Every southern winter, the coastal waters off eastern South Africa host a spectacle of almost incomprehensible scale. Billions of sardines mass into shoals that can stretch for miles, moving northward along the coast in ribbons visible from spotter planes and, when the density peaks, from space. The migration draws in nearly every large predator in the region, producing what many biologists regard as the largest concentrated feeding event in the ocean.
The event is often compared to the great wildebeest migration of East Africa, and in sheer biomass it may rival or exceed it. Yet despite decades of study, the sardine run remains only partly understood, an annual phenomenon that still surprises the scientists and fishermen who watch for it and, in some years, wait in vain for it to appear.
What happens along South Africa’s east coast
The run typically occurs from May through July, when the Southern African pilchard, Sardinops sagax, spawns in the cool waters of the Agulhas Bank and moves up the coast of KwaZulu-Natal. Individual shoals have been measured at more than seven kilometers long, over a kilometer wide, and tens of meters deep. The fish crowd together as a defense, since a lone sardine is far more likely to be picked off than one buried inside a swirling mass of millions.
That defensive instinct is precisely what fuels the feeding frenzy. Predators drive sections of the shoal toward the surface and compress them into tightly packed spheres known as bait balls, which are then hit from every direction at once. The KwaZulu-Natal sardine run ranks among the greatest gatherings of marine predators anywhere on Earth.
The predators the run summons
The sardines attract a coordinated assault. Common dolphins, sometimes numbering in the thousands, work together to herd the fish. Cape gannets fold their wings and plunge from height like living arrows, striking the water at speed. Sharks, including bronze whalers and dusky sharks, tear through the bait balls, while Bryde’s whales surge up from below and swallow enormous mouthfuls in a single lunge. Cape fur seals and game fish join the pursuit, turning a stretch of ocean into a churning arena.
For the sardines caught in a bait ball, survival odds are grim, but for the shoal as a whole the sacrifice of the outer edges buys time for the interior to escape. The interaction is a textbook example of how prey abundance can sustain an entire predator community for a season. The Southern African pilchard at the center of it is a modest fish, rarely more than 20 centimeters long, yet its collective movement powers one of the ocean’s richest seasonal food chains.
The feeding is not confined to the water. Overhead, gannets and other seabirds dive in continuous waves, and the plunging birds often signal to human observers, and to other predators, exactly where a bait ball has formed. The convergence of hunters from the air, the surface, and the depths is what gives the run its reputation as a rare, all-at-once spectacle of marine abundance.
A cold-water corridor that makes the run possible
Sardines prefer water temperatures roughly between 14 and 20 degrees Celsius, and the migration is hemmed in by that preference. Offshore, the warm, fast-moving Agulhas Current flows in the opposite direction, forming a barrier the fish will not cross. The run depends instead on a narrow band of cooler coastal water hugging the shore, which opens a temporary path northward.
This corridor is not guaranteed. It relies on seasonal drops in nearshore temperature and on oceanographic features such as the Durban Eddy and cold-water pulses that briefly widen the habitable zone. Where the continental shelf is narrowest, these enabling conditions matter most, which helps explain why the run is so tightly pressed against the coastline.
Why some years the sardines fail to run
The migration is far from reliable. In some years coastal observers see little or nothing, and the phenomenon appeared to fail in 2003 for the third time in 23 years. A non-run can mean several things: the sea may simply have stayed too warm for the fish to move inshore, or the sardines may have traveled farther offshore and deeper than usual, slipping past unseen. Because the run is defined largely by what people can observe from the shore and the surface, an unobserved run and an absent one can be difficult to distinguish.
Genomic research has added a further wrinkle. Data indicate that the sardines taking part originate from South Africa’s cool-temperate Atlantic coast and are drawn to seasonal cold-water upwelling on the south-east coast, sometimes ending up trapped in subtropical waters too warm for them to thrive.
An ecological event still full of open questions
For all its visibility, the sardine run is described by researchers as poorly understood from an ecological standpoint, with competing and sometimes contradictory hypotheses about why and how it occurs. The leading interpretation frames it as a seasonal reproductive migration by a genetically distinct subpopulation, supported by the presence of sardine eggs off the KwaZulu-Natal coast that suggest the fish linger there for months before a quiet, deep-water return south in late winter.
That uncertainty is part of what makes the run compelling. A migration large enough to be seen from orbit, involving a fish central to commercial fisheries and a food web spanning dolphins to whales, still resists a complete scientific explanation, and shifting ocean temperatures raise questions about how long the phenomenon will continue in its current form.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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