As his plane climbs above the vast Rufiji delta on the Tanzanian coast, Peter Byrne tells me about the region's deep history. "In the Miocene era, the Rufiji was one of the biggest rivers on earth," he says. "Zanzibar and MafiaIsland broke away from mainland Africa - Mafia was much later but it was part of the same process." That massive geological shift is one of the reasons oil companies have the fabled spice island of Zanzibar in their sights. But it looks as if Zanzibar's smaller cousin, Mafia - where Byrne runs Kinasi Lodge, a luxury hotel - will be the first place in Tanzania to see serious oil exploration.
The Dutch arm of Shell is in negotiations with the Tanzanian government for licences to prospect four deep-sea areas or "blocks" in the Rufiji delta and another four off Zanzibar. Petrobras of Brazil is bidding for a block about 15 miles (24km) off Mafia, while the French company Maurel & Prom hopes to drill on Mafia itself and areas of Mkuranga district on the coastal mainland. In time, the whole western flank of the Rift Valley inland may be drilled, as seismic and hydrocarbon tests have shown that this too has potential for oil.
The oil in Tanzania's coastal belt was discovered in the 1960s but it is only recently, with western governments searching for alternative sources to the Middle East, that these paradise isles are being taken seriously as drilling sites. Withnegotiations on Zanzibar bogged down between the island and the mainland over which should benefit (semi- autonomous Zanzibar is unhappy with a proposed 60:40 split of profits), Mafia and its tiny neighbour Chole seem likely to be the first to see exploration, perhaps within a year.
Mafia and Zanzibar are part of a lush reef-based network of islands and atolls dotted along Tanzania's Indian ocean seaboard. A slowly growing tourist destination, Mafia is about 30 miles (50km) long and 10 miles (17km) wide, surrounded by a host of tiny islets. It has a population of 50,000. The capital, Kilindoni, is a one-horse, or half-a-horse town. There are no telephones and only a few cars.
Mafia is one of the world's richest marine habitats - home to a marine reserve run by the Tanzanian government with support from the World Wildlife Fund. As well as fish (more than 400 species) and other marine life, from dolphins to both green and hawksbill turtles, the area is home to many species of birds, including black kites and lilac-breasted rollers. There are also said to be dugongs (sea cows), among the world's rarest creatures, in these islands.
Now economically sleepy, Mafia was once a busy entrept dealing in gold and ivory from the interior, coconuts, mangrove poles for housebuilding and tortoise-shell. The last two had serious ecological impacts, but slavery was Mafia's darkest business. It was legally abolished only in 1922, four years after the first world war and the establishment of British rule on Mafia. That came after the ousting of the Germans, who had ruled from 1890, after long periods of Arab and Portuguese dominance.
Much of the archipelago's commerce, including slavery, depended on the monsoon winds that blow variously across the Indian ocean: the north-east monsoon (the kaskazi) from December to March and the south-east monsoon (the kusi) from April to November. It was these winds, filling the sails of dhows, which once made the area rich. Oil may do so again, but at what ecological cost? And will oil revenues supplement the meagre incomes of local people?
Another factor in the mix is that the region is host to two Unesco world heritage sites: Zanzibar's Stone Town and the ruins of the coastal city of Kilwa on the mainland. Shell said at the end of August that the company henceforth would avoid exploring or drilling on sites that carry these designations.
The oil business does not run smoothly - or cleanly - elsewhere in Africa, especially in Nigeria where the bulk of exploration takes place. Vast tracts of countryside have been despoiled. In the past fortnight, crude oil spilling from a ruptured pipeline burst into flames near the southeastern Nigerian village of Gio, torching crops and spreading thick, black smoke for miles. Shell workers raced to the scene to extinguish the flames but were denied access by angry villagers demanding compensation. Congo, Angola and Cameroon (where residents say exploration is already ruining fishing) are other African countries facing these issues.
At the moment Tanzania has few resources to draw on if there were an oil spill or explosion - events which test western resources in the best possible of contexts. And it is as liable to corruption as any other African country. Julius Nyeyere, the late "father of the nation" and the country's first president, was scathing: "Corruption in Tanzania has no bounds. Every country I visit they talk about corruption in Tanzania. Tanzania is stinking with corruption," he said in 1995. Things have got much worse since then, with increased economic growth offering more opportunities for graft; what's to prevent future oil revenues in Tanzania going the way of those in the Democratic Republic of Congo, where the IMF has accused officials of creaming off as much as $1bn (650m) a year from oil revenues.
Flying above the Rufiji delta, all appears calm. The idea that oil slicks might strangle these wetlands seems impossible. But the idea of a paradise is an illusion. The area's existing economy is by no means free of ecological impacts, especially on coral. Pointing out a pair of prawn trawlers, which cause great damage to seagrass and corals - I can see the trail of brown sediment thrown up by their scraping boards - Byrne says that how Tanzania deals with its existing marine resources is of much more concern to him than threats from oil exploration. "Modern technology makes drilling far less of a worry than corruption and mismanagement by central government."
