The common dolphin (Delphinus delphis) is also known as the short-beaked common dolphin and is one of the smallest of the dolphins, measuring 2.1 - 2.4 metres in length and weighing 75 - 85 kg. The body is long and slender, as is the beak, and the dorsal fin is tall and pointed. The species is often confused with the striped dolphin Stenella coeruleoalba, but the common dolphin’s haracteristic hourglass or criss-cross pattern on its flanks is a good distinguishing feature. This patch is tan or yellowish in colour before the dorsal fin, and pale grey behind. Common dolphins are very agile and active.
They commonly bow-ride, often accompanying boats for many miles, and are capable of swimming at great speed, as well as engaging in energetic aerial acrobatics.
Mainly opportunistic feeders, the common dolphin diet is very varied, consisting chiefly of small schooling fish such as cod, hake, mackerel, sardine, pilchard, horse mackerel, scad, sprat, sand eel, herring, whiting and blue whiting, as well as squid - the type of food taken depends on local availability. Groups of dolphins often use co-operative feeding techniques to herd schools of fish, panicking the fish through frenzied activity and taking them in the confusion.
The common dolphin is a gregarious animal, often found in large, active schools. In British waters, most herds consist of less than 30 individuals, and animals often occur solitarily or in pairs, although occasional schools of more than one hundred dolphins can be seen. School size increases in mid-summer and midwinter, possibly linked to the dolphins following prey moving inshore. They are highly vocal, emitting high-pitched squeals that can often be heard easily above the surface of the water.
Common dolphins develop strong social bonds, particularly between mother and young, and males and females; if one animal is captured or injured, the other will remain in attendance, and frequently shows much distress at its companion’s plight, squeaking and squealing.
Common dolphins appear to have two calving peaks - spring and autumn - with a gestation period of 10 - 11 months. Other females may assist the mother with the birth and also take part in ‘baby-sitting’ while the mother feeds. Calves are 80 - 90 cm long at birth. They are weaned at the age of around 19 months, and the mother has a resting period of about four months before her next pregnancy so that calving intervals are generally 2-3 years or more. Males become sexually mature at 5-7 years of age, and females at around six years. Common
dolphins can live to 30-35 years.
STATUS AND DISTRIBUTION
In the British Isles, they are common in the western approaches to the Channel and the southern Irish Sea (particularly around the celtic Deep) and around the Inner Hebrides north to Skye. In recent years, the species has occurred further north and east in shelf seas - around Shetland and Orkney, and in the northern North Sea, reflecting changes in the strength of the Gulf Stream. It is generally rare to see them in the southern North Sea and eastern portion of the Channel.
The major threat facing common dolphins in British waters in recent years appears to be entanglement in trawl and purse seine nets in the Celtic Sea and Bay of Biscay. This has resulted in large numbers dying and subsequently being washed ashore, particularly in the southwest of Britain, due to the fact that they often prey on the same species as the fisheries, thus becoming a prime target for accidental capture. Although there is no evidence of serious organochlorine contamination in eastern North Atlantic common dolphins, specimens from the Atlantic coast of France have been found with high levels of methyl mercury (max 631 Cg/g dry weight in the liver), with levels of total mercury increasing with age.