The world's largest sardine migration

The annual migration of giant schools of sardines from eastern South Africa to the Indian Ocean is the largest on the planet in terms of biomass.

The wildebeest's round-trip migration across the Serengeti plains is one of the most impressive events on Earth, but it's not the largest animal migration. Measured by biomass, in this case the total mass of an animal in a particular area, the annual sardine migration actually dwarfs the wildebeest, according to IFL Science .

A school of sardines swims past a shark on their migration.

Sardines migrate each year along the eastern coast of South Africa, with huge schools of fish moving north from the cold waters off Cape Agulhas towards KwaZulu-Natal and the Indian Ocean. Schools can stretch more than 7km long, 1.5km wide and 30m deep. Collectively, billions of sardines take part in the migration. While it is one of the largest migrations on the planet, it does have its downsides. The large numbers of sardines attract a host of predators, from dolphins and sharks to seabirds and fur seals.

If sardines are vulnerable to predators, why do they migrate year after year? Using genetic data, a 2021 study determined that the majority of migrating sardines originate in the colder Atlantic. A brief upwelling of cold water in the normally warm southern ocean encourages the sardines to migrate. When the upwelling ends, they find themselves stranded in an area they haven't adapted to and exposed to predators.

The team concluded that the sardine migration was 'a rare and unproductive example of mass migration' and was essentially a trap. However, William Sydeman, an ecologist and president of the Farallon Institute for Advanced Ecosystem Studies in Petaluma, said the sardines migrate to take advantage of temporarily favorable ocean conditions in the Western Cape.

Professor Peter Teske of the University of Johannesburg suggests that the sardine migration could be a relic of spawning behaviour dating back to the Ice Age. The subtropical Indian Ocean, which today is home to the fish, was once a key breeding ground for young sardines in its cold water conditions. If so, climate change could spell trouble for the future of sardines.