A large dolphin with robust body, the Atlantic white-sided dolphin is so called due to the pale white patch on the sides below the dorsal fin. The animal exhibits the identifying yellow spot by the rear of the flanks. This dolphin is sometimes taken for the white-beaked dolphin, though the back of the latter is much lighter in color. Also, the species has similar coloration with the common dolphin - grey, white, black and yellow; these two dolphin species are sometimes confused, though, unlike the common dolphin, the Atlantic white-sided dolphin does not have the identifying hourglass coloration.
The species inhabits cool temperate and sub-arctic waters of the North Atlantic, along the continental shelf. The Atlantic white-sided dolphins usually occur in waters with low salinity, being found at the depth of 40 - 270 meters. They can often be seen in the UK waters, especially around the Hebrides and the Northern Isles as well as northern parts of the North Sea. They also inhabit waters offshore the west coast of Ireland and can occur in the south-western waters of the island, close to the English Channel.
These dolphins are highly social animals, gathering into large groups of up to several hundred animals. Meanwhile, those in the western Atlantic, and especially along the Canadian Atlantic coast, usually form small herds, consisting of 6 - 8 dolphins, whereas those, living in the inshore areas, live in larger groups of 10 - 60 individuals. At feeding periods, these dolphins are known to share their habitat with fin whales, humpback whales, long-finned pilot whales and other cetacean species. The Atlantic white-sided dolphin is a fast swimmer and a very active animal, often splashing the tail flukes onto the water surface with a noise. Bow-riding is a common behavior in this species: this is when the animal rides waves at the front of a boat, and, sometimes, a large whale. Also, the Atlantic white-sided dolphins can sometimes be observed breaching or clearing the water.
Currently, there is no information about the mating system of this species. However, it is known, that the gestation period lasts about 10 months, yielding a single baby, usually in June-July. Females give birth with an interval of 2 - 3 years. By the age of 18 months, the calf is weaned. Male individuals reach sexual maturity, when they are 2.1 - 2.4 meters in length, whereas females are sexually mature at a length of 1.94 - 2.22 meters, when they are possibly about 12 years old.
Presently, the species hugely suffer from hunting as well as drowning as a result of being entangled in fishing tackles. Other notable concerns to the population of the Atlantic white-sided dolphin are environmental changes and chemical pollution.
According to IUCN, the Atlantic white-sided dolphin is abundant and widespread throughout its range but no overall population estimate is available. However, there are estimates of its populations in specific areas: off the eastern North American shoreline - 51,640 dolphins; off the west coast of Scotland - about 96,000 dolphins; in the western North Atlantic - about 27,000 dolphins and in the Gulf of St. Lawrence - 11,740 dolphins. Currently Atlantic white-sided dolphins are classified as Least Concern (LC) on the IUCN Red List.