Thursday, September 9, 2010

Millions of sardines create an astonishing natural spectacle in their yearly pilgrimage northwards, writes Ocean Correspondent Cheryl-Samantha Owen

Every year between June and July, the waters off the east coast of South Africa spring to life with the arrival of a congregation of fish so densely packed that they form a visibly solid black-blue slick flowing parallel to the shore.

At this time of year, the already wild seas of this coastline bubble and swirl with an intensity rivalled on land only by the great Serengeti-Mara wildebeest migration.

Trapped in a giant finger-shaped cauldron, the humble sardine swims northwards creating South Africa’s great sardine run.

The sardine is a pelagic shoaling fish of the herring family that prefers the cool water and feeds on plankton.

For ten to 11 months, sardines inhabit the coastal shelf region between northern Namibia and Port Elizabeth, but for the remaining part of the year their distribution changes dramatically.

In apparent defiance of ocean dynamics and animal physiology, millions upon millions of these cold-water fish stream northwards into the subtropical waters off South Africa’s east coast.

Oceanography explains this errant behaviour. In June and July the warm, south-flowing Agulhas current moves off shore, allowing a tongue of cold water from the south to lick the country’s eastern flank.

This develops into a south-to-north current that enables the sardines to swim up the wild coast without having to fight the powerful Agulhas.

Once on the run, shoals can cover 60kms per day, but their movements are unpredictable.

Finding the fish often involves the use of spotter aircrafts and remote-sensing satellites and backbreaking days on small inflatable boats, battling some of the planet’s most treacherous seas. How far north the fish make it depends on how far the cold water extends.

Some years, if water temperatures remain too high, the run’s extension is cut short or takes place out of sight in the deeper waters off the shore and the sardines barely swim 50kms past their normal range.

In favourable conditions, the shoals race as far as the north coast, 800kms away, vanishing into deeper water on reaching their Durban finish line.

The topography of the wild coast creates further challenges, ensuring that anyone joining the sardine run is as dedicated as the little fish themselves.

Each boat has fuel capacity for a 100km round trip and fortresses of high sea cliffs allow for a handful of launch and recovery sites. Boats often have to turn back before reaching the sardines for fear of running out of fuel.

The crafts then have to be transported overland on dirt tracks to the next safe launch site, which may be up to 200km away.
Even making it to the water doesn’t guar-antee seeing this spectacle, as the region’s numerous rivers empty their silt loads into these seas, creating poor visibility.

Though the odds are stacked firmly in nature’s favour, the sardine run can make even the most Herculean effort to witness
it worthwhile. Providing the east coast with an extravagant injection of life, this mass congregation of fish attracts marine predators of all affinities that take advantage of the annual feast.

Long-beaked common dolphins, Bronze whaler sharks, Bryde’s whales, and Cape gannets alter their ecology or behaviour just to dine on the sardines.

The common dolphin is the sardine run’s most important predator, and between 15,000 and 20,000 individuals leave their hunting grounds around the southern Cape to travel inwards and chase the shoals north. For most of the year these dolphins live on squid, but during the run their diet comprises 85 per cent sardines.

Being the only hunters that occur in numbers intelligent enough to anticipate the behaviour of the sardines, the common dolphin is the true architect of the sardine run. They are behind the creation of the bait ball, a phenomenon that makes this abundant resource accessible to other animals, especially sharks and gannets.

Sardine shoals can stretch for kilometres, and a 12km long, 2km wide and 30m deep shoal poses a problem even for super-pods of more then 5,000 dolphins.

So the dolphins split into smaller pods, and fill their underwater world with screams of echolocation.

Working together, they carve smaller shoals off the main body, which they then push upwards. Trapped against the surface, the sardines swim in an ever-tightening shoal - and a bait ball is born.

The dolphins then switch gear, altering between herding the shoal and rushing in to snatch sardines. Every time they strike, the ball explodes and scatters in patterns that should be impossible to track.

Yet the dolphins’ sonar, intelligence and speed mean they grab a hearty meal and provide a feeding opportunity for plenty of fellow sardine-lovers.