As winter approaches at the end of winter in South Africa, anticipation begins to build up for one of the world’s biggest and most stunning migrations: the KwaZulu Natal sardine run. In the waters near the southern end of Africa millions of slim, silvery fish congregate into huge shoals, and then begin to move hundreds of kilometers to the northeast across the nation’s east coast. The massive migration draws an array of predators including sharks, dive-bombing cormorants Bryde’s whales, and dolphin megapods. The subsequent feeding frenzy draws divers and tourists from all over 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.
The most popular argument is that the sardines that are part of the run are part of an exclusive eastern subpopulation that originated within the Indian Ocean, so the fish are moving to their ancestral spawning areas through instinct. A genomic study on several hundred South African sardines reveals that the sardines in the run come directly from an area of the Atlantic Ocean, suggesting that the dazzling migration may be of no benefit for these fish and could instead trap the fish in dangerous waters The researchers published their findings this autumn in Science Advances.
“We’re used to the idea of adaptations that benefit an organism, however it’s not always that in the same way.” states Jessica Glass who is a marine biologist at the University of Alaska Fairbanks who did not participate in the research. “These creatures are doing something that isn’t really benefiting their long-term health.”
The researchers looked at the genetic sequences for 284 of the sardines that were caught on the South African coast. They included fish that resided in the Atlantic Ocean and Indian Ocean and also individuals from 2015, 2018,, and 2019 sardine runs, which the researchers believed were part of a third east coast population from the east coast. A large number of markers were analyzed for common genetic traits to help identify large subpopulations.
A trend of geographic distribution emerged in DNA segments linked to adaptations to specific temperatures of water: Atlantic fish were genetically adaptable to cold water however, Indian Ocean fish were genetically adaptable to warmer water.
To the researchers’ delight There was no evidence of genetics indicating another subpopulation living and traveling along South Africa’s east coast. The fish that were collected from the runs had the same genetic traits as Atlantic Sardines, suggesting they were sourced from far away from the Atlantic Ocean. “Finding that these sardines originate from a totally different region is quite a shock,” says Peter Teske as the principal researcher of the study and an ocean biologist at the University of Johannesburg.
While sardines are known to prefer different areas of the ocean but it’s not uncommon for the fish to move around, Teske explains. Sardines in across the Atlantic Ocean sometimes stray too far to warm waters in the south which could result in short bursts of nutrient-rich, cold water rising along the coast that remind them of their homeland in the Atlantic Teske says. The cold-adapted fish may not realize that they are following the waters northward, and then end up trapped within more arid Indian Ocean waters when the upwelling stops. Already suffering biologically due to the warmer conditions, Sardines are apex predators from the air and the sea by numerous predators.
This is an example that an eco-trap is in place Teske declares. Teske cites the case of “hawks who move into urban areas due to the fact that there are lots of the pigeons. This is beneficial for a time due to the abundance of food available, however, they’re acquiring so many illnesses that their children don’t live to see.” When it comes to Sardines, he states, “we have a natural one of a massive size–perhaps the biggest ever recorded. It could be going on for hundreds many years.”
The results also address additional questions regarding Sardines, a major commercial fish species found in South Africa. In recent years the sardine industry on the coast of the country’s Atlantic coast has slowed. Many fisheries managers had believed they could benefit from sardines coming out of in the Indian Ocean might help Atlantic coastal sardine populations recover However, new research suggest that this is unlikely due to the fact that both populations of sardines have been able to adapt to different temperatures in the water. “This could explain why the Atlanticstocks have dwindled and the reason there wasn’t any replenishment on the southern coast” Teske says. “We must protect west coast sardines in order to prevent further problems,” he adds. “Essentially simply, cut down on fishing in the region. It’s that simple.”
As the ocean temperatures continue to rise due to climate change The fate of the KwaZulu-Natal sardine run remains uncertain. If cold rising waters continue to happen and warm waters are forming along the eastern coast could cause the sardine run’s run to stall out over time, and before the fish reach sharks, dolphins, and other predators that usually appear to feast. Climate change could have a variety of negative effects for Earth’s ecosystems. However, warmer oceans can help keep the swarming Atlantic Sardines by slowing the dreadful movement, Glass says. She says: “It’s kind of comforting.”
