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The sardine run of Southern Africa occurs from May through to July. Billions of sardines (Sardinops sagax) spawn in the cool waters of the Agulhas Bank and move northward along the east coast of South Africa. Their numbers of sardine create a feeding frenzy along the coastline. The run, containing millions of individual sardines, occurs annually between May and August when a coastal current of cold water heads north from the Agulhas Bank up to Mozambique.
In terms of biomass, researchers estimate the sardine run could rival East Africa’s great wildebeest migration. However, little is known of the phenomenon. It is believed that the water temperature has to drop below 21 °C in order for the migration to take place. The sardine shoals are often more than 7 km long, 1.5 km wide and 30 meters deep. These shoals are clearly visible from spotter planes or from the surface.
Drawn to this mass migration is a diverse array of marine predators. These include the Common dolphin (Delphinus capensis), Cape gannet (Morus capensis), Cape fur seals (Arctocephalus pusillus pusillus), Blacktip shark (Carcharhinus limbatus), Dusky shark (Carcharhinus obscurus) and Bryde’s whale (Balaenoptera brydei) amongst others. In addition, Humpback whales (Megaptera novaeangliae) are continuously encountered on the sardine run. Whilst not attracted to the sardines per se, these leviathans pass through the sardine run on their northern migration from Antarctica to Mozambique’s summer breeding grounds.
The sardine run is still poorly understood from an ecological point of view! Various research hypotheses, sometimes contradictory, that try to explain why and how the run occurs.
A recent interpretation of the causes of the Sardine Run is that it is most likely a seasonal reproductive migration of a genetically distinct sub population of sardine that moves along the coast from the eastern Agulhas Bank, along South Africa’s wild coast to the coast of KwaZulu-Natal. The migration is restricted to the inshore waters by the preference of sardine in the cooler current. Sardines avoid the strong and warm offshore Agulhas Current, which flows in the opposite direction to the migration. A band of cooler coastal water and the occurrence of Natal Pulses and break-away eddies make it possible for sardine shoals to overcome their habitat constraints and migrate northwards. The importance of these enabling factors is greatest where the continental shelf is narrowest.
Sardine prefer water temperatures between 14 and 20 °C. Each southern winter the nearshore sea temperature along the South African south east coast drops to within this range. Along the KwaZulu-Natal coast, sardine may be found in water warmer than 20 °C. Between May and August, the warm Argulas current slackens, and a cooler counter current from the intermixing zone off the Agulhas banks drives cooler water along a thin corridor up South Africa’s Wild Coast. It is this counter current of cooler water that the sardines utilise to migrate northwards towards KwaZulu Natal.
Each year many SCUBA divers and adventurers travel to South Africa to experience the Sardine Run. Expeditions typically exit from established launch sites including East London, Port Saint Johns, Coffee Bay and Port Elizabeth. These expeditions are offered by experienced tour and dive operators and typically run for between 3 and 7 days. Sardine Run trips are known as physically demanding, with guests spending upwards of 8 hours on the ocean each day. Many operators utilise a network of fishing boats, aerial observers and land based observers to increase the likelihood of encountering sardine run bait balls.