Atlantic white-sided dolphins are found in the temperate and sub-arctic waters of the North Atlantic, from southern Greenland as far south as Chesapeake Bay. They inhabit open waters and are rarely seen from shore. Being nomadic and rarely staying in one spot for long, the Atlantic white-sided dolphins do not show any patterned seasonal migration. They are social animals, and likely to swim in the company of other whales, dolphins and porpoises.
The Atlantic White-sided dolphin is one of the most colorfully marked, with a combination of bold well-defined white and yellowish-tan patches on the sides. The markings make them highly visible at sea. The have a single blowhole, a sharply pointed, erect dorsal fin mid-way on the back and sickle shaped fins. They come to the surface for breath every fifteen to twenty seconds. Like other oceanic dolphins, they will swim with slower boats and bow-ride with faster ones, often leaping high above the water’s surface. Adults range in size from five to eight feet and weigh approximately three to six hundred pounds, the males being larger.
The Atlantic white-sided dolphin female gives birth every two to three years. The calves are born in June and July following a gestation period of ten months. They stay with their mother and are nursed for about eighteen months. Mature at twelve years of age, they can live for at least twenty-five years. Inshore they are usually sighted in groups of ten to sixty. Offshore they are occasionally seen in larger schools of as large as one thousand.
They have a widely varied diet. Herring, silver hake, smelt, and squid are staple food, but these dolphins consume a fairly large amount of small mackerel and various bottom fish.
Although no data is available, the species is considered to be regionally abundant. Common threats are from hunting, and fishing net entanglement and pollutants. Also, individual and mass strandings are relatively common.