Mystery of why annual 'sardine run' happens revealed

The annual winter migration of millions of sardines up the country’s east coast is a spectacular event that many South Africans and visitors flock to KZN and the Eastern Cape to witness. But up to now the reason behind the “sardine run” has been a puzzle, even to scientists.

A new study has, however, revealed how one of the world’s biggest migration events works.

According to the study, by South African and Australian scientists, and published in the journal Science Advances, there are two sardine populations in SA, 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 it experiences in its native region.

“Surprisingly, we also discovered that sardines participating in the migration run are primarily of Atlantic origin and prefer colder water,” said one of the study authors, Prof Luciano Beheregaray at the Flinders University molecular ecology lab based in South Australia.

Scientists tested the hypothesis that the sardine run represents the spawning migration of a distinct east coast stock adapted to warm subtropical conditions. The scientists generated genomic data for hundreds of sardines from around SA, including data from regions of the genome associated with differences in water temperature along the coast.

They found that the run is triggered by the upwelling of cold water on the southeast coast. As the fish swarm north they get sandwiched between the coast and a southward-flowing hot current that exceeds the sardines' physiological capacity.

They are then predated by huge numbers of dolphins, sharks, seabirds and even whales, an event that has featured in many nature documentaries.

Prof Peter Teske from the University of Johannesburg said the 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,” he said.

Scientists also suggest that this visually breathtaking migration, which is considered the “greatest shoal on earth” and attracts tourists from around the world keen to get a glimpse of the underwater spectacle, may not be around forever.

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

Despite the huge numbers of fish involved, the run involves only a small portion of the South African sardine population. So while its end would mean the loss of one of nature’s most spectacular migrations, the effects on the population as a whole are likely to be negligible.

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