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Atlantic White Sided Dolphin

Physical description


Growing up to 3 metres, the Atlantic white-sided dolphin is a sturdy dolphin with a small head and a short, inconspicuous beak. The beak is clearly differentiated. The dorsal fin is mid-way down the back and is large and falcate. The flippers are short and the tail fluke has a marked median notch. The caudal peduncle (between the dorsal fin and the tail) is keeled. The entire dorsal surface, including the upper jaw, head and dorsal fin, is dark grey or black. The belly and throat are white and the flippers and fluke dark. Along the side, below the dorsal fin is a white flash behind which is a narrow bright yellow patch extending almost to the tail. These zones of colour are sharply delineated.

Distribution
The Atlantic white-sided dolphin is found in the cold waters of the North Atlantic Ocean from the tip of Greenland and around the North Cape in the north to Cape Cod and the mouth of the Mediterranean in the south. It tends to migrate in winter along with the movement of cooler waters southwards and in summer moves north. It appears to be found mostly in areas with a high seafloor relief.

Behaviour
In the western Atlantic groups of 50 to 60 dolphins are typical and sometimes up to several hundred, but off Europe they are usually found in groups of less than 10 individuals. There appears to be some segregation in the schooling with some groups made up of juveniles. Mixed schools of white-sided and white-beaked dolphins have been observed and they are also found in association with feeding fin and humpback whales.


Breeding
Male sexual maturity is reached at the age of 10 years when they measure about 2.30 metres with females attaining maturity a little earlier at 8 or 9 years. The gestation period is about 11 months with mating taking place in summer. Calves are born in June and July and lactation probably lasts about 18 months. It is thought that healthy females are able to give birth every other year. Life expectancy is of at least 27 years.

Feeding
Atlantic white-sided dolphins are probably not deep divers with most dives being recorded as less than a minute in duration. Their major prey species are herring, small mackerel, cod, hake and several species of squid. However they also consume many other prey such as bottom dwelling fish and sand lances. They are sometimes seen co-operatively surrounding schooling fish near the surface.

Current situation
The Atlantic white-sided dolphin is common off the coasts of Newfoundland and Norway and the total Atlantic population is estimated at a few hundred thousand. This species is not currently hunted on a large scale but many are killed in drive hunts. In recent years these numbers have been estimated to be up to 500. Considerable numbers are killed in trawl nets particularly off Iceland and they are often entangled in gillnets.