One of the most spectacular ocean walks in the world is the KwaZulu-Natal Sardine Run. The so-called “largest swarm on earth” takes place in the southern hemisphere during winter.
Dozens to hundreds of millions of sardines are transported from the warm, temperate waters of the south coast of South Africa to the subtropical waters of the east coast more than a thousand kilometers away.
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MASS MIGRATION OF SARDINES
This annual mass migration, first reported in 1853, is triggered by cold water upwelling on the southeast coast of South Africa. Cold, nutrient-rich water rises from the depths and forms a highly productive food web.
The hike attracts numerous predators: the swarms of sardines are followed by sea birds, sharks, seals, dolphins and even large baleen whales.
These eat as many of the helpless sardines as possible, which is made easier by the fact that their prey is sandwiched between dry land and the hot, tropical waters of the south-flowing Agulhas Current, which exceed the sardines’ physiological tolerances.
To make matters worse, it is still not easy for the fish that survive the predators: The journey is so strenuous that the sardines that finally arrive on the east coast are emaciated.
This contradicts what scientists understand about animal migrations – such large-scale population movements usually offer a “selective advantage” in allowing animals to make the most of environmental resources.
Surely the obvious disadvantages of participating in the sardine run have to be far outweighed by some fitness benefits to make it all worth it? The answer, according to our new research, is “no” – and the reason why sardines behave lies in their genes.
AN EXCELLENT EAST COAST POPULATION?
A popular explanation for the sardine outbreak is that the migration could be a relic of spawning behavior that dates back to the last ice age about 10,000 years ago. What is now the sub-tropical habitat of the Indian Ocean could have been an important growing area with cooler waters.
With the end of the Ice Age, the sardines would have adapted physiologically to the subtropical conditions in this region and developed into a distinct east coast population that spawns there to this day.
These sardines mix with south coast sardines in summer and then separate from them in winter as they migrate up the east coast. The presence of sardine eggs in plankton confirms that spawning is taking place in this region.
Surprisingly, we found that the sardines involved in the migration are not part of any particular east coast population. Instead, they mainly come from the colder waters off the Atlantic west coast of South Africa.
Why do these sardines migrate to the other end of the country only to end up in a habitat that is obviously too warm for them? We suggest that the fish be drawn into an ecological trap – a rare example of mass migration that has no obvious fitness benefits.
Our research was based on the assumption that the sardine run represents the spawning migration of a certain sardine population that is physiologically well adapted to subtropical conditions.
Physical characteristics and other data suggest that sardines are indeed different on the east coast. However, this can result from various environmental pressures, including the stress of participating in migration.
We knew that understanding the hereditary genetic traits of sardines would provide stronger evidence for this hypothesis – or debunk it.
So we used thousands of genetic markers from the genomes of hundreds of sardines caught across the species’ range in South Africa. Although most of these markers showed little differentiation, a number of genetic markers with a signal of adaptation to water temperature showed regional differences.
We found evidence of two regional populations – but it wasn’t the east coast sardines that were distinguished.
Instead, we found genetic differences within the temperate core of the species: one population was associated with the cool-temperate west coast of South Africa (Atlantic Ocean) and the other with the warm-temperate south coast (Indian Ocean).
The strong association with water temperature suggests that thermal adaptation maintains these regional patterns; Each population cluster is adapted to the temperature range that it experiences in its home region.
The sardines participating in the run showed a clear affiliation with the west coast population. Not only are these sardines not well adapted to subtropical conditions, but they even prefer the colder, swelling waters of the southeastern Atlantic.
IMPORTANT PUZZLE ABOUT SARDINE RUN SOLVED
This study solves some of the big puzzles surrounding the sardine run that make perfect sense in light of the new findings.
Our results explain why only a small fraction of the sardines found on the south coast take part in the run. The majority of these sardines come from this region and are adapted to warm, temperate conditions. For this reason, they show little interest in the cold, swelling water.
The results also provide an explanation as to why sardine outbreaks do not occur in years without cold water buoyancy. The upwelling on the southeast coast attracts west coast sardines, which have scattered to the south coast but are not well adapted to the warmer water temperatures in this region.
They consider the upwelling areas in the southeast essentially to be the habitat of the west coast. For a short time it is as if they were back home in the Atlantic – but when the upwelling ends and the water temperatures rise, their fateful error is revealed.
At this point the predators have gotten wind of their presence, and as the sardines try to flee, they travel further and further north to unbearably warm subtropical habitats. The fate of the fish that survived the sardine run is uncertain.
Our genomic explanation shows that there is still much to be discovered about how marine life interacts with its environment.
Much integrative, multidisciplinary research is still needed before humans can benefit efficiently and sustainably from the incredible diversity of life and the resources of the oceans.
This article was republished by The Conversation under a Creative Commons license. Read the original article.