Common Dolphin

Latin: Delphinus delphis
Gaelic: Deilf

Physical Description

The common dolphin has a distinctive creamy yellow hourglass pattern along the sides, with a dark grey back, tail and flippers and a cream coloured belly. The beak is relatively long and slender. Adult common dolphins measure between 1.7 to 2.7 metres long and weigh about 150 kg.  Lifespan is about 20 to 30 years. Worldwide, there are currently two species of common dolphins recognised by scientists – short-beaked (Delphinus delphis) and long-beaked (Delphinus capensis). Common dolphins seen in Hebridean waters are short-beaked common dolphins.

Habitat and Distribution

Globally, short-beaked common dolphins inhabit tropical and warm temperate seas, and prefer deeper waters and shelf regions. In the waters off western Scotland, common dolphins are summer visitors recorded between May and October, when food is most abundant. The species is capable of travelling vast distances in a short time. They can be found in coastal and offshore waters.

Behaviour

In Hebridean waters, common dolphins are usually found in groups of about 10 to 30 individuals, but sometimes in large, active groups of several hundred. Their leaping and splashing can sometimes be seen from several kilometres away, and is often what gives away their presence. They are fast swimmers, reaching speeds in excess of 15 mph. Common dolphins are very acrobatic and can leap clear of the water. Their high-pitched vocalisations can, at times, be heard by humans above the surface of the water. These dolphins are inquisitive and sociable animals and often approach boats to ride the bow wave. Common dolphins with young calves have been observed during summer months in the Hebrides.

Food and Foraging

The common dolphin has 82 to 108 sharp, pointed teeth on each jaw, and eats a varied diet of squid and fish, such as herring, mackerel and other mid-water schooling fish. Individuals will co-operate to herd fish in order to catch them more easily. They are often seen in association with diving gannets feeding on the same fish. Toothed cetaceans (odontocetes) like common dolphins usually swallow their prey whole, using their teeth to grasp their prey but not to chew it.

Status and Conservation

There has been an apparent increase in common dolphin numbers in the Hebrides in recent years, and ongoing research is important to monitor this trend. Common dolphins are thought to be one of the most abundant cetacean species, with population estimates suggesting that there are several hundred thousand animals globally, yet overall numbers have declined due to a combination of factors. There is evidence that significant numbers of common dolphins are accidentally caught in open sea trawl and drift nets; they seem particularly vulnerable to this threat because they are attracted by the fish inside the nets but do not jump over them to escape. Common dolphins are also subject to the same threats as other cetacean species including the pollution and degradation of the marine environment, injury and disturbance from vessels, and decreasing food resources due to overfishing. Common dolphins are protected under UK and EU law, principally under Schedule 5 of the Wildlife and Countryside Act 1981, the Nature Conservation (Scotland) Act 2004 and by the 1992 EU Habitats and Species Directive.