Sardine Run

Sardine Run

Subject – Environment

Context – South Africa’s massive ‘sardine run’ leads fish into an ecological trap

Concept –

• One of the world’s most spectacular marine migrations is the KwaZulu-Natal sardine run. The so-called “greatest shoal on Earth” takes place during the southern hemisphere’s winter.

• It involves the movement of tens to hundreds of millions of sardines from the warm-temperate waters of South Africa’s south coast to the subtropical waters of the east coast, over a thousand kilometres away.
• This annual mass migration, first reported in 1853, is triggered by cold water upwelling on South Africa’s south-east coast. In this process, cold, nutrient-rich water rises up from the deep, creating a highly productive food web.
• The migration attracts vast numbers of predators: the sardine schools are followed northwards by seabirds, sharks, seals, dolphins and even large baleen whales. These devour as many of the helpless sardines as they can, which is made easier by the fact that their prey is sandwiched between dry land and the hot, tropical waters of the southward-flowing Agulhas Current, which exceed the sardines’ physiological tolerances.
• The journey is so strenuous that the sardines which eventually arrive on the east coast are emaciated. This goes against what scientists understand about animal migrations – such large-scale population movements normally provide some “selective advantage” by allowing animals to make optimal use of environmental resources.
• One popular explanation for why the sardine run occurs is that the migration might be a relic of spawning behaviour dating back to the last glacial period, about 10,000 years ago. What is now subtropical Indian Ocean habitat may have been an important nursery area with cooler waters.
• When the ice age ended, the sardines would have physiologically adapted to tolerate the subtropical conditions in this region, and evolved into a distinct east coast population that continues to spawn there to this day.
• These sardines mix with south coast sardines during summer, then separate from them in winter as they migrate up the east coast. The presence of sardine eggs in the plankton confirms that spawning does occur in this region.