South African Sardine Run: Big Animals in Congregation
The sardine run along the east coast of South Africa is a spectacular and well-known phenomenon, but the seeds of our understanding are only now beginning to emerge.
We now know that this is one of the largest marine events on the planet, involving many species of fish, sharks, marine mammals and birds. Fishermen have been making the most of this annual winter event for decades and gradually, the media and tourism potential of this occurrence is being developed.
More recently international film crews have converged on the Kwa-Zulu Natal (KZN) and Eastern cape coastline to capture images for worldwide broadcast, and specialist operators have created excursions to allow the more adventurous to witness this exciting coastal exhibition of frenzied marine interaction.
The sardine run is much more than the simply numerous glistening shoals of sardines moving up the coast, for which the local tourism industry has coined the phrase 'the Greatest Shoal on Earth'. It is a dynamic and complex event that involves and affects many marine animals.
Copper sharks, common dolphins and Cape gannets are three key predators of the sardines and follow the shoals of sardines northwards along the east coast. The feeding displays that result are spectacular. Pods of common dolphin join together to form 'super-pods' several thousand strong. Cape gannets plunge into the water like fighter planes to feed on the sardine 'bait balls' that have been rounded up by the sharks and dolphins.
This breathtaking spectacle is an event that is unique in magnitude and complexity to the eastern coast of South Africa. It is perhaps not merely the greatest shoal on earth ' but also the 'Greatest Show on Earth'.
The Run Sardines, also known as pilchards, are cold-water fish and are typically found in areas of cold ocean upwelling, such as off the west coast of Australia, California, Japan, Peru/Chile and Southern Africa.
In these areas cold, nutrient-rich water is up welled from the deep and provides suitable conditions for minute free floating aquatic plants, called phytoplankton, to bloom. Sardines rely on phytoplankton and other minute free floating aquatic animals called zooplankton for food, as do their close relatives the anchovies and herrings. Collectively these small fish comprise 25% of the world's fish catch (by weight) and thus form the most valuable group of fish.
In South Africa there is a large sardine fishery off the Western Cape coast and approximately 100 000 tonnes are caught annually. Off the Eastern Cape coast the annual catch drops to about 7700 tons whilst it is only up to a maximum of 700 tons in Kwa-Zulu Natal.
Sardines have a short life-cycle and only live to 2-3 years of age. Adult sardines, which are about 18-20cm long and two years old, aggregate on the Agulhas Banks off the southern Cape coast. Here spawning takes place in the spring and summer months when each female releases tens of thousands of eggs into the water which are then fertilized by males. These eggs drift with the current in westerly and northerly directions into the nutrient-rich up welled waters off the west coast. Here the larvae mature and develop into juvenile fish which once strong enough, aggregate into dense shoals and migrate southwards, returning to the Agulhas banks in order to complete their life cycle.
Sardines are typically found in water between 14 ' 20 C/ 57 ' 68 F. During the winter months of June and July, the penetration of cooler water eastwards along the Eastern Cape coast towards Port St Johns, effectively expands the suitable habitat available for sardines. From the Port St Johns region northwards, it is likely that a cool, northerly flowing counter-current, flowing inshore of the southerly flowing Agulhas current, may be one of the factors responsible for the 'leakage' of large shoals of sardines further north in what has traditionally been known as the 'Sardine Run.'
Upwelling of cool water along this section of coast caused by northeasterly winds may also assist in the movement of large shoals of sardines northwards. The cool band of water inshore is critical to the run. If the water is too warm (over 20 C/ 68 F) the sardines will remain in the cooler water further south or move northwards further offshore and at greater depths where the water is cooler, consequently making themselves unavailable to the seine-net fisherman and many of the predators associated with them. This was the case in 2003 when unseasonably warm sea surface temperatures (21 ' 23 C/ 70 ' 74 F) were recorded off southern KZN coast during the months of June and July.
North of Port St Johns the sardines become concentrated in a narrow band of cool inshore water and as a result are easily located by predators. Predators, including various species of sharks, marine mammals, predatory fish and sea-birds, are quick to take advantage of this time of plenty in what are otherwise relatively unproductive waters. It is likely that once the sardines reach the Port St Johns stretch of coastline the predators may play an important role in driving the sardines close to the surface and inshore, making them accessible to seine-net fisherman in KZN.
