Taxonomy

Linneaus described the genus Delphinus in 1758. Since then a number of species have been named, but most of them are not considered to be valid. Some of them have been moved to the genus Stenella. D. delphis was generally considered to be the only valid species, although there now is strong support for considering Delphinus capensis (Gray, 1828) a distinct species. This is the long-beaked form. According to Rice (1998), the Northern Indian Ocean and Red Sea form, Delphinus tropicalis (van Bree, 1971) should also be considered a separate species. The latter is a long-beaked form. Common names for D. delphis include: shortbeaked common dolphin, whitebelly dolphin and saddleback. Common names for D. capensis include: longbeaked common (or saddleback) dolphin and Cape dolphin. D. tropicalis is referred to as: Arabian or Malabar common (or saddleback) dolphin (Rice, 1998).

Appearance

The color pattern of this species is highly variable. The dorsal side is dark grey, the ventral side light grey to creamy white. There is an hourglass shaped pattern on the flanks, where the dark dorsal and light ventral colors meet, just caudal of the dorsal fin. In front of this "meeting point" there is a yellow-ochre patch and behind it a grey patch (Peet et al, 1992). There is a dark stripe from the flipper to the lower jaw and from the eye to the base of the beak (Evans, 1987). There is sexual dimorphism in the coloration near the genital area. In males, there is a black blaze just above the genital area, whereas in females there is a narrow band of black with grey countershading in the same area (Evans, 1994). The body is slender, with a long slender beak. The flippers are tapering and the dorsal fin is sickle-shaped and erect.

The common dolphin is on average less than 2.3 m long (maximum 2.6 m). Males are slightly larger than females. Adult weigh 80-136 kg. The smallest form lives in the Black Sea (1.7-1.8 m for males and 1.5-1.7 m for females). The largest form lives in the Indian Ocean (males up to 2.42 m and females up to 2.12 m) (Klinowska, 1991). They have 40-55 pairs of small pointed teeth in each jaw.

Distribution

The common dolphin can be found in the offshore waters of nearly all tropical, subtropical and warm temperate seas. In the Atlantic, they come as far north as Nova Scotia and Iceland and are also found in the Mediterranean and Black Seas, the Red Sea and the Persian Gulf. In the Pacific the northern limit is from Northern California to Japan. The southern limits reach to Peninsula Valdéz, Argentina, the southern tip of Africa, South Australia and New Zealand (Evans, 1987). The common dolphin is occassionally seen in the North Sea (Peet et al, 1992). Its temperature range is 10°-28°C (Klinowska, 1991). On the Northeast US coast, common dolphins are most abundant near the 100-200 m contour of the continental slope (Selzer and Payne, 1988).

Population dynamics and life history

At birth, calves are 76-86 cm long. They are born in spring and fall (see also table below) The gestation period is about 10-11 months. Weaning starts at an age of 6 months (Evans, 1994). Only milk was found in the stomachs of 80-100 cm calves. Milk and squid was found in the stomachs of 100-130 cm calves. Sexual maturity is reached at a length of 1.67 - 1.8 m (at a probable age of 3-4 years) (Minasian et al, 1984) There appear to be regional differences in the age at maturity: 3 GLGs (teeth Growth Layer Groups, probably equivalent to years) in males in the Black Sea and 2-4 GLGs for females. In the North Atlantic, the age at maturity is 5-7 GLGs for males and 6-7 GLGs for females. In the Eastern Tropical Pacific (ETP) males this is 6-7 GLGs and in the North Pacific 7-12 GLGs. Maximum ages found are 22 GLGs for males and 20 GLGs for females, both for the Black Sea population (Klinowska, 1991).

Population status

In Southern California waters, the population is about 15,500 in winter-spring and 57,000 in summer-autumn. For the ETP, estimates range from 220,770 for the west central zone to 1,300,300 in the southern zone. A total of 3,112,300 was estimated in 1990 for the whole ETP. In the Black Sea, there were probably about 50,000 common dolphins in 1986 (Evans, 1994). The National Research Council (1992) quotes the average abundance for 1986-1990 in the ETP as 467,400 for the Northern Stock, 594,300 for the Central Stock and 2,117,500 for the Southern Stock.

Exploitation

There has been an extensive cull of this species in the Black Sea from 1870 through 1983. In 1966 all countries except Turkey discontinued their fisheries due to a collaps of the stocks. Between 1976 and 1981, an estimated 250,000 dolphins were killed (averaging 41,221 per year). The Turkish fisheries officially closed in 1983, but there may still be a low level hunt. There are also small takes in the Azores and Venezuela and possibly in the Mediterranean (Spain and France) (Klinowska, 1991).

A lot of common dolphins are also killed as incidental catches in tuna fisheries. In 1986, about 25,000 common dolphins were killed in the ETP tuna fisheries. On average, four times as many common dolphins are killed per set than spotted dolphins. Changes in fishing techniques have reduced the incidental take. In 1990, 700 dolphins of the Northern ETP stock were taken (0.7%), 4,100 of the Central ETP stock (0.7%) and 300 of the Southern ETP stock (0.014%) (National Research Council, 1992). The IWC (1996) mentioned a reported incidental take of 4250-4342 in 1992, 1249 in 1993 and 142 in 1994.

Feeding

The diet varies per region and per season. In the Black Sea, the common dolphin feeds in large groups (10,000 or more) on pelagic fish. In California, they feed on fish (Engraulis in autumn-winter, Leuroglossus in spring-summer) and cephalopods (Loligo in autumn-winter and Onychoteuthidae in spring-summer). In the North Atlantic their main prey appears to be Gadidae (cod familiy) (Klinowska, 1991). They are probably opportunistic feeders. While feeding they dive to 9-50 m. Maximum dive depths are around 200 m (Evans, 1994).

References

Evans, P.G.H. (1987)

The natural history of whales and dolphins. Christopher Helm, London.

Evans, W.E. (1994)

Common Dolphin, White-bellied porpoise Delphinus delphis Linnaeus, 1758 In: S.H. Ridgway and R. Harrison: Handbook of Marine Mammals. Volume 5: The first book of dolphins, pp. 191-224. Academic Press, San Diego

International Whaling Commission (1996)

Report of the sub-committee on small cetaceans. Rep. Int. Whal. Commn. 46:160-179

Klinowska, M. (1991)

Dolphins, Porpoises and Whales of the World. The IUCN Red Data Book. IUCN, Gland, Switzerland.

Minasian, S.M., Balcomb III, K.C. and Foster, L. (1984)

The world's whales. The complete illustrated guide. Smithsonian Books, Washington D.C.

National Research Council (1992)

Dolphins and the Tuna Industry. National Academy Press, Washington D.C.

Peet, G., Nijkamp, H., Nelissen, P.-H. and Maas, F.-J. (1992)

Bruinvissen dolfijnen en walvissen van de Noordzee. Uitgeverij M&P, Weert, the Netherlands.

Rice, D.W. (1998)

Marine mammals of the world - Systematics and distribution
Society of Marine Mammalogy Special Publication Number 4, 231 pp.

Selzer, L.A. and Payne, P.M. (1988)

The distribution of white-sided (Lagenorhynchus acutus) and common dolphins (Delphinus delphis) vs. environmental features of the continental shelf of the Northeastern United States. Marine Mammal Science 4(2):141-153

About the pictures

The pictures on this page were taken in November 1996 during a whale-watching trip out of Eden, New South Wales, Australia (in Twofold Bay, the home of the infamous "Old Tom" the friendly killer whale).

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