Common dolphin in the Sardine Run
As their name implies, the Common Dolphin (Delphinus delphis) is one of the most abundant of all dolphin species, found world-wide in tropical and warm temperate waters. They often occur in very large schools and are sometimes associated with diving gannets and feeding whales and penguins. Generally, schools move at about 7 km/h, but common dolphins are fast swimmers and can attain speeds of over 35 km/h. Some schools of Common Dolphin estimated to be over 5000 animals have been observed chasing large fish shoals out at sea. Like most dolphins, the Common Dolphin are preyed upon by orcas and various sharks, their only other threats being man-made (pollution, toxins and fishing activities). As with many species, there are sub-divisions into the Short-beaked Common Dolphin and the Long-beaked Common Dolphin to try to cater for variations in sizes and coloration, but even within these two groups, variations still exist.
In South Africa, the common dolphin is a primary predator of sardines and other baitfish during South Africa’s annual Sardine Run. This migration of fish is a critical feeding opportunity for many predators of sardine and anchovy including the common dolphin. The common dolphins are thought to be critical players because they do most of the hard work by herding the fish into tight bait balls which are easier to feed from for all predators concerned. They successfully form ‘bait balls’ by using cooperative herding and bubble netting to drive pockets of baitfish towards the surface in tight bait balls. Once a bait ball has formed, dolphins will swim through the bait ball picking out individual baitfish to feed on. Other opportunistic predators, such as shark, gannets, fur seals and whales will utilise these bait balls to also feed on the balls of bait fish.
Experienced tour operators will frequently shadow the movements of common dolphin pods waiting for them to drive part of a sardine shoal to the surface and form a bait ball. It is these dolphin made bait balls that allow divers to view and witness the sardine run in South Africa
Common dolphins are stream-lined animals with a prominent, sickle-shaped dorsal fin. It’s markings are complex and elaborate, with a dark back, a striking creamy yellow to buff (at the front) and grey (towards the tail) figure of eight on the flank, a lighter tail and belly and a distinct dark stripe from the slender, black beak to around the eye. When observed at sea in large schools, the yellow side flashes are quite distinctive and can be seen from large distances.
Common Dolphins have 40 to 58 pairs of small, pointed teeth and feed on small, shoaling fish such as pilchard and herring as well as squid. They appear to rest in large schools during the middle of the day, and then disperse into feeding areas in the afternoon. This behaviour corresponds to the movement of zooplankton which lay low in the water column during mid-day to ovoid the light, and then rise to the surface when the sun sets in order to graze. The zooplankton attract fish and squid, which in turn become the dolphin’s prey. When day breaks and the zooplankton sink into the darkness, the dolphins once again form large groups and rest.
The gestation period for Common Dolphins is about one year, after which the female gives birth to a single calf that is about 95 cm in length. Calves are suckled for almost 6 months, with mothers (such as in South Africa) using the sardine run and other abundant food sources to wean their calves. The peak calving season is in summer.
The best time of year to see common dolphins is in late summer, autumn and early winter when they are lured from the deeper ocean to follow the migration of fish along the coastlines. The famous Sardine Run off the east coast of South Africa attracts large schools of all species, and particularly the Common Dolphin. They form huge schools for short period of time, of up to 10,000 animals in some areas, although most regular and stable groups are composed of about 200 individuals. The best places in South Africa to spot common dolphin are Plettenberg Bay from Robberg Peninsula, St. Francis and Algoa bays and East London, especially while the pilchards are moving slowly up the east coast. At this time, they may be seen as far north as Richards Bay.