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Pod of common dolphins working together to round up sardines and push them toward the surface so that the dolphins can feed on them. The image was taken during the sardine run off the east coast of South Africa’s province of Kwazulu-Natal. Credit: wildestanimal/Getty Images

Africa - Mystery of Doomed Sardine Migration Is Finally Solved

Pulses of cold seawater mislead millions of sardines into swimming along the South African coast to their death

As winter arrives in South Africa, anticipation begins to build for one of the planet’s largest and most spectacular migrations: the KwaZulu-Natal sardine run. In the waters off the southern tip of Africa, hundreds of millions of the slender, silvery fish coalesce into massive shoals and begin to travel hundreds of kilometers northeast along the rainbow nation’s eastern coast. The mass migration attracts a deep roster of predators that includes sharks, dive-bombing cormorants, Bryde’s whales and dolphin megapods. The ensuing feeding frenzy itself attracts divers and tourists from around the world. But why the feeble fish take up this gauntlet is not well understood—especially because only around 10 percent of South African sardines participate in the run, and some years the fish fail to run at all.

One popular explanation holds that the sardines participating in the run belong to a distinct eastern subpopulation that evolved in the Indian Ocean, so the fish are migrating to ancestral spawning territories there by instinct. But a genomic analysis of hundreds of South African sardines reveals that those in the run hail from the Atlantic Ocean, suggesting that the spectacular migration may serve no benefit to the fish and instead traps them in hostile waters, researchers wrote this fall in Science Advances.

“We're used to adaptations benefitting an organism, but it doesn't always work that way,” says Jessica Glass, a marine biologist at the University of Alaska Fairbanks who was not involved in the study. “These animals are doing something that doesn't actually benefit them in the long term.”

The researchers examined the DNA sequences of 284 sardines captured along the South African coast. These included fish residing in the Atlantic Ocean and Indian Ocean as well as fish from the 2015, 2018 and 2019 sardine runs that the researchers anticipated were members of a third, east coast population. Thousands of markers were examined for genetic commonalities in an effort to identify broad subpopulations.

A geographic trend emerged among DNA segments associated with adaptations to specific water temperatures: Atlantic fish were genetically adapted to cold water, whereas Indian Ocean fish were genetically adapted to warm water.

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