Atlantic White-sided Dolphin


Class: Mammalia
Order: Cetacea
Suborder: Odontoceti
Family: Delphinidae
Genus: Lagenorhynchus
Species: acutus
Common names: Atlantic white-sided dolphin
Irish name: An deilf bhánchliathánach Atlantach

Key Identification Features

Maximum body length: Adult male 2.8m, female 2.5m
Average body length: Adult 2.4 - 2.8m
Head shape: Gently sloping forehead.
Beak: Beak is short, black above and white below. Teeth are sharp and conical, 58 - 80 in the upper row, 58 - 80 lower jaw.
Dorsal fin: Tall, falcate, sharply pointed with narrow base, located mid-back.
Their name derives from the Latin acutus ("sharp"), referring to the acutely pointing dorsal fin.
Colouration: Black back, top of beak, flippers and flukes. Grey sides, white belly. White band below dorsal fin connects with yellow band on tailstock.
Body: The body is robust, with a gently sloping forehead and the tailstock is strongly truncated. Flippers are sickle shaped and pointed.

Field Identification

A prominent dorsal fin, narrow white patch on flanks, yellow or tan streak above white patch along each side, behind dorsal fin and extending up toward ridge of tail.

Species Similar in Appearance

Most likely to be confused with white-beaked and common dolphins.


Atlantic white-sided dolphins are gregarious and acrobatic. Individuals and small groups are commonly encountered, but also herds of several hundreds. They are frequently seen breaching, but seldom bowriding. Associate with humpback, fin and pilot whales, and common and bottlenose dolphins. Strandings are common, including multiple live strandings.

Status and Distribution

This dolphin lives only in the northern North Atlantic and is restricted to temperate and sub-arctic areas, including the continental shelf, slope and deep canyons. The southern range for this species is Cape Cod and the mid-Atlantic canyons off the United States and the British Isles in the eastern Atlantic. It is known from well inside the Gulf of St. Lawrence, and off south Greenland, Iceland, and Norway, occasionally as far north as the Barents Sea. It has also been recorded in the North and Baltic Seas. While it has been stated that the white-sided dolphins have a more southerly distribution than white-beaked dolphins, the distribution of both overlap.

Where and When Best Seen in Ireland

Largest concentrations in Ireland along the edge of the continental shelf, and also in deeper waters. Occur inshore off the northwest in late summer and autumn and also occur occasionally in the Irish Sea. Frequently live strands including 3 “mass” strandings recorded. The best documented occurred in Killala Bay, Co Mayo in September 1994 when 19 individuals stranded and all died. Post-mortem examination showed that the largest male in the group had a perforated aortic valve resulting in heart failure (Rogan et al. 1997). This was the first time all individuals in a mass stranding in Europe had full post-mortem examinations and biological parameters determined.

Food and Feeding

Atlantic white-sided dolphins feed on a variety of schooling fish and squid species, including mackerel, herring, silvery pout and blue whiting. Species composition will vary seasonally and with geographic location.

Reproduction and Life Cycle

Max life expectancy: 22 years for males, 27 for females.
Female sexual maturity: 2.01 – 2.22m length
Male sexual maturity: 7 – 11 years at a length between 2. 15 – 2.30m
Gestation period: 10 - 12 months
Calves born: 108 – 122 cm with summer parturition

Social Structure

White-sided dolphins sometimes occur in large herds numbering in the high hundreds; groups of 10 are considered typical around Ireland although groups of 50 – 60 are sometimes seen. There is some evidence of age and sexual segregation in white-sided dolphins, based on the composition of stranded groups.

Conservation Issues

This species is one of several species that has been taken in drives also involving pilot whales in the Faroe Islands.

Incidental mortality in fishing gear has been documented off Canada, the US, the UK and Ireland. They appear particularly susceptible to capture in mid-water trawls.

A juvenile dolphin from the northwest coast of Ireland was found to have a relatively high concentration of mercury in its liver. Moderately high levels of organochlorines have also been recorded.

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