The common dolphin is a small toothed whale that measures 2.5 m long and averages more than 85 kg, males being slightly larger than females. The streamlined body is distinctly marked, with a dark brownish-gray back, white belly, and tan to ochre thoracic patch. This thoracic patch dips below the dorsal fin and combines with an area of streaked light gray on the tail stock to produce the common dolphins’ most characteristic feature, an hourglass pattern on the side. Often there are light patches on the flippers and dorsal fin. The dorsal fin is placed halfway down the common dolphin’s back, and the beak is long and black. There are 41-54 pairs of small pointed teeth in each jaw.
Can be Confused With
Until recently, the long-beaked common dolphin (D. capensis) and the short-beaked common dolphin (D. delphis) were considered as one species, and much of the research regarding common dolphins does not differentiate between the two. Short-beaked common dolphins have tall, slightly falcate dorsal fins. Additionally, the short-beaked common dolphin is more robust than the long-beaked species, with a shorter beak and a rounder melon. Common dolphins could also be confused with the Atlantic white-sided dolphin, but they can be distinguished by a “V”-shaped saddle under the dorsal fin and hour-glass coloration.
The common dolphin is possibly the most widely distributed cetacean; found world-wide in temperate, tropical, and sub-tropical seas. Common dolphins range along continental shelves, particularly over prominent bottom relief such as banks and seamounts. Although there are some seasonal shifts in distribution, common dolphins are present year round in some regions. The most northerly records are the waters off British Columbia and Norway.
Atlantic Ocean: In the western Atlantic, common dolphins are found on the continental shelf from Nova Scotia to Florida; in the Caribbean and Lesser Antilles; and along the South American coast from Venezuela to Uruguay. It is a vagrant to Greenland and Iceland. In the eastern Atlantic, the range of the common dolphin includes the North Sea and waters of the United Kingdom; the Bay of Biscay and the Azores; and the African coast from Morocco to the Gulf of Guinea. Common dolphins have also been sighted in the waters of Namibia and near the Cape of Good Hope. Separate populations inhabit the Mediterranean Sea and Black Sea.
Pacific Ocean: In the western Pacific, range includes the waters of Japan; the Philippines; the waters of Indonesia and Micronesia; the Coral Sea near Australia; and the coasts of Tasmania and New Zealand. In the eastern Pacific, range includes the west coast of the United States; the eastern tropical Pacific; and the South American coast to approximately Valparaiso, Chile.
Indian Ocean: The common dolphin’s range includes the waters of Sri Lanka and India; the Arabian Sea including the Gulf of Oman and the Gulf of Aden; and the coast of Natal, South Africa and Madagascar.
Ecology and Behavior
The common dolphin’s habitat includes shelf-slope waters. Large boisterous groups of common dolphins are often seen whipping the ocean's surface into a froth as they move along at high speed. Herds range in size from about 10 to over 10,000. Associations with other marine mammal species are not uncommon. Active and energetic bowriders (except in prime tuna fishing zones of the eastern tropical Pacific), common dolphins are very familiar to most seagoers in low latitudes. They are often aerially active and highly vocal; sometimes their squeals can be heard above the surface as they bowride.
Adult females calve every two or three years, gestating ten to eleven months and nursing up to nineteen months. At birth, common dolphins are 80-85 cm long. Age at sexual maturity varies greatly among populations. The lifespan of common dolphins is believed to be between twenty and thirty years.
Feeding and Prey
Common dolphins seize prey that composes a broad diet dominated by: Small schooling fish > squid. Large groups of common dolphin often feed in areas of prominent bottom topography. In some areas, common dolphins feed mostly at night on creatures associated with the deep scattering layer (DSL), which migrates toward the surface in the dark. In other areas, they feed mainly on epipelagic schooling fish.
The IUCN lists specific populations of common dolphins of particular concern the Bulgarian population is “vulnerable”, the Ukrainian stock is “data deficient”, and the Romanian stock is “endangered”. In the United States, the short-beaked common dolphin is not listed as endangered or threatened.
Although the species as a whole is not at risk, particular populations are of concern. In the Black Sea a fishery for common dolphins decimated their numbers, once killing up to 200,000 annually. Common dolphins continued to be taken in large numbers in the Black Sea until 1973, and even after this date a direct fishery existed to a lesser extent. Effects on the common dolphins of the Black Sea, with numbers thought to historically exceed one million, were severe and this population is of greatest concern for conservationists. Common dolphins are also taken in gillnet, trawl, tuna purse seine, and longline fisheries. Prey depletion and poor habitat quality are a concern for the population in the Mediterranean. United States stock estimates are 318,795 common dolphins in the eastern North Pacific off California, Oregon, and Washington and 22,215 in the western North Atlantic.
Amaha, A. 1994. Geographic variation of the common dolphin, Delphinus delphis (Odontoceti Delphinidae). Ph.D. thesis, Tokyo University of Fisheries.
Dizon, A.E., W.F. Perrin and P.A. Akin. 1994. Stocks of dolphins (Stenella spp. and Delphinus delphis) in the eastern tropical Pacific a phylogeographic classification. NOAA Technical Memorandum NMFS 119:1-20.
Evans, W.E. 1994. Common dolphin, white-bellied porpoise Delphinus delphis Linneaus, 1758. pp. 191-224 in S.H. Ridgway and R. Harrison, eds. Handbook of marine mammals, Volume 5: The first book of dolphins. Academic Press.
Ferrero, R.C. and W.A. Walker. 1995. Growth and reproduction of the common dolphin, Delphinus delphis Lineaus, in the offshore waters of the North Pacific Ocean. Fishery Bulletin 93:483-494.
Forney, K.A., J. Barlow, M.M. Muto, M. Lowry, J. Baker, G. Cameron, J. Mobley, C. Stinchcomb and J.U. Caretta. 2000. U.S. Pacific marine mammal stock assessments 2000. U.S. Department of Commerce, NOAA technical memorandum, SWFSC-300.
Hassani, S., L. Antoine and V. Ridoux. 1997. Journal of Northwest Atlantic Fisheries Science 22:119-123.
Heyning, J.E. and W.F. Perrin. 1994. Evidence for two species of common dolphins (genus Delphinus) from the eastern North Pacific. Natural History Museum of Los Angeles County, Contributions in Science 442:1-35.
Pauly, D., A.W. Trites, E. Capuli and V. Christensen. 1998. Diet composition and trophic levels of marine mammals. ICES Journal of Marine Science, 55:467-481.
Perrin, W.F. 2002. Common dolphins Delphinus delphis, D. capensis, and D. tropicalis. pp. 245-248 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press.
Silva, M.A. 1999. Diet of common dolphins, Delphinus delphis, off the Portuguese continental coast. Journal of the Marine Biological Association of the United Kingdom, 79:531-540.
Waring, G.T., D.L. Palka, P.J. Clapham, S. Swartz, M.C. Rossman, T.V.N. Cole, K.D. Bisack and L.J. Hansen. 1998. U.S. Atlantic marine mammal stock assessments 1998. U.S. Department of Commerce, NOAA technical memorandum, NMFS-NE-116.