South Africa’s sardine run is a spectacle to behold as thousands of predators feast on sardine mega-shoals – but climate change and over fishing are putting it at risk.
As southern winter approaches, cold water wells up and a silvery mass emerges from the ocean depths. Off the coast of South Africa, tiny torpedo-like fish amass. These densely-packed mega-shoals are often more than 7km (4 miles) long, 1.5km (1 mile) wide and 30m (100ft) deep and made up of tens to hundreds of millions of South African sardines.
“There might be a few gannets here and there, but nothing major,” says Stephanie Plön, a marine biologist at Stellenbosch University, South Africa, who studies the health of marine mammals off the KwaZulu-Natal coast. “Then all of a sudden they come together – and you see the water boiling, as dolphins and sharks follow the sardines, and the gannets start diving. You have to give up taking [identification] photos, because there are just too many dolphins!”
Following the cool nutrient-rich currents, the sardines embark on their annual migration from Cape Agulhas to Kwazulu-Natal – one, in terms of biomass, thought to rival the great herds of the Serengeti. Most animal migrations offer some benefit to the species involved, to winter in warmer climes or to return to a suitable place to lay their eggs, but the South African “sardine run” ends in disaster for most of the fish.
As these cool-water Atlantic Ocean sardines move northwards, the shoals are penned in, trapped between the eastern coastline of South Africa and the warm waters of the southward-flowing Agulhas Current. As the shoal moves north, it attracts thousands of predators.
“The sardines are trying to stay out of the Agulhas Current,” says Peter Teske, an expert in marine genomics at the University of Johannesburg, South Africa. “They don’t like this hot water – and move up the coastline in a relatively shallow area of cooler water. At Waterfall Bluff, in Lusikisiki Eastern Cape, the Agulhas Current is so close to the coast, they bottleneck.”
By its peak in June, the sardine run is a feeding frenzy. As many as 18,000 common and bottlenose dolphins round the sardines up into bait balls, stretching 20m (66ft) in diameter. Now, they are easy pickings for other predators too – and sharks, whales, seals and birds join in this most plenteous of ocean banquets.
“The sardine run is an exceptional event and an important part of the food web,” says Teske. During an upwelling event, he says – when cold, nutrient-rich water comes to the surface – phytoplankton blooms, zooplankton feeds on the phytoplankton, fish feed on the zooplankton, “and essentially everything else” feeds on the fish. “Suddenly, there is this huge source of protein.”
For the few sardines that survive the onslaught, the future remains bleak. The shoal continues north, moving with the cold water – until the upwelling ends. “They just keep moving into warmer water – the worst possible habitat,” says Teske.
These cold-water sardines find themselves trapped in a subtropical habitat, too hot for them to survive. The epic thousand-kilometre journey of this little fish ends in an “ecological trap” – making the sardine run a rare example of a mass migration that has no apparent benefits for the survival of the species.
“We don’t know the ultimate fate of the majority of them,” says Teske. “But, once the run is complete, it’s very likely a lot of them do die.”
But one thing is certain, an entire food web – including endangered species such as the African penguin, Cape cormorant, Cape gannet, and school shark – relies on the sardines to make the run. Now, over-fishing and warming oceans are causing sardine stocks to collapse and the world-famous sardine run could be at risk of ending within a few decades.