This female whale shark was small enough to capture photographically in the plankton-rich water.

TONY MANOEUVRED THE ZODIAC in his usual taciturn manner. We had been staring at the empty surface of the sea for two days. The equatorial African sun scorched us incessantly. Our hands were badly reddened and I had developed an interesting oval-shaped tan where my hood exposed my face.
It was hotter than a very hot thing, and we were not tempted to crisp other parts of our bodies. Our eyes smarted from the sea spray that showered us with every wave that met our onward progress. Our backsides ached from hours of bumping about the choppy channel.
Finally, we saw what we had come for. Go now, Tom said to me, almost under his breath. Mask on and snorkel in, I dipped my camera into the water at arms length and did a less-than-elegant duck dive quietly over the side sponson of the inflatable, trying to splash as little as possible.
My legs and fins, straightened high in the air, provided me with enough thrust to send me down into the murky green water. I swam as hard as I could in the suspected direction of my target, breath held, camera still outstretched those precious extra 60cm ahead of me.
Finger on the trigger, I was shooting five frames a second at nothing. Well, nothing yet. It was like swimming in the soup my mother used to make when I was a kid - full of nutrients, lukewarm, and slightly unpleasant. A sudden rush of tiny fish and there was what I had come to see.
It knew I was there before I saw it.
I was within a couple of metres or even less before it turned swiftly away, a tantalising glimpse of that massive spotty body, brushing me with a pectoral fin, washing me with the thrust of its tail. First time lucky!
It was a small and immature female whale shark, too small to tag but small enough to allow me to capture a reasonable image in the plankton-laden water using my super-wide-angle lens.
Instantly she swung away. I wasnt to know that one picture of many was a success until I pressed the playback button on my Nikon, but it gave me something to do while bobbing back at the surface, waves breaking over my head. Meanwhile, Tony attempted to do others in the Zodiac the same favour he had done me.

THERE FOLLOWED SEVERAL ENCOUNTERS with much bigger specimens. They always sensed my presence before I could see them and would turn away, so that I tended to get only views of their massive passing flanks.
I often had to content myself with shots of the passing pectoral fin, or a few remoras or a cobia improbably balanced on the pressure wave at the whale sharks nose.
Rhincodon typus is the largest fish in the ocean. Mature females can grow to a stupendous 18m long.
They are encountered both pan-tropically and in some temperate climes, and frequent the open ocean as well as coastal waters. Most diver encounters with whale sharks are fortuitous, as they can turn up almost anywhere.
Places noted for such encounters include the Gulf of Tadjura in Djibouti, the Seychelles, Western Australia, and South Ari Atoll in the Maldives.
They are seen at places such as Darwin and Cocos Islands and off the coast of Mozambique, too, but they are often merely passing through during long ocean wanderings.
What interests marine biologists such as Matt Potenski of the Shark Research Institute (SRI) are known aggregations of whale sharks, and this is what we had come for.
Despite their grand scale, whale sharks feed on tiny plankton and the smallest of fishes. They follow the plankton as they roam the ocean, mouths agape, filtering as they go.
As plankton is encouraged by bright daylight, it thrives in warm shallow seas, so the whale sharks feed near the surface and are often sighted by seafarers.
Despite being endangered by the demand for sharkfin, there has never been any true commercial fishery for whale sharks, so little baseline data has been collected. Life-spans and growth-rates can only be guessed at.
Unlike other sharks, these spotty monsters cannot be caught on conventional long-lines, or lifted out of the water to be measured and weighed. All this make serious study tricky.
Scientists from the SRI have been attempting to study distribution and abundance patterns in the Indian Ocean through tagging since 1993. Visual ID tags have been attached to more than 200 whale sharks off South Africa, Mozambique and the Seychelles.
Whale sharks tend to congregate in the shallow, plankton-laden waters of the Mafia Channel between Mafia Island and the coast of Tanzania, and this is now a new area of investigation.
There are two types of tag, a large one with a number that can be clearly seen and reported by any opportunistic snorkeller or scuba diver, and a satellite radio tag that finally comes unattached at some later date, bobs to the surface and downloads all the data it has collected via a satellite to SRI researchers computers. These cost US $9000 each, so are used sparingly.
But first, find your whale shark! Along with a few other interested holiday-divers, I joined handsome young American giant Matt Potenski aboard mv Kairos, a diving expedition ship, out of Dar es Salaam. It is French-owned and registered in Panama.
The first evening, Matt briefed us on SRI guidelines. We should not cause the whale shark to deviate from its normal behaviour, get in front of one causing it to change direction, or get within 4m of one. Easier said than done.
At 4m distance under water in the Mafia Channel, a full-grown whaleshark could pass you by without you ever knowing it was there. The only successful technique to achieve a visual encounter was to drop into the water some way ahead of an animal and swim furiously on a collision course.
And how did Matt attach the tags With a huge three-band speargun at point-blank range!
The skin of a shark is very tough, and covered in tiny teeth-like denticles. The skin of a whale shark near the dorsal fin is 10-15cm thick, so using a speargun causes least disturbance to the animal.
Matt deploys a tag attached to a detachable stainless-steel tip at the end of a shaft with a stopper that governs depth of penetration. I noted that the animals hardly noticed the impact, although they certainly noticed when Matt tugged the spear away once the tag was attached. On one occasion, with a tag that was, unusually, rather firmly fixed to the spear, I thought we might have lost him when a shark swept its massive tail hard across him as he pulled, before going into warp drive and disappearing over the horizon.
On another occasion, trying a new type of spear, an animal set off with it still attached and it took some time to relocate her and get it back.
More usually, Matt would slip from the Zodiac with his speargun just as an animal passed by, hit it under the dorsal fin on its port side, wrench away the spear shaft, climb back into the boat, and then get back in with his camera to record visual details of the animal that was still feeding and, apparently, seemed undisturbed.
Whale sharks do tend to shy away from another large animal in the water, such as a snorkeller, so we tended to get side-on and rear three-quarter shots in rapidly receding perspective.
The other hazard is that the Mafia Channel is busy with dhows sailing quite fast between Killigali and the mainland. We often found ourselves shadowing an animal in the Zodiac, or even bobbing at the surface waiting to be picked up, with one of these vessels bearing down on us.
We always made way for them when in the rubber boat, but I wondered if the crew would give the same cheery wave after they had split a swimmers head open with their hull.
After the first two days of fruitless searching, and with Thierry unable to fly his Microlight on a spotting mission because of windy conditions, the animals began turning up in numbers.
The bigger they were, the harder they were to photograph, simply because of what they were there for - the thick layer of plankton. After the third day, we set sail to do something else.
Expedition diving comes with a permanent problem attached. You may well discover where things are not, rather than where they are. The recent fiasco with wreck guide Peter Collings (British Diver In Red Sea Battle Of The Bottles, News, March) is understandable when you consider that he attempts to give his customers an expedition-diving experience with guaranteed results.
Burying a few artefacts at a dive site before producing them in front of an audience rather like a rabbit out of a hat can give the illusion that there are still things to be found.
The reality is that dive operators tend to take their customers to tried and tested dive sites. I imagine that most of the people who pay to go on such expeditions would have asked Christopher Columbus for their money back if he had failed to discover the new route to China by the third day!
The operators of Kairos offer real expedition-diving, but most of the dive sites they visit are therefore undived, untested - and often disappointing.
Some readers may recognise Kairos. Formerly the Martin Knutsen, built in 1974 as a submarine patrol ship or spy trawler for Denmark, she was designed for North Sea conditions, with a covered forecastle, watertight doors and plenty of places to bang your head.
Her 35m steel hull is pushed along sedately by a massive 18-tonne Alfa five-cylinder diesel engine that revs at a stately glopiter-glopiter 350rpm.
In the 90s she found a new lease of life as Sea Surveyor in the Red Sea, used by Schlomo Cohen to research his second Red Sea Divers Guide.
One more owner later, the 186-tonne vessel was bought in 2004 by Frenchman Thierry Terevet and his wife Catherine to satisfy their urge to operate a liveaboard dive boat and expedition-dive off Africa. Refitted in Toulon, it was renamed Kairos in 2005. The couple discovered the whale sharks in the Mafia Channel and contacted the SRI.

