The sardine run along
east coast of South Africa is a spectacular and well-known phenomenon, but seeds of our understanding are only now beginning to emerge.We now know that this is one of largest marine events on planet, involving many species of fish, sharks, marine mammals and birds. Fishermen have been making most of this annual winter event for decades and gradually, media and tourism potential of this occurrence is being developed.
More recently international film crews have converged on Kwa-Zulu Natal (KZN) and Eastern cape coastline to capture images for worldwide broadcast, and specialist operators have created excursions to allow more adventurous to witness this exciting coastal exhibition of frenzied marine interaction.
The sardine run is much more than simply numerous glistening shoals of sardines moving up coast, for which local tourism industry has coined phrase “the Greatest Shoal on Earth”. 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 of sardines and follow shoals of sardines northwards along east coast. The feeding displays that result are spectacular. Pods of common dolphin join together to form “super-pods” several thousand strong. Cape gannets plunge into water like fighter planes to feed on sardine ‘bait balls’ that have been rounded up by sharks and dolphins.
This breathtaking spectacle is an event that is unique in magnitude and complexity to eastern coast of South Africa. It is perhaps not merely greatest shoal on earth – but also “Greatest Show on Earth”.
The Run Sardines, also known as pilchards, are cold-water fish and are typically found in areas of cold ocean upwelling, such as off west coast of Australia, California, Japan, Peru/Chile and Southern Africa.
In these areas cold, nutrient-rich water is up welled from deep and provides suitable conditions for minute free floating aquatic plants, called phytoplankton, to bloom. Sardines rely on phytoplankton and other minute free floating aquatic animals called zooplankton for food, as do their close relatives anchovies and herrings. Collectively these small fish comprise 25% of world’s fish catch (by weight) and thus form most valuable group of fish.
In South Africa there is a large sardine fishery off Western Cape coast and approximately 100 000 tonnes are caught annually. Off Eastern Cape coast annual catch drops to about 7700 tons whilst it is only up to a maximum of 700 tons in Kwa-Zulu Natal.
Sardines have a short life-cycle and only live to 2-3 years of age. Adult sardines, which are about 18-20cm long and two years old, aggregate on Agulhas Banks off southern Cape coast. Here spawning takes place in spring and summer months when each female releases tens of thousands of eggs into water which are then fertilized by males. These eggs drift with current in westerly and northerly directions into nutrient-rich up welled waters off west coast. Here larvae mature and develop into juvenile fish which once strong enough, aggregate into dense shoals and migrate southwards, returning to Agulhas banks in order to complete their life cycle.
Sardines are typically found in water between 14 – 20 C/ 57 – 68 F. During winter months of June and July, penetration of cooler water eastwards along Eastern Cape coast towards Port St Johns, effectively expands suitable habitat available for sardines. From Port St Johns region northwards, it is likely that a cool, northerly flowing counter-current, flowing inshore of southerly flowing Agulhas current, may be one of factors responsible for “leakage” of large shoals of sardines further north in what has traditionally been known as “Sardine Run.”
Upwelling of cool water along this section of coast caused by northeasterly winds may also assist in movement of large shoals of sardines northwards. The cool band of water inshore is critical to run. If water is too warm (over 20 C/ 68 F) sardines will remain in cooler water further south or move northwards further offshore and at greater depths where water is cooler, consequently making themselves unavailable to seine-net fisherman and many of predators associated with them. This was case in 2003 when unseasonably warm sea surface temperatures (21 – 23 C/ 70 – 74 F) were recorded off southern KZN coast during months of June and July.
North of Port St Johns sardines become concentrated in a narrow band of cool inshore water and as a result are easily located by predators. Predators, including various species of sharks, marine mammals, predatory fish and sea-birds, are quick to take advantage of this time of plenty in what are otherwise relatively unproductive waters. It is likely that once sardines reach Port St Johns stretch of coastline predators may play an important role in driving sardines close to surface and inshore, making them accessible to seine-net fisherman in KZN.
Although numerous species of marine mammals take advantage of bountiful supply of food, there are three key predators that follow sardines north into KZN waters. These are common dolphin, copper shark (bronze whaler) and Cape gannet. In fact common dolphins and Cape gannets are thought to time their breeding cycles with sardine run so that their young are weaned or fledged at time of event. This allows youngsters to be exposed to an abundant source of food during a crucial learning phase of their lives, thus increasing their rates of survival.
