THE TELL-TALE SIGNS of a sardine gathering are hundreds of Cape gannets wheeling overhead. They are alerted by the splashing of common dolphins, rounding up a shoal of sardines. The gannets have now spotted the dark shadow of a baitball, moving several metres beneath the sea.
Rob Nettleton, our skipper, and Dietmar Posch, the trip organiser, are watching. When the gannets start plunging in a focused stream into a patch of water a few metres across, its time to manoeuvre our RIB alongside and plunge in. Diving in these conditions is a high-octane pursuit.
Our day begins at 7 on a misty July morning on the Mkambati River near Port St Johns on South Africas Eastern Cape. Air temperature hovers around 5°C. Wearing wetsuits, woollen hats and waterproofs, we hang on determinedly as Rob steers the RIB carefully through the surf. Its like riding a bucking bronco.
Breaking through beyond the surf, ours is the first RIB of the day to launch. We at once start looking for baitballs.
Approaching this first one, amid the plunging gannets and arcing dolphins, were all kitted up. After checking with Rob and dive leader Raffaella, and at Dietmars shout Go!, we back-roll and tumble negatively buoyant into the water.
At 5m we swim rapidly towards the glittering baitball. Its ducking and weaving like some phosphorescent energy cloud from an early Star Trek.
A feeding frenzy is underway. I hear clicks and whistles as three common dolphins hurtle past my shoulder and sweep into the sardines.
The shoal closes up and moves jerkily. The dolphins scythe through and the school splits, like torn fabric. It then reforms behind the dolphins, as though being zipped up.

THIS FIRST DIVE IS POTENTIALLY the most hazardous of my trip. Eight 2m bronze whalers are circling the baitball - and us. Ive dived with plenty of curious sharks, and Im used to eyeballing them, but these do not seem to be responding to our back off signals.
Two are heavily scarred females, perhaps nursing wounds after recent bouts of courtship.
The group converges on us like a gang of thugs. As we stare them out, each shark swims away, arcs around and then sneaks up behind us. They seem more interested in us than in the sardines.
We break away from the baitball and gather in defensive formation, our backs to each other. We rise slowly to the surface. As the RIB arrives, the sharks disappear, but its all a wee bit unnerving.
Fortunately, there is no repeat of such thuggish behaviour on this trip.
Sardine Run diving is hard work but, wow, exhilarating! When sardines are gathering, you can expect to plunge into the water six, seven or more times in a day. None of the diving is deep - often around 5m, 10m maximum.
Depending on how long the baitball remains intact, or how long you can keep up with it, each dive lasts from a few minutes to half an hour.
Often its easier, less disruptive and less restrictive to snorkel and freedive.
Our group of seven consists of veterans of hundreds or thousands of dives, all accomplished underwater photographers, so on the first day I had felt like a novice, and rather unfit.
The first two fast-action dives leave me pretty exhausted. But by the afternoon I am in snorkelling mode and have worked out the best way to heave myself into the RIB without assistance.

WHY DO SARDINES GATHER off the Wild Coast to form such a tempting feast for predators None of the various explanations seems entirely satisfactory.
Most South African sardines spawn in cool Southern Cape waters, and the young mature off the Western Cape before returning south to breed.
Fewer than a quarter of adult sardines travel up the Eastern Cape. Sardines prefer moderately cool water of 14-20°C, and a combination of events - scientists are still trying to work this out - causes a spur of cold water to sidle northwards up the Eastern Cape, along the narrow continental shelf.
Sardines massing in comparatively shallow water make it worthwhile for dolphins and sharks to gather in their thousands on order to to herd them into catchable baitballs.
So why these sardines migrate up the east coast, and not the safer west, is unclear. Perhaps it is simply to exploit a rich food resource in cool waters.
Watching the sardines in calmer moments, I noticed that many swim with mouths wide open, filtering the water for animal plankton.
For me, the second day is the peak experience. On one occasion, Daniela and I snorkel out towards some baitball activity and, to my great fortune, the sardines gather below me for protection.
For the next half-hour, up to six common dolphins make scything runs back and forth across the baitball. Bronze whalers rise slowly from below and begin their own slicing runs.
Many times the dolphins sweep past within centimetres of my face, snatching a sardine as they pass. Several times a dolphin jumps right over me.
At one point a sardine lands on my back, flaps about for a moment, and then flops back into the water.
It is only later, seeing the playback of a video taken at about 8m, that I realise how persistent the dive-bombing gannets had been. Floating at the surface, I heard the thud as a bird hit the water, then saw the bubble trail as it plunged to about 5m and swam to grab a fish. I hadnt realised how close the gannets had come. Many hit the water only a metre or so from me. What would have happened had one miscalculated
If youre planning the holiday, be warned: the Sardine Run is unpredictable. On my third day the sardines seem to have disappeared, but at one point a gigantic pod of common dolphins come by. It must number at least 1000 - in all directions, as far as the eye can see.
Humpback whales, which have been passing the RIB at the rate of 15-20 a day, seem to be everywhere too.
We follow a pod of eight for about an hour. In relaxed fashion, travelling at only 2-3 knots, they treat us to a wide selection from their acrobatic repertoire, including breaching and flipper-slapping. At one point they rise together, with heads rubbing, as though in a group hug. On several occasions they come to within 50m of the boat.

