Atlantic White Sided Dolphin | cornwallmammalgroup

Recognition:

Atlantic white-sided dolphin-(Lagenorhynchus acutus)

Order: Cetacea, Cornish Name: Morhogh

Cornish Tales

The adult male Atlantic white-sided dolphin can reach just over 2.8 m in length, and up to 230 kg in weight; the female is a little smaller.

Similar to the white-beaked dolphin,
although smaller, it is somewhat more stocky in appearance than most
other dolphin species, having a short beak that tapers up to a stocky body.

The tail stock is much wider vertically and narrows just ahead of the tail flukes. The dorsal fin is relatively tall and centrally located, whilst the pectoral fins are pointed, curved, and black.

The body is dark grey on the back, shading to mid grey on the flanks, and white on the underside. An irregular white patch may also be present on the flanks below the dorsal fin, while a distinctive yellow stripe begins in a similar place and stretches back along the tail stock, widening as it nears the tail flukes. The pale patch does not extend
across the back, which it does in some white-beaked dolphins.

There is usually also a small dark grey patch encircling the eyes with a thin line that joins the dark grey that comes down from the back to the tip of the
upper jaw.

Ecology and behaviour

The Atlantic white-sided dolphin generally inhabits deeper offshore waters, so they are rarely spotted close to the shore. However, seasonal movements have been recorded in parts of its range, with some animals coming inshore in summer. The diet of Atlantic white-sided dolphins includes a wide variety of fish species as well as squid. They are not considered to be deep divers, with most dives lasting less than 4 minutes. They hunt just below the surface, where they pursue small shoaling fish such as mackerel, herring, and pout. Atlantic white-sided dolphins have been observed cooperating to herd schools of fish towards the surface where the prey can be caught more easily.

The ecology of the Atlantic white-sided dolphin is similar to that of the white-beaked dolphin, but is better understood. Life expectancy is about twenty years, or more, while the peak calving months appear to be June and July following 11 months of gestation. Females probably nurse their young for up to 18 months, meaning that they could give birth every other year.

The Atlantic white-sided dolphin is a very sociable animal and groups of up to 100 individuals are frequently seen in the North Atlantic. Off Ireland superpods of 1000 have been recorded. The pods are mixed sexes but evidence from mass strandings suggest that they may be segregated into breeding and non-breeding animals. Atlantic white-sided dolphins frequently form mixed herds with white-beaked dolphins and sometimes other dolphin or whale species.

Natural predators come in the form of larger shark species and orcas. Humans still take large numbers in drive hunts in the Faroe Islands, with between 150 and 550 killed annually. Bycatch in mid-water trawl fisheries may also accidentally kill a significant number, particularly off south-west Ireland while trawling for horse
mackerel.

Only six stranded animals of this species have been recorded in Cornwall since 1992. The two most recent records of this species in Cornwall, at the time of writing, were of a dead animal on Hayle beach in 2002, followed by a live stranded elderly male dolphin, also on Hayle beach in 2008. These were attended to by Cornwall Wildlife Trust Marine Strandings Network and British Divers Marine Life Rescue respectively.

Did you know?:

The Atlantic white-sided dolphin often associates with other cetacean species
including white-beaked dolphin, fin whale and humpback whale. The Atlantic white-sided dolphin is currently classified as a member of the same genus, Lagenorhynchus, as the white-beaked dolphin, but recent research has proposed placing the species in a separate genus, Leucopleurus.

Survey Methods:

Land-based surveys are mostly unproductive in the UK due to the offshore distribution of this species. Boat-based surveys, such as those by Marinelife and Organisation Cetacea (ORCA) around Europe on vessels of opportunity, are more suited to surveying this pelagic species.

Distribution:

As an occupant of cool temperate and sub-arctic waters, the Atlantic white-sided dolphin is a rare sight in Cornwall. It occurs in the central and northern North Sea and across north-west Scotland and across the Atlantic to Greenland and south to the northeast USA.

Records:

2007-2012: 2

2002-2007: 1

Pre-2001: 28

Total: 31

Conservation:

Despite the large number killed by humans every year, the species is not thought to be threatened overall, even though no accurate total abundance estimates are available at present. The population off the US coast is estimated to be approximately 30,000, with a further 12,000 in Canadian waters. In the UK they are protected against intentional harm and disturbance under the Wildlife and Countryside Act 1981. Atlantic white-sided dolphins are currently listed as ‘least concern’ in the IUCN Red List. There are concerns in the US about high levels of bioaccumulated toxins, especially flame retardants and mercury, in this species.