Between May and July each year, millions of sardine move northward along the East coast of South Africa to reach spawning sites where they release their eggs. This is the spectacular Sardine Run. The migrating fish come under relentless attack from dolphins, sea lions, whales, tuna, birds and fishermen during their headlong, even suicidal race. Why do the sardine brave such formidable dangers? How did this dogged migration arise?
A great event, it has been stimulating scientists' imagination for many decades. Many hypotheses have been put forward, often contradictory ones. IRD researchers and their partners reviewed these different theories and tested them by comparing and combining a range of biological, acoustic, oceanographic and satellite data.
Inevitable return to their hatching place
Only a variably-sized proportion of the population of the South African sardine, Sardinops sagax, undertakes this long voyage, from the Agulhas Bank to grounds further North off Durban, in KwaZulu-Natal province. Like Emperor penguins in Antarctica, salmon in the rivers or indeed antelope across the African deserts, a single objective drives them: perpetuation of the species. Their reproductive instinct takes precedence over that for survival and pushes them along to overcome the many challenges in their way to return to their native area to scatter their eggs in the spawning grounds.
The logic is unyielding: if this homing strategy has worked for them, it will also succeed for their descendants. The resulting population balance justifies their efforts in the end. Great reproductive success wins out against the high mortality suffered during the migration. Releasing eggs further North, upstream of the ocean currents, ensures a better yield. Eggs, then larvae, thus have enough time to develop before they reach the Agulhas Bank. This bank is an extremely dispersive environment, where eggs or larvae become exposed to a high risk of being carried out to the open ocean, beyond the continental waters, and therefore lost to the species.
Permeation: the sixth sense of the sardine
How do the sardine manage to arrive each year exact at their own hatching site? Exchange mechanisms across its membrane allow permeation of the egg with components characteristic of the local marine ecosystem and with the terrestrial input from rivers. Then the same happens to the larvae. Olfactory processes are probably involved. Once adult, demonstrating exceptional sensory ability, the sardine pick up chemical stimuli coming from the local environment to guide themselves to their exact hatching site, in the KwaZulu-Natal.
In-bred heritage or loss of direction?
How have the sardine become fixed on an area so far North, near Durban, an environment so hostile to them? The research team decided on two hypotheses as being the most plausible.
The first suggests that this migration is a relict behaviour, probably inherited from the Last Ice Age. These sardine prefer sea temperatures of between 18 and 22°C. In those cold times they lived further North, off KwaZulu-Natal, where the ocean was colder than it is now. Then, the last deglaciation, brought on an overall warming of the seas. The fish therefore had to migrate towards the South, nearer the Pole. However, each year, in the reproductive season, they continued to come to the same place to spawn. That is why this ancient seasonal migration continues.
The second theory postulates that at a particular moment a shoal strayed off its normal migration route. Following some unusual oceanic conditions or a fault developed in the sardine's direction-finding system, their drift off-course led them to KwaZulu-Natal and resulted in an exceptional reproductive success. Two years later, the numerous descendants of this shoal, in their mature stage, repeated this epic voyage. From year to year, the population then grew, until it generated the enormous shoals we see today.
The efforts put in by the scientific community over the past five decades, summarized by this recent work, have substantially improved understanding of the ecological processes associated with the Sardine Run. This great migration route attracts a host of higher predators such as sharks, dolphins, whales and tuna, and generates a highly active fishing industry. Also, as a visually spectacular feature, it also stimulates tourism. Understanding how this extraordinary event of animal behaviour arose and why it persists would therefore help to meet some important economic necessities for the local communities.
These research studies were conducted jointly with the South African Departments of Agriculture, Forestry and Fisheries and of Environmental Affairs, the Universities of Cape Town and KwaZulu-Natal in South Africa and the South African Institute for Aquatic Biodiversity.
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