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The Greatest Shoal on Earth: Scientists discover why sardines run

The Greatest Shoal on Earth: Scientists discover why sardines run
Locals gather during the annual Sardine Run on Durban's Addington Beach on 26 June 2021. (Photo: Gallo Images/Darren Stewart)

It’s sometimes called the ‘Greatest Shoal on Earth’, and for good reason with hundreds of millions of small fish moving around in giant bands causing a feeding frenzy wherever they go. A new study in the journal 'Science Advances' by South African and Australian scientists explains why the sardines run.

Every year in June or July along the KwaZulu-Natal coast an incredible spectacle plays out. Hundreds of millions of Sardines move in a band — often more than 7km long, 1.5km wide and 30 metres deep — up the coast in what is commonly known as the annual “Sardine Run”. Now, a recent study using genomics explains exactly why Sardines ‘run’ at all.  

The study in the journal Science Advances was conducted by South African and Australian scientists who tested the hypothesis that the Sardine Run represents the spawning migration of a distinct east coast stock adapted to warm subtropical conditions.

The results of the study showed that there exist two distinct sardine populations in South Africa; one in the cool-temperate west coast (Atlantic Ocean) and the other in warmer east coast waters (Indian Ocean). Each regional population appears adapted to the temperature range that it experiences in its native region.

 

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“Surprisingly, we also discovered that sardines participating in the migration run are primarily of Atlantic origin and prefer colder water,” said Professor Luciano Beheregaray at the Flinders University Molecular Ecology Lab, one of the study authors.

Professor Peter Teske from the University of Johannesburg (UJ) and an author in the study said “[t]he cold water of the brief upwelling periods attracts the west coast sardines, which are not adapted to the warmer Indian Ocean habitat. 

“This is a rare finding in nature, since there are no obvious fitness benefits for the migration, so why do they do it? We think the sardine migration might be a relic of spawning behaviour dating back to the glacial period. What is now subtropical Indian Ocean habitat was then an important sardine nursery area with cold waters”, Professor Teske said.

“Given the colder water origins of sardines participating in the run, projected warming could lead to the end of the sardine run”, says Professor Beheregaray. 

Daily Maverick previously reported that there has been a “significant and progressive delay” in the arrival of these fish in recent years according to a recent study published in the South African Journal of Science

Professor Jennifer Fitchett from the School of Geography, Archaeology and Environmental Studies at Wits University said “this coincides with a poleward shift in the location of sea surface temperatures below 21C – as the sea surface temperatures warm up, it is too warm for the sardines to migrate and they wait for cooler temperatures which occur later into the middle of the winter season.”

The most recent study, however, suggests that while the numbers of fish are significant, the run itself involves only a small portion of the South African Sardine population. Accordingly, while its end might mean the loss of one of nature’s most dazzling mass migrations, the effects on the species’ population as a whole are likely to be negligible. OBP/DM

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