The greatest shoal on earth is a no-show

Birds, dolphins, sharks and tourists all turn up for the annual migration of the South African pilchard: so where is it this year?

South Africans from Sezela, about 80 kilometres north of Durban, have in the past used the Sardine Run to supplement their incomes.

In the Indian Ocean off Port St Johns, South Africa, the first sign of one of the underwater world's most extraordinary sights is in mid-air. Flocks of gannets gather above the waves of the Indian Ocean a few kilometres off the shores of South Africa's Wild Coast, a starkly beautiful landscape so rugged that waterfalls plunge directly into the sea. From above, ragged squadrons of the birds dive towards the water in an avian tornado, stretching their wings straight behind them for extra streamlining a split-second before impact, turning the surface to foam.

They travel fast enough to launch themselves as far as 10m down, into a world of dolphins, sharks and passing whales, joining in the feeding frenzy that is the Sardine Run. Every austral winter, the sea temperatures off the Eastern Cape province drop significantly. Whether the change is the result of cold water moving north from the sub-Antarctic, or the warm Agulhas current from the Indian Ocean shifting a few degrees eastward - or both - is unclear, but the effect is that billions of the Sardinops Sagax Sardine, also known as the Southern African pilchard, which can only live in waters below 20 degrees Celsius, migrate north towards land.

Described as "the greatest shoal on earth", they can form black slicks of fish 20km long, and the biggest army of predators anywhere on the planet gathers from across the oceans to plunder them. Groups of dolphins join to form vast super-pods, sometimes thousands strong, and break off sections of the sardine shoal, herding them towards the surface in "bait-balls" where they are confused by the light and become a concentrated mass of easy prey.

Copper sharks, sometimes almost as numerous as the dolphins, join in the meal, as do the gannets, and occasionally a Bryde's whale rises from the depths to swallow the bait-ball whole. Within minutes, all that is left of the thousands of sardines that made the bait-ball are glinting scales drifting through the water. By coincidence, humpback whales migrate through the area at the same time towards their breeding grounds off Mozambique, at times throwing themselves above the waves in spectacular "breeches", making the sardine run one of nature's most stunning phenomena.

"My whole life I have been fishing or diving [for] them," said Antony Diplock, 36, of African Ocean Charters, a diving and marine research company. It's the biggest biomass movement of creatures on our planet. It's unique, just one of the things Mother Nature makes perfect for us. I will do the Sardine Run every year until the day I die." "It's very, very special," said Carl Elkington, of Oceanworx, who is also based in Umkomaas, in KwaZulu-Natal (KZN) province south of Durban. He moves his diving operation to Port St Johns in the Eastern Cape province every year for the event.

But it is in KZN that the Sardine Run has the biggest impact on humans - and where, for reasons that have yet to be established, it has become unreliable in recent years. As the shoal moves north from Eastern Cape towards KZN, the area of cold water forms an ever-narrower tongue along the shore, forcing the fish into the shallows until they can be caught from the beaches. The fish sometimes strand themselves on the sands in huge numbers as they try to flee the sharks. It is a bonanza for netters, who can make a small fortune if they are lucky enough to make the first catch, and a draw for mid-winter tourists, who are worth millions of dollars to the local economy.

But as the period for the run drew to a close this week, the sardines had yet to appear. "There have been no confirmed nettings to date of any Natal sardines so far this season," said the KwaZulu-Natal Sharks Board, which monitors the fish as it has to remove its protective shark nets, which protect human swimmers, to avoid a wholesale slaughter of the predators. Last year, a promising start to the run in KZN was ruined by unseasonal rains, which silted up the inshore waters and kept the sardines away; the run in 2007 was also poor.

It is a slightly difficult situation for South Coast Tourism, covering the area from Durban to Port Edward, which promotes an annual Sardine Festival that this year has taken place without any sardines. "We have to accept that with global warming and everything that's happening in the oceans it may not happen every year," said Michael Bertram, its chief executive. "It's not like going to the movies when you know you are going to see a film.

"We are saying treasure it, we don't know long it's going to last, whether it's going to be there for our children and grandchildren. It would be a blow [if it ended] but we have been trying to get people focused on the [other] marine experiences that we have." Theories abound about possible causes for the fish not reaching the KZN shores, with global warming, changing ocean currents, and overfishing all cited as possibilities.

But Mike Anderson-Reade, deputy chief executive of the KZN Sharks Board, pointed out that there have been several periods of bad runs in the last 30 years, and the five years preceding the last three were all good. It is too early for alarm, he said. "People have got very short memories. Obviously some years are better than others, it's nature's cycles. It's just hit and miss, it's bad luck." Nonetheless, he added: "If next year we don't have anything again, I think the following year I will start getting nervous."