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It is thought that
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 surface, resulting in 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 dolphins have done rounding up, other predators are quick to capitalize on opportunity. Gamefish such as shad, garrick, geelbeck and eastern little tuna dart in and out of frenzy making most of “fast-food” on offer.
Copper sharks are usually found in cooler waters off southern Cape and Namibian coasts but travel along coast in their thousands to take advantage of easy food sources shoals have to offer. Other sharks include blacktip, spinner, dusky and Zambezi also appear out of deep blue to join in feast. It is amazing how 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 sharks radiating in towards it.
Although it is not clear to what extent sharks and gamefish rely on common dolphins to round up sardines in order for them to feed – seabirds certainly do. Unless fish are near surface they are inaccessible to sea birds such as cormorants, gulls, terns, and tens of thousands of Cape gannets that have followed sardines northwards from Algoa Bay.
Once sardines have been driven up from depths, spectacular displays of feeding activity take place as Cape gannets launch aerial assaults on sardines as they dive into surface waters to feed. The height from which gannets dive depends on depth of fish. If fish are quite deep, say at a depth of 5 – 10 m/ 15 – 30 ft, 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 KZN coastline. These are 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 shoals of sardines that move along KZN coast. It is however possible that 2000 plus bottlenose dolphin that migrate into KZN waters from Eastern Cape during winter months make more use of sardines.
Humpback whales are also spotted regularly during 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 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 prey. The whales then lunge to surface opening their jaws up to 4.5m/ 14 ft wide to engulf both water and krill. The water is then sieved out through baleen plates (comb-like structures that hang from upper jaws of these creatures) and krill can then be swallowed.
After summer feeding has taken place humpback whales migrate north to give birth and mate off northern KZN and Mozambican coasts during winter and spring months. During this migration these whales may travel up to 8000 km in what is probably longest mammal migration known to man. Humpback whales can often be seen performing spectacular leaps out of water known as breaching, on sardine run.
Southern right whales are also observed off eastern coast during winter months but are much less common than humpback whales. Like humpbacks, southern right whales do not appear to feed off KZN coast but migrate from their summer feeding grounds in Antarctica to give birth and mate off southern Cape and eastern coast of southern Africa.
Unlike humpback and southern right whales, Bryde’s whales do feed on sardines. Although these whales are present off 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 top third of their bodies often clearing water as whales race vertically up through shoals of fish toward surface.
Another marine mammal that feeds on sardines is Cape fur seal. Many of these animals follow sardines from southern Cape along Eastern Cape coast as far north as Port St Johns.
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