ON THE FOURTH DAY I stay ashore. At midday I head for the vantage point of the Cape Hermes Lighthouse.
The lighthouse-keeper is staring out to sea. Several hundred metres offshore, just beyond where the Mkambati Rivers water is staining the sea brown, a dark shadow is moving - a strange attractor for hundreds of Cape gannets.
I see a RIB weaving beyond the large shoals farthest edge, and wonder if it is Dietmars. Then a grey submarine shape erupts along one flank of the shoal - a 12m Brydes whale, its open, baleen-filled mouth gulping in the sardines.
I watch for half an hour as squadrons of gannets take off from the sea surface, plummet into the shoal, pop up, rest, and then take off on their next sortie.
The combined actions of dolphins, gannets and unseen sharks is pushing the shoal into the murky limits of the river water. I know that those on the RIB will now give up diving or snorkelling. Such poor visibility would be inviting a shark attack - accidental or otherwise.
The shoal slowly fragments into a dozen dark shadows. Eventually, the gannets are satiated. Only the occasional back of a dolphin or thrash of a sharks tail reveals the drama still being played out below.
Dietmar and Raffaella later say that they estimated the shoal to be about three-quarters the size of a football pitch - and the baitballs that splintered off it were the biggest they had seen in their five Sardine Run seasons. They had seen sharks up to 4m-long in the shoal, but couldnt identify them.
Raffaella and the other divers had plunged in alongside a baitball many metres across. It had suddenly changed direction and engulfed them.
Being inside a baitball is dangerous. Your visibility is almost zero, and so it is for the sharks, which might accidentally bump into you, mouth open.
At one point Raffaella found herself with a 2m bronze whaler mouthing her shoulder. It didnt bite, but three jagged rips in her wetsuit show just how close she had come. The shark had probably mistaken the silver insignia on her shoulder for a sardine.
We toast Raffaellas survival with a round of drinks - a blue spirit that tastes like medicine, and which the barmaid is reluctant to give a name.
Intercepting the Sardine Run can be a bit of a lottery. This years was particularly good, says Sheldon Dudley, shark specialist with the Natal Sharks Board, but going with a reputable trip organiser in June or early July gives you the best chances of success.
Couple your trip with a visit north to Umkomaas and Aliwal Shoal, about 30 miles from Durban, and you are almost guaranteed unforgettable shark action.
With dozens of whales and hundreds of dolphins as a backdrop, there will always be plenty to experience.

FACTFILE
GETTING THERE: BA, SAA, Virgin Atlantic and other airlines fly from the UK to Durban via Johannesburg. Port St Johns is about 250 miles south, 5-7 hours by road.
DIVING: Blue Rush, www.bittenbysharks.com
ACCOMMODATION: There are several B&Bs and other accommodation in and around Port St Johns on the warmer south side of the Mkambati River, www.portstjohns.org. Many divers visiting Umkomaas to dive Aliwal Shoal stay at Agulhas House, www.agulhashouse.com.
MONEY: South African rand.
WHEN TO GO: June and early July for the Sardine Run. Raggies breed at Aliwal Shoal at this time, and dolphins and whales are common in both places.
PRICES: Return flights to Durban start at around £750 and Sardine Run packages from around £1200, with B&B accommodation £26-£28 per night. Contact Africa Dive & Safari, 0208 879 6178, www.africa-dive-safari.co.uk
FURTHER INFORMATION: South African Tourism 0870 1550044, www.southafrica.net, www.portstjohns.com