Although numerous species of marine mammals take advantage of the bountiful supply of food, there are three key predators that follow the sardines north into KZN waters. These are the common dolphin, copper shark (bronze whaler) and Cape gannet. In fact the common dolphins and Cape gannets are thought to time their breeding cycles with the sardine run so that their young are weaned or fledged at the time of the event. This allows the youngsters to be exposed to an abundant source of food during a crucial learning phase of their lives, thus increasing their rates of survival.
It is thought that the common dolphin, of which over 20 000 migrate north into KZN waters, are responsible for rounding up pockets of sardines and driving them up toward the surface, resulting in the formation of 'bait balls'. When threatened sardines instinctively group together as a defense mechanism, as an individual fish it is at much lower risk of being eaten if it is part of a large group. These bait balls are typically 10 ' 20 m/ 30 ' 60 ft in diameter and extend to a depth of about 10 m/ 30 ft. Bait balls are normally relatively short lived events and an individual bait ball seldom lasts longer than 10 ' 20 minutes. Once the dolphins have done the rounding up, other predators are quick to capitalize on the opportunity. Gamefish such as shad, garrick, geelbeck and eastern little tuna dart in and out of the frenzy making the most of the 'fast-food' on offer.
Copper sharks are usually found in the cooler waters off the southern Cape and Namibian coasts but travel along the coast in their thousands to take advantage of the easy food sources the shoals have to offer. Other sharks include the blacktip, spinner, dusky and Zambezi also appear out of the deep blue to join in the feast. It is amazing how the sharks are able to home in on a particular bait ball from a considerable distance away ' at least several kilometers. One only has to fly over a well-established bait ball to see the sharks radiating in towards it.
Although it is not clear to what extent the sharks and gamefish rely on the common dolphins to round up the sardines in order for them to feed ' the seabirds certainly do. Unless the fish are near the surface they are inaccessible to the sea birds such as cormorants, gulls, terns, and the tens of thousands of Cape gannets that have followed the sardines northwards from Algoa Bay.
Once the sardines have been driven up from the depths, spectacular displays of feeding activity take place as Cape gannets launch aerial assaults on the sardines as they dive into the surface waters to feed. The height from which the gannets dive depends on the depth of the fish. If the fish are quite deep, say at a depth of 5 ' 10 m/ 15 ' 30 ft, the birds may dive from as high as 30 m/ 90 ft. The birds may only dive to a depth of 5 m/ 15 ft or so but are able to swim down to depths of about 8m/ 24 ft in order to obtain food.
There is approximately 1000 resident bottlenose dolphin along the KZN coastline. These are the dolphins most commonly seen as they are often found close inshore in groups of 10 ' 60 and regularly surf waves. It appears that these dolphins do not feed to a large extent on the shoals of sardines that move along the KZN coast. It is however possible that the 2000 plus bottlenose dolphin that migrate into KZN waters from the Eastern Cape during the winter months make more use of the sardines.
Humpback whales are also spotted regularly during the sardine run. Their presence is however merely co-incidental as oddly enough, they have not been observed feeding on sardines. The humpback whales feed in Antarctica during the summer months. There they feed on krill (a small shrimp-like crustacean) by emitting a stream of bubbles through their blowholes forming a 'net' or 'curtain', which confuses and traps the prey. The whales then lunge to the surface opening their jaws up to 4.5m/ 14 ft wide to engulf both water and krill. The water is then sieved out through the baleen plates (comb-like structures that hang from the upper jaws of these creatures) and the krill can then be swallowed.
After summer feeding has taken place the humpback whales migrate north to give birth and mate off the northern KZN and Mozambican coasts during the winter and spring months. During this migration these whales may travel up to 8000 km in what is probably the longest mammal migration known to man. Humpback whales can often be seen performing spectacular leaps out of the water known as breaching, on the sardine run.
Southern right whales are also observed off the eastern coast during the winter months but are much less common than the humpback whales. Like the humpbacks, the southern right whales do not appear to feed off the KZN coast but migrate from their summer feeding grounds in Antarctica to give birth and mate off the southern Cape and eastern coast of southern Africa.
Unlike the humpback and southern right whales, the Bryde's whales do feed on the sardines. Although these whales are present off the southern African coast year round and regularly feed of shoaling fish, they are not often seen in KZN waters. They have however been observed moving through patches of surface shoaling fish at a speed of 8 ' 12 knots with the top third of their bodies often clearing the water as the whales race vertically up through the shoals of fish toward the surface.
Another marine mammal that feeds on sardines is the Cape fur seal. Many of these animals follow the sardines from the southern Cape along the Eastern Cape coast as far north as Port St Johns.