17 June 2010

Sardine bait ball swims into action

Two Oceans Aquarium

The sardine run has arrived at the Two Oceans Aquarium! A large shoal of sardines (also known as pilchards) was introduced into the I&J Predator Exhibit yesterday, 17 June 2010. The silvery shoal has transfixed staff and visitors alike with its shimmering underwater ballet.

The old adage “safety in numbers” has never been so apt. Sardines form a tight ball to protect themselves against potential predators, which – in our exhibit – include yellowtail, dusky kob and leervis.

When larger fish approach the shoal, the silvery fish move in the opposite direction in a single ribbon-like motion. Should an individual become separated from the shoal, it runs the risk of becoming a snack for one of the predators. 

“Watching the way the sardines move is like poetry in motion,” says Communications & Sustainability Manager for the Aquarium Helen Lockhart. “We would love to have them on display permanently as they are so beautiful to watch. The other inhabitants of the I&J Predator Exhibit would also be pleased as the sardines are such tasty snacks for them!”

Southern Africa is famous for the sardine run that happens here between May and July every year. Millions of sardines (Sardinops sagax) spawn in the colder Agulhas Bank and then swim northwards along the east coast in a giant ball, which results in a feeding frenzy along the coastline. The shoals are often more than seven kilometres long, one-and-a-half kilometres wide and thirty metres deep, and are clearly visible from spotter planes or from the water’s surface.

The sardine run is so large, in fact, that researchers reckon it could even rival the size of East Africa’s wildebeest migration! But the run, about which relatively little is known, seems to be under threat (possibly due to global warming and other environmental factors). Marine scientists believe that water temperature needs to be below 21°C in order for the migration to take place; but in 2003 and 2006 no run occurred. Could it be because the water was too warm then?

These pictures depict yesterday’s release of the sardine shoal at the Aquarium. “The sardines are released in batches, so the bait ball grows when the groups merge,” says Natasha Townsend, assistant to Aquarium Curator Michael Farquhar. “The yellowtail are already having a feast.”

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