Every autumn a small portion of the Cape sardine stock Sardinops sagax (present on the Agulhas banks off the southern Cape Coast), breaks off and heads eastwards along the South African coastline. As the coastal waters along the east coast cool to temperatures of 20C or less, the sardines move further northwards, and by late May to mid-June the shoals of sardines have usually reached as far north as KwaZulu-Natal. The temperature of the water is critical. If the water is too warm (over 20C) the sardines will remain further south or move northwards further offshore and at greater depths where the water is cooler.
The movement of sardines northwards along the east coast of South Africa is often termed a ‘migration’ which implies that the sardines move for feeding or breeding purposes. This is actually not the case, and scientifically it is more correct to think of the movement as a ‘range extension’.
The sardine run, however, is about much more than the enormous shoals of fish that move along the east coast, some of which are up to 17 kilometres long. It is a dynamic and complex event that involves and affects many marine animals. Copper sharks, common dolphins and Cape gannets are three key predators that follow the sardines northwards and interact to produce feeding displays that are spectacular.
Humpback whales are regularly seen performing their spectacular leaps out of the water (also known as breaching) during the time of the sardine run. Their presence, however, is merely coincidental, as peculiarly, they have not been observed feeding on sardines. They arrive from their summer feeding grounds in Antarctica to calve and mate in the warmer waters off the southern African east coast.
The sardine run and it associated activity, like any natural phenomenon, is unpredictable and sardine shoals may disappear as quickly as they arrived. The event itself, however, is over a month long, and numerous shoals of sardines along with scores of predators may pass a particular point during the period. For those who have not yet witnessed the sardine run, it is well worth a little bit of effort in order to catch a glimpse of the frenzied marine interaction that is unique in magnitude and complexity to the east coast of South Africa.
