Where it is, which is not where people assume
Tanzania’s whale shark aggregation sits in Kilindoni Bay, on the inshore western side of Mafia — the opposite side of the island from the diving. The bay runs roughly between Ras Kisimani in the south and Ras Mbisi in the north, is shallow at under thirty metres, has a sandy bottom and a wide mangrove-lined intertidal zone, and lies about twenty kilometres east of the Rufiji delta.
The core habitat is close in. Research tracking tagged sharks found it extending only about a kilometre and a half from Kilindoni harbour, with two activity hotspots, one inshore at around fifteen metres depth and one offshore at around twenty-five, inside an array spanning under ten kilometres from inshore to offshore.
The “ten to fifteen kilometres offshore” figure attached to this aggregation is real but is measured from the mainland coast, not from Mafia. Repeated as a distance from the island it puts the boats in the wrong water.
The fact that ought to be better known
The aggregation is outside Mafia Island Marine Park. The study that mapped the sharks’ home ranges states that none of the core home range falls within the park’s boundary, and notes why: the boundaries were drawn before the presence of whale sharks in Kilindoni Bay was widely known, and were designed to protect coral reef habitat rather than megafauna. The word “whale” does not appear anywhere in the park’s 2011 management plan.
So the protection a visitor might assume is covering this activity is not. What governs it instead is a code of conduct produced by the marine conservation organisation working here with a megafauna research foundation, held at island hotels and carried on the boats, alongside a later management strategy and an operators’ network. The whale shark guide sets out the code’s substance and how the morning actually runs; the version briefed to you on the boat is the operative one.
Why the sharks are here, stated carefully
The Rufiji is the usual explanation: nutrient-rich outflow drives plankton, plankton draws sharks. The researchers are less certain than the brochures. The prey study found the sharks targeting dense surface patches of a sergestid shrimp and did not invoke the river at all; a later paper proposes wind as the driver of the bay’s productivity; and a conservation report working here noted that the origin of the productivity in Kilindoni Bay remains to be determined, and that the seasonality of the Rufiji’s peak flow and of high whale shark sightings do not overlap.
What is not in doubt is what it produces: green water rather than blue, because the plankton that brings the sharks is the same thing that reduces visibility.
The population
Small, resident and very faithful to this bay. Photographic identification from 2012 to 2019 catalogued 207 individual sharks, 89 per cent of them immature males, with acoustic tagging showing individuals present for up to sixty consecutive days and only one long-distance movement recorded across the whole catalogue.
Two cautions follow. Rising catalogue totals are sometimes reported as a growing population; they are cumulative identifications and grow with survey effort, and no study claims an increase. And the same site fidelity that makes a sighting likely is what makes this group unusually exposed to boat strike, entanglement and crowding — the same animals come back to the same water, so the pressure lands on a known population rather than being diluted across an ocean.
Common questions about the whale shark area
Where do the boats go?
Kilindoni Bay, on the western side of Mafia, close inshore — the core habitat extends about a kilometre and a half from the harbour. It is the opposite side of the island from Chole Bay and the diving.
Is it inside the marine park?
No. The study that mapped the sharks found none of their core home range inside the park, whose boundaries predate knowledge of the aggregation and were drawn for coral reefs. The activity is governed by a code of conduct rather than by park rules.
Is the Rufiji why they are here?
It is the usual explanation and it is less settled than it sounds. The prey study found the sharks feeding on sergestid shrimp without invoking the river, later work proposes wind, and one report notes the river’s peak flow and the peak sighting season do not overlap.
How many sharks are there?
207 individuals were photo-identified between 2012 and 2019, 89 per cent immature males. That is a cumulative catalogue rather than a population trend, and no study describes the population as growing.