The greatest shoal on Earth
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When it comes to mass migrations, the most famous is probably the annual event on the African Serengeti, where a herd of a million wildebeest, along with a supporting cast of zebras and gazelles, roam hundreds of miles across the plains in search of water and lush grass - trailed, as you might expect, by predators like lions and hyenas. It’s so large it can be seen from space, with keen wildlife photographers often planning their safari around the best time to see the animals on the move.
But there’s an even bigger animal migration based around Africa that deserves just as much attention. Popularly known as the “greatest shoal on Earth”, the KwaZulu-Natal sardine run involves tens to hundreds of millions of Pacific sardines packed into high density shoals to make the annual trip from the coast of South Africa to spend the winter in the Indian Ocean.
For sheer size and spectacle, this marine migration is no less impressive than the wildebeests on terra firma. But unlike the Serengeti herds, which migrate in search of food, the underlying reason for the sardine run is much less clear.
In search of answers, a team led by Peter Teske from the University of Johannesburg set about analysing DNA from hundreds of sardines collected around the South African coast.
Curiously, the team found two genetically distinct populations of sardines - one in cooler climes on the western Atlantic coast and the other favouring the warmer waters of the eastern Indian Ocean coast - each with specific genetic adaptations that enable it to thrive in its favoured climate.
However, while you might expect the warm sardines to be the ones that like to go walkabout - this isn’t the case. The fish taking part in the annual migration are mainly the Atlantic, cooler water variety.
Every year, the sardine run is triggered by cold water welling up on the southeastern coast of South Africa, attracting the cold water-loving fish and pushing them northeastwards. Not only does the warmer water have a negative impact on their fitness - combined with the exhaustion of moving along with less food availability - they also end up sandwiched between the coast and a layer of warmer water coming down from the Indian Ocean, packing them in like, err, sardines and making them easy snacks for predators like dolphins, sharks, seabirds and even whales.
It doesn’t seem to be for food, and it’s not for the weather, so why would millions upon millions of these little fishies make such a dangerous journey, for no apparent reason?
The researchers suspect that the great sardine migration is an ingrained spawning behaviour left over from the last glacial period tens of thousands of years ago, when the now sub-tropical Indian ocean would have been much cooler. So it would make sense for cold-water sardines to head over there to spawn, when things got too nippy in the winter. Despite what was then a very good idea now being a very bad one, for some reason the sardines are still compelled to migrate.
Finally, having been tricked once by changing temperatures into an unnecessary and damaging migration, it’s looking likely that the great sardine run may be finally stymied altogether by modern climate change. As the planet warms, the cold water sardines may not be able to adapt and survive. Although the number of fish taking part in the migration is impressive, it’s actually less than 10% of all the sardines in the South African region. So while there are likely to be plenty of warm water adapted sardines, they don’t have the same urge to ‘get up and go’ as their cold water cousins. So this incredible spectacle may not be around for much longer. Appropriately for sardines, if you want to see them on the move, you’ll have to catch it while you… can.
