Scientists believe the sardine run may be a relic of spawning behaviour dating back thousands of years to the glacial period.
They have unpacked, using genomics, how one of the world’s biggest migration events works.
The event, referred to the “greatest shoal on Earth”, involves the movement of hundreds of millions of sardines from their cool-temperate core range into the warmer subtropical waters of the Indian Ocean, on SA’s east coast.
They are followed by huge numbers of predators, including dolphins, sharks, seabirds and whales — an event that has featured in many nature documentaries.
A new study in the journal Science Advances by SA and Australian scientists tested the hypothesis that the sardine run represents the spawning migration of a distinct east coast stock adapted to warm subtropical conditions.
They generated genomic data for hundreds of sardines from around SA, including data from regions of the genome that are primarily associated with differences in water temperature along the coast.
The results showed 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 that 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 Prof Luciano Beheregaray, from the Flinders University Molecular Ecology Lab, one of the study authors.
Prof Peter Teske, from the University of Johannesburg, and another author 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,” said Teske.
This visually breathtaking migration attracts tourists from around the world who are keen to get a glimpse of the underwater spectacle, but it 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 Prof Beheregaray.
The scientists said despite the huge numbers of fish involved, the run involves only a small portion of the SA 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.
The KZN Sharks Board said: “Fresh, frozen, canned, pickled or bait — whatever way you consider them, sardines (also known as pilchards) will have featured somewhere in the lives of many South Africans.
“Although these fish are small, collectively they comprise nearly a quarter of the world’s fish catch by weight, making them one of our most valuable groups of fish.
“The sardine run is an annual phenomenon sparked by the entry of large shoals of sardines into the waters of southern KZN during the winter months. Although the great bulk of SA’s sardine stock is to be found in the cooler Cape waters, each winter a small proportion of the stock moves eastward up the Wild Coast.
“These shoals take advantage of cool water on the continental shelf of the east coast that occurs seasonally as a narrow band between the coast and the warm, southward-flowing Agulhas current.
“It is not clear what advantage the sardines gain by entering KZN waters. On the contrary, in fact, local waters are less food-rich than are Cape waters, the favourable cooler conditions are only temporary and, to make matters worse for the sardines, they are accompanied by many predators which prey on them heavily,” said the board.
Would you like to comment on this article?
Sign up (it's quick and free) or sign in now.
Please read our Comment Policy before commenting.