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Facts on Common dolphins.

Commond_Dolphins_Plettenberg_Bay

Off Southern Africa two species of common dolphin can be found, long-beaked (Delphinus capensis) and short-beaked (Delphinus delphis). Until fairly recently, both were considered to be the same species with great variation in appearance, but due to many years of research, are now treated as separate species. The home ranges of the two species over-lap, but the short-beaked is primarily oceanic, while the long-beaked is more tropical, living within the continental shelf and in coastal waters. In Plettenberg Bay common dolphins (long-beaked) can be seen throughout the year, but predominantly in summer and early autumn. Their presence in the bay has been related to the seasonal movements of their prey (mainly sardine and anchovy).

Both long and short-beaked common dolphins are easy to distinguish from other dolphins by the ‘hourglass’ marking on the side. This is a golden-yellow colour nearest the head and light grey towards the tail. They are dark grey dorsally (on the back) and pale underneath (belly). They reach lengths of 2.2 m (females) to 2.5 m (males) in length and can weigh up to 140 kg. They have a gestation period of around 11 months and calves are approximately 1m at birth. Although they can give birth all year round, there is a peak in births during summer (Feb/Mar). Females reach sexual maturity at around 8 years of age and they will calve every 2 -3 years. Males are sexually mature at 10 – 12 years of age.

School sizes can range from 1 – 10 000 individuals, but the average group size is around 600. They can travel at high speeds for extended periods of time and regularly bow ride boats and ships.. They can often be seen travelling with Bryde’s whales and when going at speed they make the ocean look like it’s boiling. The average speed common dolphins keep is 7 km/h, but they can reach speeds of 35 km/h and have been reported to cover 120km in 24 hours.

Common dolphins feed primarily on pelagic shoaling fish such as pilchards (Sardinops ocellatus), mackerel (Scomber japonicus), squid (Loligo spp) and anchovies (Engraulis japonicus). They are also seen following the spectacular natural event known as the ‘Sardine Run’ into Kwazulu-Natal waters in winter. This migration of fish is a critical feeding opportunity for many predators of sardine and anchovy and ‘feeding frenzies’ involving Bryde’s whales, sharks, fur seals, seabirds, predatory fish and common dolphins occur. 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.
Neither of the two species of common dolphin are threatened, they are classified as Least Concern and Data Deficient  by the IUCN, but because they are marine mammals they are protected in South African waters by the Marine Mammal Protection Act of 1972. In some countries common dolphins are hunted for food and bait in fisheries. The largest threat to their survival is from entanglement in gillnets, trawlers and the purse seine fisheries. A large number are also killed in bather protection nets along the KZN coast.

 

References:
– http://www.iucnredlist.org/details/6337/0
– Whales dolphins and other marine mammals of the world (2006) by Hadoram Shirihai and Brett Jarrett. Prinston field guides 2006
– Smithers’ Mammals of Southern Africa a field Guide by Peter Apps Struik Nature edition 2012
– Best, PB (2007) Whales and Dolphins of the Southern African Subregion. Cambridge University Press.

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