Mass migrations are a crucial feature of the natural world, with millions of animals undertaking great journeys in search of food or mating opportunities. One of the largest, by number if not distance, represents an oddity, undertaken for no obvious reasons. New research shows it is even more curious than previously thought.

Every year hundreds of millions of sardines migrate up the coast of South Africa, packed like... err sardines into a narrow band of water. Predators of sea and sky make their own migrations to feed on the bounty. Documentaries celebrate it and tourists flock to watch.

Scientists have long wondered why the sardines do this. A study in Science Advances adds to evidence it is a historical legacy that persists despite harming the fish who make the journey more than it helps.

Offshore of South Africa cold Atlantic waters fresh from the Antarctic meet tropical Indian Ocean currents, producing an astonishing temperature gap in the space of a few kilometers. Sardines thrive in both waters, but there has long been a suspicion the two populations were separate, adapted to different conditions.

Professor Luciano Beheregaray of Flinders University told IFLScience that genetic studies had failed to distinguish between the two oceans' sardines, but his team could. “[Ours] was the first study to use genomics not genetics so we compared thousands and thousands of regions of the genome. It showed two distinct populations, but with some connectivity.” The team, also showed the two were adapted to different water temperatures.

Nevertheless, all was not as expected. “Surprisingly, we also discovered that sardines participating in the migration run are primarily of Atlantic origin and prefer colder water”, Beheregaray said in a statement. 

This appears like madness of the sort falsely attributed to lemmings. “The cold water of the brief upwelling periods attracts the west coast sardines, which are not adapted to the warmer Indian Ocean habitat”, said first author Professor Peter Teske of the University of Johannesburg.

When the upwelling stops, the Atlantic sardines find themselves in uncomfortably warm water.

Beheregaray told IFLScience; “Our study was not designed to find out what happens to them.” Certainly, many of the sardines are eaten by the whales, sharks, dolphins, and birds that make the event famous, but Beheregaray doesn't know how many others make it back to colder waters compared to those that die of heat stress. “Sardines in South African waters can live to five,” he added, so theoretically some might be foolish enough to make the journey twice.

The behavior seems so self-defeating the authors think it might be a legacy of times past. During the last Ice Age, the waters of the area were much cooler, and Atlantic sardines might have enjoyed an Indian Ocean sojourn even when the upwelling stopped. The South African tourism industry is benefiting from something that stopped making sense 10,000 years ago.

Global heating is expected to stop the “Greatest Shoal On Earth” with the waters becoming too warm for the majority of the Atlantic population to venture in. This isn't as bad news as it sounds. Less than 10 percent of the region's sardines participate. Even the animals that feast on the run close to shore only do so briefly, so Beheregaray doubts their populations will suffer too badly. On the other hand, he agreed, the tourism industry might be hit hard.