'Greatest shoal on Earth' hits town

By Zoe Murphy
BBC News, South Africa

Bronze whaler shark cuts through a "baitball" of sardines

The ocean was boiling with activity. Thousands of panic-stricken fish turned and convulsed below, herded by predatory dolphins and sharks. And an aerial assault was under way as gannets plummeted into the writhing mass.

After weeks of anticipation the "greatest shoal on Earth" had arrived.

The "sardine run" is one of the biggest marine events on the planet, taking place from May to July along the east coast of South Africa.

There are no accurate figures on how many sardines make the passage, but huge shoals 15km long and 4km wide have been known to hug the coastline for more than 1,000km.

The sardines, however, are not always the main attraction.

It is not a migration in the true sense as the sardines do not travel for feeding or breeding purposes Andrew Aitken, marine scientist

Following them are a host of predators, including around 20,000 common dolphins, thousands of bronze whaler sharks (copper sharks), Cape fur seals and tens of thousands of Cape gannets.

Despite the incredible density of marine life, witnessing the sardine run remains a hit-or-miss business. Along with the marine predators, I was stalking the culmination of the action - the elusive "baitball".

The chase

According to folklore, the sardines will arrive when the last aloe plant has bloomed.

However, operators in the Eastern Cape have a number of alternative methods to shorten the odds on locating the main event.

The hunt begins at first light as the spotter plane scours the former Transkei coastline looking for bird activity or the dark stain of sardines.

Dolphins are responsible for herding the sardines

The pilot then directs our boat's skipper towards the action and the vessel is loaded with dive equipment and enough diesel for several hours at sea.

These usually unproductive waters are alive. The skipper heads north following a steady stream of Cape gannets to the spot where hundreds of common dolphins have been sighted.

Below the waves, the noise is deafening as the dolphins shoot past at an incredible pace, communicating with their distinctive echo-locating clicks.

Within minutes, 20 bronze whaler sharks surround us.

These magnificent creatures have us mesmerised until an inquisitive female swings directly towards one of the divers and veers off at the last moment.

We ascend to the boat, hearts pounding. We were getting close.

Feeding frenzy

Baitballs are formed as a result of dolphins rounding up pockets of sardines using streams of bubbles and trapping them against the ocean's surface.

When threatened, sardines instinctively align their bodies with those of their neighbours in order to prevent themselves from becoming isolated from the group (the best form of defence from predators).

These baitballs are short-lived events, seldom lasting longer than 10 to 20 minutes as marauding predators, whipped into a feeding frenzy by this brief period of plenty, pick off the sardines.

As the sardines are driven into shallower water, Cape gannets strike.

To see hundreds of birds dive-bombing is spectacular. They plunge in formation from heights of almost 30m, hitting the water at 90km/h and reaching depths of 10m or more.

A minute or two later, they pop to the surface, sardine in mouth, while the frantic bubbles of the dolphins foam the surface of the ocean, amid a thrash of tails and dorsal fins.

After the activity has diminished, the satiated gannets float on the surface too full to fly. As our boat approached, they were forced to regurgitate part of their fishy meal in order to take off.

Unanswered questions

Marine science has no definitive explanation for the sardine run phenomenon.

Sardines are known to migrate close to the shore only along South Africa's east coast. The configuration of the coastline and the currents that flow down them are believed to be the key to the event.

The sardine is a coldwater species, which aggregates off the southern coast of South Africa.

With the onset of winter, a cool band of water from the south (14-20 degrees) penetrates the warm south-flowing Agulhas current. This band of cool water allows the sardines to extend their range eastwards.

This "leakage" of sardines is believed to be only 2% of the holding stock based off the southern Cape coast.

Marine and shark research scientist Andrew Aitken says there are still many unanswered questions. "We don't really know why they do it," he told me.

"It is not a migration in the true sense as the sardines do not travel for feeding or breeding purposes.

"There is evidence that some of the sardines return south later in the year. It is not clear what factors other than cooler water temperatures may trigger their movement up the coast."

Inevitably, the sardines vanished into deeper water as suddenly as they had appeared, reminding us that this is a truly wild event whose secrets are only just beginning to be understood.

The sardine run is featured in Massive Nature: The Deep, broadcast on BBC One, on Thursday, 15 July, at 2030BST.