Diving in South Africa/Sardine run
The sardine run occurs most years between May and July when billions of sardines (Southern African pilchard Sardinops sagax) spawn in the cool waters of the Agulhas Bank then and move along the southern coast of South Africa, eastward at first, then northward as they follow the coastline. This migration is followed by large numbers of predators, which provide the main attraction for divers.
The run, containing millions of individual sardines, occurs when during the winter months a cold south to north-flowing current develops off the east coast, moving inshore and counter to the Aghulas current. A part of the sardine population follows this narrow band of cool water north to Port Edward, swimming up between the coast and warm Aghulas current. The numbers vary from year to year, and it is only considered a "run" when the shoals are big enough to be visible at the surface.
North of Port Edward the cold current is restricted by the narrowing continental shelf and the shoals become concentrated in a narrow inshore band of water, as far as Mozambique where it then leaves the coastline and goes further east into the Indian Ocean.
The shoals may be more than 7 km long, 1.5 km wide and 30 meters deep and are clearly visible from spotter planes or from the surface.
Sardines group more closely together when they are threatened. This instinctual behaviour is a defense mechanism, as lone individuals are more likely to be eaten than large groups, but this behaviour also encourages large numbers of predators to follow the shoals.
Thousands of dolphins are largely responsible for rounding up the sardines into bait balls. These bait balls are densely packed masses of fish and can be 10–20 metres in diameter and extend to a depth of 10 metres and may last for up to about 10 minutes. Once the sardines are rounded up, sharks , game fish birds and the occasional whale take advantage of the opportunity. This is the most desirable time for a diver to join the action.
The Cape Fur Seal follows the shoals as far as Port St Johns.