Already there have been rumblings of discontent among locals on nearby Songo Songo island, where there is a gasfield, and staff at the Mafia Island Marine Park (MIMP) are circumspect about the proposed developments. "As a conservator I'm against it," says Sylvester Kazimoto, enforcement warden at the park, "as there's bound to be negative effects. But economically - we have to look at it."
He's right. If the deals come off, they could make an enormous difference to Tanzania, one of the world's poorest countries, with an average annual income of less than 160 per person.
There are no easy answers, says Ali-Rashid Mgeni, whose role in MIMP is to persuade fishermen in the park to seek alternative livelihoods such as beekeeping, handicrafts, or building from mudbricks rather than coral, the traditional construction material. "It's a tough question. You know we are fighting to eradicate poverty here. One of the reasons we are protecting this environment is because we want it to sustain our lives and those of future generations. We need a special plan or else local people won't benefit."
Hearing that oil surveyors have been to the nearby island of Chole, I jump on a MIMP patrol-boat going there. I find a man called Miwadi Juma in the harbour. Now 30, he has been fishing these waters since he was 11. He's sitting on the spar of his boat Mwafaka - meaning "reconciliation" in Swahili. Behind me, throttled by a mass of fig-tree roots, loom the ruins of the German customs house and an old prison.
Spearing white cockles with its long black beak, a wading-bird pokes about in the shallows as Miwadi talks. "I have heard about this oil, and it is worrisome. I am afraid it will be like Songo Songo, where the island was divided into two parts. One part is where the gas is extracted and the other is where people live; some of them have been moved. They have sold their farms and houses, but not very willingly. This island is very small and people will be angry if it happens here."
Ibrahim Imani, MIMP's liaison officer on Chole, recalls the visit by the oil surveyors. "They used bombs to do the survey. I remember there were explosions right round the island." Hassan Nahuda, a fisherman, remembers them too. "During that time, some fish died from the explosions."
Thomas Chale, who runs the WWF micro-loans scheme which enabled Hassan to trade in his damaging seine net for a more environmentally-friendly gill net, is too aware of the economic benefits of the oil to dismiss it. "We know that fuel resources are one of the best ways for a country to improve itself. It's a matter of finding the best utilisation. If environmental impact studies can be done, then the whole thing might be managed safely. Once that has been taken care of there is the question of local people. We wouldn't want it to be like the case of those Ogoni people in Nigeria. All they see of the oil money is pipes flying over them."
Under a tree heavy with fruit bats, I wait for Johany Rajabo, Chole's nursery-school teacher, to finish class, then we chat as the kids run about around us. You can see western influence here (one child has a Teletubbies rucksack) but the survey team didn't exactly bond with the locals. "That group of oil explorers who came here," says Rajabo, "they didn't talk to anyone." He says he doesn't know anything about the ecological effects of oil drilling, but understands that it can be a way of raising a country's standard of living.
I make my way back to the harbour. Miwadi Juma and the Mwafaka have set sail. Other fishermen are cutting up stingrays on the sand - their livers are a delicacy. The dried tails were once used to lash criminals in the prison. As I board the MIMP patrol boat, I wonder whether the appearance of oil rigs will bring pain or pleasure to these islands. The signs aren't that good, though most people seem prepared to welcome the development. "I think it would be better to get the oil and be rich," says Abdul Bwaki, a security guard. Even if it destroys the beauty of the islands? "Yes, even if it means that, though it would be better for that not to happen."
Commerce or conservation? It is not a simple stand-off, not least because oil companies are now much more alert to environmental issues than they used to be. Many sponsor environmental programmes. And as I learn on my return to Mafia, deep-sea rigs can sometimes be an ecological benefit. "Fish collect round structures like rigs," says Audie Murphy, a diving instructor at Kinasi, "they can act as artificial reefs, which is important when coral is being damaged, as a lack of coral has a massive effect on marine diversity."
Back in the MIMP offices, enforcement warden Sylvester Kazimoto isn't persuaded. "It all depends on how much of your natural resources you are willing to sacrifice for the national economy, and what dangers you are prepared to expose yourself to. Yes, a rig can act as a fish-aggregating device but it's not 100% safe. Anything can happen over time, such as big spills or gradual leakages."
It may be that the monsoons which sustained Mafia in the past are what will protect it when oil drilling begins, as the drift currents they produce are not directly onshore. But more general trade-wind patterns do come onshore, and air pollution can be as much of a problem from oil rigs as water pollution.
If there is to be a mwafaka , or reconciliation, between economics and conservation, the ecology of the whole coastline needs to be considered, not just that of the marine park. This process will involve understanding that money and ecology must work in concert. Once again the environmental dictum holds: "It's all one."