Every diver and ocean lover has at least heard of the famous sardine run in South Africa where billions of sardines pass through the cooler waters of the Agulhas Bank and move north along South Africa’s east coast from May through July annually. The sardines are met by numerous species of sharks, dolphins and whales on their migration which makes the spectacle one of the greatest dive events on planet Ocean.
A new study, however, argues that the annual KwaZulu-Natal sardine migration, commonly called ‘the greatest shoal on earth’ might not actually benefit the fish itself. The study, published in the journal Science Advances, finds that the run is an ecological trap where the fish’s behaviour drives the population into an unfavourable habitat that ultimately decreases their chances of survival.
While the fish swim from their temperate core range into the subtropical Indian Ocean, it is believed that “this migration represents the spawning migration of a distinct subtropical stock”, the study says. The researchers behind the study sampled sardines from runs in 2015, 2018 and 2019 as well as sardines from their normal range, and compared the fish’s DNA and mRNA. Through this, they identified two stocks – a cool temperate, or Atlantic, stock and a warm temperate, or Indian Ocean, one.
The sardines in the annual migrations are primarily of Atlantic origin and thus prefer colder water. The study goes on to say that “these sardines separate from the warm-temperate stock and move into temporarily favourable Indian Ocean habitat during brief cold-water upwelling periods. Once the upwelling ends, they find themselves trapped in physiologically challenging subtropical habitat and subject to intense predation pressure. This makes the sardine run a rare example of a mass migration that has no apparent fitness benefits”.
Ultimately, the annual migrations seem to be a dead end for the Atlantic fish which are driven into a trap due to their innate desire to seek cooler waters. Instead of finding cooler water, they swim up the eastern coast and find themselves in warm, tropical water that is swept over from the Indian Ocean instead, trapping them between the shore and the warm current and limiting their ability to survive.
Other researchers and biologists have argued, however, that the fish do the migrations for reasons still unknown, saying that more research needs to be done before committing to the idea that it is actually an ecological trap.
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Photography courtesy of Unsplash.