m in length, they are the biggest fish that has ever existed, and the world’s largest living cold-blooded animal. 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.
Whale sharks are so placid that 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 around the world. Major fisheries have operated in India, the Philippines and Taiwan, and are still ongoing in mainland China.
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 at Mafia, and to identify the factors that influence their abundance.
We teamed up with a research group from King Abdullah University of Science and Technology (KAUST) and 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 about October to February. Whale sharks are routinely seen feeding at the surface over these months, heads protruding and dorsal fins carving through the water as they’re propelled by wide sweeps of their enormous tails. The sharks are easy to find. We can sometimes watch them swimming at the surface from our breakfast table.
For the sharks, this surface-feeding requires a lot of energy. Whale sharks are, ordinarily, 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. Together, that means less phytoplankton, the tiny ocean plants that form the base of the food web. The system can only support a few large animals.
Whale sharks are ectothermic, meaning that they don’t have to burn energy maintaining a constant body temperature. Instead, their internal temperature naturally varies with the environment. A whale shark’s metabolism is far slower than a whale of the same size.
Nevertheless, a giant shark still needs a lot of food. Whale sharks seem to be less concerned about what they’re specifically eating, and more focused on how much. That’s why whale sharks migrate in most areas. They travel vast distances to take advantage of short-lived explosions of productivity, whether that be plankton blooms, fish spawning events, or schooling baitfish.
Mafia is popular with whale sharks because of shrimp. Lots and lots of shrimp. Small sergestid Lucifer shrimp, to be exact. The shrimp group together in incredible densities. These one-centimetre translucent shrimps are impressive swimmers in their own right. For plankton, they’re quick. For the sharks, the sheer quantity of shrimp present makes it worth the effort to chase down these highly-mobile patches.
Jens Paulsen, Chris’s former masters’ student, did some preliminary tagging work with multi-sensor ‘behavioural’ tags, temporarily affixed to the dorsal fins of the sharks. These tags use a battery of recording devices, including accelerometers, gyroscopes and pressure sensors, to reconstruct the shark’s three-dimensional movements over hours to a few days.
Some back-of-the-envelope calculations indicate that, with the density of shrimp seen at Mafia, the sharks need to actively feed for only 10 minutes each day to meet their basic daily metabolic requirement. Anything after that is bonus time. The sharks spend around 4.5 hours feeding each day while the shrimp are abundant. That additional energy can be channelled into growth.
Almost three-quarters of the sharks we see at Mafia are feeding, so it’s clear that the shrimps are a powerful attraction for the sharks. Around February, though, the shrimp patches dissipate. The sharks disappear. Whale shark tourism closes down, and so does research. Everyone assumed that the sharks were leaving Mafia.
Fortunately, our research collaborators at KAUST had deployed small acoustic pinger tags on some sharks in 2012/13, and installed an array of passive acoustic receivers adjacent to the island. Each tag was uniquely coded. Whenever a tagged shark swam within a few hundred metres of receiver we knew who it was, and when it was there.
When we returned to Mafia the following October, we recovered and downloaded the receivers. It was then that we realised just how resident these sharks actually are. Many of the sharks were still there, just off Mafia, the whole time. They simply weren’t at the surface. They had moved slightly further offshore, around the limits of the receiver array.
While it’s easy to explain the sharks’ presence during “shrimp buffet season”, we don’t know much about their habits during these months in between. Even the fishermen working year-round off Mafia rarely see sharks over these months. Elsewhere, when the food source disperses, so do the sharks.
Clearly, they are likely to be switching to alternate prey. However, we don’t know what that could be. The sharks will feed on baitfish given the opportunity. They probably also feed at night, on the zooplankton that emerges from the sandy bottom at dusk. But we don’t know for sure. Ultimately, their behaviour remains a mystery at this stage.
Even in season, though, the local whale shark population is small. While we’ve counted up to 24 sharks feeding close together, less than 200 whale sharks have been identified from Mafia in total.
Only a specific subset of whale sharks make use of Mafia’s waters. Almost 90% of the sharks we’ve identified are males, nearly all between three and nine metres in length.
Whale sharks reach adulthood at about nine metres. They’re born at 40-65 centimetres. For some reason, then, Mafia is missing the youngest whale sharks, most of the adults, and almost all the female sharks. Though little is known about whale shark reproduction, I’d expect there to be an approximately equal number of male and female pups. The female sharks are out there, somewhere.
Nevertheless, despite the bromantic population structure at Mafia, the juvenile male sharks tend to loiter. Matthew Potenski, then at the Shark Research Institute, spent some time on the island tagging and photo-identifying the sharks for a couple of seasons prior to 2009. A decade on, many of the 21 sharks first identified by Matt and others during that time are still at Mafia. In fact, we re-identified more than half of those sharks during our 2016 and 2017 field periods.
Since our own work began in 2012, we’ve seen some individual sharks every single year. These days, it’s like visiting old friends. Several sharks have been re-sighted over 40 times. While most whale shark hotspots do have a few regulars, such as ‘Stumpy’ off Western Australia, who was seen at Ningaloo Reef in 19 of 22 years between 1995 and 2016, the Mafia sharks are unusually agoraphobic. Whatever the reason, many sharks spend years, or even decades, close to the island.
For management and conservation, that presents both challenge and opportunity.
Whale sharks are unprotected in Tanzania. There are multiple active threats that put this small population at risk. Several sharks are caught and killed each year off Zanzibar. In Mafia itself, there is some conflict between sharks and fishers. The dense shrimp patches that attract the sharks are also important prey for several species of small anchovies and mackerel which are, in turn, targeted by people.
The fishers aren’t trying to catch the sharks, but their large ring-nets can trap and entangle whale sharks. Of the 74 sharks we saw in 2017, 85% had scars. Most were minor, but several bore major amputations or propeller cuts that could lead to death or long-term disability. Most of the serious scars were from boat strikes, likely from when vessels were travelling too fast through feeding areas.
Others appeared to be purposeful wounds, where fins had been cut off by knives. While there is a possibility that these fins could be sold, we think it is more likely that the fins were removed while untangling the shark from being enmeshed in fishing nets.
The fishermen don’t want to catch sharks. They can wreck the net, for one thing. Because the sharks are only seasonally at the surface, and generally in a predictable area, the prospects are still good for creating a managed zone where sharks can feed safely with minimal change to local fishing and boating practices.
There is also a major upside to the sharks’ high residency. Whale shark tourism has doubled at Mafia since our study began in 2012. While this has introduced some management issues itself, more and more people are employed by this industry. The whale sharks are a major draw for tourists. There is excellent potential for the island to develop as a marine ecotourism destination. Our long-term data shows an average of 4.6 whale sharks seen per trip.
We still know little about the life of this endangered species. The fact that we can see the same sharks, year to year, potentially for the next few decades, makes Mafia a fantastic ‘natural laboratory’ for whale sharks. It’s a great opportunity to learn more about these remarkably enigmatic fish.
Mafia has long been known as a healthy dwelling place for people. With a bit of work, we can ensure that the same is true for the whale sharks as well.
Thanks to our long-term funders for this work: WWF Tanzania, Aqua-Firma, Shark Foundation, Waterlust, and two private trusts.
Like geeky animal facts and dad jokes?
I write a few articles just for my mailing list. They normally focus on something interesting, and possibly hilarious, that I've learnt about sharks (or other random animals) that week. There may also be groan-inducing jokes.
Real talk: there will be groan-inducing jokes.