The Greatest Shoal on Earth, less prosaically known at the Sardine Run, has been an annual event along the east coast of South Africa for decades. What was simply a natural resource that provided an easily accessible protein source for local inhabitants and an income for fisherman has become a tourist event, classified with the Serengeti Migration and Iceland Atlantic Puffins as “Must See Nature Sites“.
The Sardines
The sardines, which are more correctly known as the Southern African Pilchard (Sardinops sagaxis), are the most fished species in South African waters, although not the most valuable industry. The pilchards are harvested on the Agulhas Bank, a broad, shallow part of the continental shelf which extends 250 kilometres (160 miles) south of Cape Agulhas, the southern most tip of Africa. The meeting of the warm Indian and cold Atlantic oceans fuels the nutrient cycle for marine life, resulting in one of the best fishing grounds in South Africa.

Catches vary significantly from year to year as a result on conditions and with time as authorities monitor and manage the resources, but in 2004 some 100,000 tons of pilchards were fished with only about 1% of these being off the KwaZulu Natal coast. Most pilchards are canned in tomato sauce or converted into fish meal.

The Sardine Run

The precise reason for the phenomenon is not really understood, but it is linked to the temperature along the east cost, as the run only starts when the water temperature drops below 21ºC. It is, however, not linked to the life cycle of the sardine since spawning occurs before the run. It is also only a small fraction of the sardines take the run, which can’t be explained.

The massive numbers of small fish, attract thousands of predators. The common dolphin, copper shark and Cape gannet are the most numerous but even whales feed on the shoals. The dolphins are known to force sardines to crowd together in small areas, forming “bait balls” as a defensive tactic. These balls can be up to 20 metres (65 feet) in diameter, but only last for five or ten minutes as they make such obvious targets for predators.

There are many stunning underwater photographs of bait balls but most can not be published - there are however, some good images on flickr by pats0n.
Frequency of the Sardine Run
In the last 20 years of the twentieth century, authorities agree that the sardines failed to run only twice. This seemed to mean that although there were most likely sardines moving north they were not readily visible from the KwaZulu Natal coast either because there were fewer sardines or they were further off land.

With the sardine run becoming established as a “tourism destination” and the fact that divers go out in boats to find shoaling sardines and bait balls it is likely that sardine activity is called the sardine run when it doesn’t match the criteria used previously.

However, in the new millennium there have already been three times when the sardines failed to run in just nine years. So even talking into account the vagueness of declaring a sardine run, there has definitely been a significant decrease in the number of sardine runs in recent years.

Global Warming
Given that everyone agrees the sardine run is triggered by the sea temperature adjacent to the coast, dropping below 21ºC, it would seem to be obvious to conclude that the change in the frequency of the sardine run is related to global warming.

The consequences would be fewer runs and possibly the end of the sardine run altogether. This would have little effect on South Africa’s fishing industry but would definitely effect the livelihoods of the poor and unemployed on the east coast. There would be a direct effect due to the loss of a free food source and sale income but also an indirect loss of income resulting from a drop off in tourism. 

Source: GO Media - Written by Dave Harcourt - Photograph by unforth on flickr under a under a Creative Commons license.