Sardine run

21 of may -homepage

The sardine run, the largest biomass migration in the world, is even larger than the migration of the wildebeests. This is an annual migration of Pacific sardines (Sardinops sagax) from the Agulhas bank Northwards along the East coast of South Africa to the KwaZulu-Natal (KZN) area in winter. The Pacific sardine is a small pelagic, plankton-feeding fish that reproduces rapid and frequently.

 How does it happen? Where do they come from? And where do they go to? The annual sardine run is corresponding with an offshore movement of the warm Agulhas Current in the region of the Wild Coast during autumn through to winter and its replacement by a cool, narrow band of surface water inshore. This event provides a corridor for cold-water-loving sardines to migrate Northward in large shoals that may be 15 kilometres long, three kilometres wide and 40 metres deep.

 Where do these sardines come from? One hypothesis is that the sardine run has this as a seasonal reproductive migration of a genetically distinct sub-population of sardines. This means that the sardines taking part in the run are sexually matured and they do the run to mate and lay thousands of eggs. Then where do they go to after the run is finished? The sardines appear to remain in KwaZulu-Natal (KZN) waters after the run, indicated by the presence of their eggs in samples taken between June and December by scientists. The adults are however, no longer observed close inshore or in near-surface waters during this time, but may be in the cold deeper water of KZN.

 Is the sardine run threatened? It has become a commercial thing with divers going into the ocean to see the bait balls and fishermen fishing the sardines out. With fishermen overfishing the ocean, will this also affect the sardines? Most scientists don’t believe it will. If we completely deplete the fish yes, but no sign of that is apparent. In 2003, the sardines failed to ‘run’ for the third time in 23 years. While 2005 was a good run, 2006 marked another non-run. This is not believed to be due to overfishing but more to the water temperature rising. It is believed that the water temperature has to drop below 21 °C in order for the migration to take place. The fishing of the sardines has not had a great effect on the stock. The stock is on a steep decline, and this is most likely due to the natural oceanographic cycles. Fossil evidence suggests that the Pacific sardines have experienced “boom-and-bust” cycles with lows every 60 years over the last 1,700 years, independent of fishing.

 When fishing the Pacific sardines they are usually taken with purse-seines, a method that is dragging a net in the area the fish is seen. This way of fishing produces a little by-catch and negligible habitat damage. The shoals attract thousands of predators such as gannets, dolphins and sharks, but when fishing with a seine net very few of the predators get caught. Why do the sardines form such big schools?The instinctual behavior of a lot of animals is to group together when migrating.  This is a great protect mechanism, because one singular individual are more likely to be eaten than when in a large group.



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