The bay
Chole Bay is the large sheltered basin on the eastern side of Mafia, the island’s natural deep-water harbour, protected from the open ocean by Juani and by the reefs along its eastern edge. Almost all of Mafia’s diving and snorkelling happens in it or in the passes that drain it.
It is shallow. Mafia Island Marine Park’s management plan records that more than half the park’s marine area lies in less than twenty metres of water, Chole Bay included, with a deeper channel of twenty to thirty metres carrying tidal water in through Kinasi Pass and across the bay as far as the Chole channel. Spring tides average 3.3 metres and reach 4; neaps average around 1.5. That is a great deal of water moving through a narrow gap twice a day, which is what makes the passes dive sites and the bay calm.
A figure to avoid: an area of 50 square kilometres and a maximum depth of 28 metres are widely quoted for the bay. Neither could be traced to a source that opens. The management plan gives no area for it at all.
Why it dives so well
The park’s own assessment is unambiguous. Chole Bay and the outer part of Kinasi Pass have higher habitat and species diversity than the rest of the park and are less disturbed; the deeper reef walls in the bay and on the outer reef are described as still relatively pristine; and because the bay holds coral reef, mangrove, seagrass, algae, sponge and soft coral beds together, it contains examples of the majority of tropical marine ecosystems.
The zoning follows that. Most of the bay is a Specified-use Zone rather than a core zone, taking in the Kulawe and Msumbiji reefs, the Utumbi reef and the Chole and Kinai walls along Kinasi Pass, and the shallower reefs between the pass and Juani. The plan calls these collectively the most species-rich coral reef habitats in the marine park. The stricter Outer Kinasi Pass core zone begins at the bay’s mouth.
The park is candid about what has damaged them: dynamite fishing historically, and more recently bottom-dragging seine nets.
The fish count, and which number to use
Mafia is commonly advertised as having more than 400 fish species, a figure that comes from Mafia District Council’s own list of the island’s attractions. The marine park’s management plan gives 380 species of fish and 48 coral genera, alongside 12 seagrass species, 134 marine algae, 130 birds and 656 higher plants. Both are official; the management plan is the more specific document and 380 is the number this site uses.
What it means for a trip
The practical consequence of the shelter is the single most useful fact about diving in Tanzania: Chole Bay keeps working through the strong south-easterly kusi winds from roughly June to September, when exposed and ocean-facing sites elsewhere on the coast become unworkable. Diving and snorkelling cover that in full, and the bay is also among the best sheltered paddling in the country.
Marine park rules apply throughout, and a boat should be able to say which zone it is in and what is permitted there.
Common questions about Chole Bay
How deep is it?
Mostly under twenty metres, with a channel of twenty to thirty metres running in through Kinasi Pass and across to the Chole channel. The often-quoted maximum of 28 metres has no traceable source.
Is the whole bay a no-take zone?
No. Most of it is a specified-use zone. The stricter core zone, where restrictions are tightest, is at the mouth around the outer part of Kinasi Pass.
How many fish species are there?
The marine park’s management plan says 380, with 48 coral genera. The district council’s attraction list says more than 400. This site uses 380, from the more specific document.
Why can you dive here when other sites are closed?
Because the bay is enclosed by Juani and its reefs, so it stays workable through the kusi winds of roughly June to September that make ocean-facing reefs rough. It is Mafia’s structural advantage over the rest of the coast.