One of the world’s most spectacular marine migrations is the KwaZulu-Natal sardine run. The so-called “greatest shoal on Earth” takes place during the southern hemisphere’s winter. It involves the movement of tens to hundreds of millions of sardines from the warm-temperate waters of South Africa’s south coast to the subtropical waters of the east coast, over a thousand kilometres away. This annual mass migration, first reported in 1853, is triggered by cold water upwelling on South Africa’s south-east coast. In this process, cold, nutrient-rich water rises up from the deep, creating a highly productive food web. The migration attracts vast numbers of predators: the sardine schools are followed northwards by seabirds, sharks, seals, dolphins and even large baleen whales. These devour as many of the helpless sardines as they can, which is made easier by the fact that their prey is sandwiched between dry land and the hot, tropical waters of the southward-flowing Agulhas Current, which exceed the sardines’ physiological tolerances.
SOURCE: THE CONVERSATION