Sardine Run - Causes of The Sardine Run

Causes of The Sardine Run

The sardine run is still poorly understood from an ecological point of view. There have been various hypotheses, sometimes contradictory, that try to explain why and how the run occurs.

A recent interpretation of the causes is that the sardine run is most likely a seasonal reproductive migration of a genetically distinct subpopulation of sardine that moves along the coast from the eastern Agulhas Bank to the coast of KwaZulu-Natal in most years if not in every year.

The migration is restricted to the inshore waters by the preference of sardine for cooler water and the strong and warm offshore Agulhas Current, which flows in the opposite direction to the migration, and is strongest just off the continental shelf. 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. The importance of these enabling factors is greatest where the continental shelf is narrowest.

The presence of eggs off the KwaZulu-Natal coast suggests that sardine stay there for several months and their return migration during late winter to spring is nearly always unnoticeable because it probably occurs at depths where the water is cooler than at the surface.

In some years there does not appear to be a sardine run. This may be because it is not detected by coastal observers either because it actually does not occur due to high water temperatures and/or other hydrographic barriers, or the migration may occur farther offshore and possibly deeper due to unusual conditions.