THE PROBLEMS OF COMBINING a research vessel with the needs of fare-paying passengers are obvious. The holiday-makers may take turns at watch-keeping on night passages without complaint, but after a few days of uneventful activity, they get bored. They need to go diving, and virgin sites can be a let-down.
Thierry provides distractions with nitrox and semi-closed-circuit rebreather courses, plus the opportunity to use full-face masks with communications units, diver-propulsion vehicles (banned from use with whale sharks), and big video lights on surface supply.
This can give the illusion of an expedition but is merely capricious if there is no sensible objective. Kairos has a two-man recompression chamber on board and I spent an enjoyable evening demonstrating how quickly I could clear my ears while getting to know a fellow passenger really well while under the scrutiny of those who looked on through the observation ports.
Incidentally, I took the early version of the all-new Galileo Sol computer in with me to follow the progress of our dry dive.
If you fancy a trip on a research vessel such as this, you must be fit and not overweight. You need to be able to swim under water quickly, breath-holding.
Climbing quickly back aboard an inflatable several times a day can be tiring, and be aware that the main vessel is not a purpose-built dive-boat, or in any way luxurious. It has difficult access to and from the sea, and the plumbing arrangements were state-of-the-art only back in the early 90s.
You must also be open-minded about the type of diving youll be doing. Finally, if you dont like hot and steamy conditions, its not for you.

Rebreather training is one of the diversions offered aboard Kairos when whale shark tagging is not on the agenda.
In and out of the Zodiac
the onboard hyperbaric chamber provides hours of fun
built for Arctic seas, but a luxury liveaboard the Kairos isnt.
This whale shark has been tagged but is thick-skinned enough barely to have noticed.
Matt Potenski is adept at tagging whale sharks, but here he is conducting a rebreather course


GETTING THERE: Emirates provides a daily service to Tanzania via Dubai. Kairos is based at Dar es Salaam. UK passport-holders can apply for a visa on arrival but avoid long queues by getting one from the Tanzanian consulate in London. Download the application form online - the visa costs around 60.
DIVING: mv Kairos, Some cabins have en-suite facilities. Food consists mainly of fish dishes. A certain amount of diving equipment is available free - book what you need ahead.
WHEN TO GO: Whale sharks are regularly sighted from December to March.
HEALTH: Malaria is endemic in Tanzania. Malarone is recommended. There are hyperbaric facilities on board. And cover up!
COSTS: A nine-day trip costs around £1750, excluding flights. Course costs and additional drinks onboard can be paid for with any international currencies (cash machines onshore).