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 migration on its round trip around the Serengeti plains is one of the most impressive events on Earth, but not the largest animal migration. If measured by biomass, in this case the total mass of an animal in a specialized area, the annual sardine migration actually beats wildebeest, according to IFL Science.
Sardines migrate each year along South Africa’s east coast, with huge schools of fish moving from the cold waters off Cape Agulhas north towards KwaZulu-Natal and the Indian Ocean. Sardine schools can stretch more than 7 km long, 1.5 km wide and 30 m deep. Combined, billions of sardines participate in the migration. Although this is one of the largest migrations on the planet, it also entails disadvantages. Large numbers of sardines attract many predators to feed, from dolphins and sharks to seabirds and fur seals.
If they are easy targets for predators, why do sardines still migrate year after year? Using genetic data, a 2021 study determined the majority of sardines participating in the migration originated in the Atlantic Ocean, where the water is colder. The brief upwelling of cold water in normally warm southern waters prompts sardines to move. When the upwelling ended, they found themselves trapped in an area they had not yet adapted to and facing many predators.
The team concluded that the sardine migration was a “rare and unmerited 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, believes that sardines migrate to take advantage of temporarily favorable ocean conditions in the Western Cape.
Professor Peter Teske at the University of Johannesburg said that the sardine migration could be evidence of egg-laying behavior dating back to the Ice Age. The present-day habitat of the subtropical Indian Ocean was once an important breeding ground for young sardines with cold water conditions. If so, climate change could affect the future of sardines.