Linnaeus, 1758 - Short-beaked common dolphin
Common dolphins have only recently been split into two species, and it may sometimes be difficult to distinguish the two species at sea. Both species are characterized by their slightly falcate dorsal fins, but the short-beaked common dolphin is somewhat more robust than the long-beaked species, with a shorter (but still moderately long) beak, and a more rounded melon. It has 41 to 55 pairs of small, pointed teeth in each jaw.
Both species are strikingly 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. The lips are black, and there is a dark, distinct stripe running from the apex of the melon to encircle the eye. In short-beaked common dolphins, the thoracic patch is relatively light, contrasting strongly with the dark cape. The flipper-to-anus stripe is weakly developed or absent. Often there are light patches on the flippers and dorsal fin. The chin-to-flipper stripe does not approach the gape, and it narrows anterior to the eye.
Can be confused with
The hourglass pattern on the side should allow common dolphins to be distinguished from other species. Clymene dolphins, especially when viewed from above, can look very much like short-snouted forms of common dolphins. A good look at the cape should allow the 2 to be distinguished.
At birth, common dolphins are 80 to 85 cm long. Adult short-beaked common dolphins are 1.6 to 2.2 m (females) or 1.7 to 2.3 m (males).
The short-beaked common dolphin is an oceanic species that is widely distributed from tropical to temperate waters of the Atlantic and Pacific oceans. There are no reliable records of it occuring in the Indian Ocean or in southeast Asian waters. It occurs from nearshore waters to thousands of kilometres offshore, but almost always in deep (>200 m depth) waters.
Biology and Behaviour
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 ten 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.
The prey of common dolphins consists largely of small schooling fishes and squid. 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. Squid may form a more important part of the diet of short-beaked dolphins than it does for long-beaked dolphins.
Short-beaked common dolphins are taken in many fisheries worldwide. Huge catches by Turkish and Russian fishermen formerly occurred in the Black Sea. Common dolphin stocks there have declined and the fishery has not operated recently; however, there are some reports that it has started up again. The tropical Pacific tuna fishery takes common dolphins from several stocks, and some of these may have been depleted by past levels of mortality. There is also an incidental catch off southern California in various gillnet fisheries. Mortality also occurs at least off Japan and in the Mediterranean.