Marine scientists scratch heads over sardines
Once a year the normally-tranquil waters off South Africa's balmy east coast bubble as billions of pilchards migrate north with sharks and dolphins in hot pursuit.
The "sardine run" is one of the planet's great natural wonders and an annual bonanza for small-time fishermen who wade into the waters to scoop the bounty from the sea.
Yet scientists know little about the phenomenon.
"It's really quite bizarre that there is this major biological event we know so little about," said Vic Peddemors, a marine biologist who has been observing the run for more than 15 years.
Every year the small, silvery fish leave the cold depths off Cape Agulhas, Africa's most southerly point, during the southern hemisphere's winter and swim north, swarming the beaches of KwaZulu-Natal.
In terms of biomass, researchers estimate the spectacle could rival East Africa's great wildebeest migration, but experts are puzzled why the fish leave nutrient-rich feeding grounds for emptier, sub-tropical climes.
From a spotter plane, the sheer size of the migration is staggering. sardine shoals often 7km long, 1,5km wide and 30 metres deep, are clearly visible against the sandy shallows.
Such an immense mass of fish, dubbed "the greatest shoal on earth", attracts predators in vast numbers.
As the pilchards head north, about 18 000 dolphins, sharks in their untold thousands, and whales devour the fish - yet they make little impact on the overall shoal size.
Attacked from below, the fish are driven towards the surface where they come in range of gannets, seabirds whose arrow-like attacks on the hapless plankton feeders pepper the surface like small explosions.
Peddemors said the sardine run was the only place in the world where scientists and tourists could see such extensive predation so close to shore.
"You are out there on a boat and the entire sea is just dolphins. You can see 2 500 bottle-nosed dolphins swimming past," said Peddemors, who is leading a study into the sardine run at the University of KwaZulu-Natal.
"From a shark's point of view, it is impossible to know how many there are, but you might see up to 300 on just one bait ball 30 metres across."
Peddemors said part of the study looked at how sharks and dolphins whipped pilchards into shimmering "bait balls" and how slower sharks managed to catch their more agile prey.
From the air, the sharks appear to never actually catch the pilchards - their position betrayed by a "doughnut" of empty water around the hunter as it moves through the shoal.
Earlier this year, Peddemors suggested to National Geographic television that a film crew attach one of their innovative "critter-cam" cameras to the dorsal fin of four bronze whaler sharks to solve the mystery.
The project failed to capture footage of the sharks feeding, an indication, as Peddemors explained, that filming the frenzied activity raised some tricky, and unusual, challenges.
"You need to film at the leading edge of the shoal, otherwise you are swimming in faeces of 15 000 common dolphins, thousands of gannets and little bits of sardine mush," he said.
Thermal satellite images, part of Peddemors' study, now suggest currents rather than spawning or large plankton blooms trigger the migration.
"We believe it is primarily the cool water coming up the coast in the winter months that allows them to extend their range," said Peddemors. "Last year, when there was warm water close to shore, there was no sardine run."
Whatever its cause, the sardine run is a huge bonus for poor communities along Natal's southern coast, as the spectacle draws increasing numbers of tourists, many for extreme diving, and provides extra protein for local diets.
This year's run was particularly good, with fishermen hauling in tons, using nets cast from the beach, almost two months after the shoals first appeared.
In an era of dwindling fish stocks, it is astonishing that the shoals represent just a fraction of South Africa's sardine fishery. Just 800 tons are caught off KwaZulu-Natal every year compared with 100 000 tons on the Agulhas Banks. - Reuters