Key Facts

Length: Up to 3.8 metres
Range: Widely distributed in all major oceans, absent in polar regions
Threats: Marine litter, pollution, acoustic disturbance
Diet: Mainly squid, some octopus and cuttlefish

Visual sightings of Risso’s dolphins from Silurian, 2003-2010

Adult Risso's dolphins have scratches and scars on their skin

Risso's dolphins have a tall, curved dorsal fins and a bulbous head with no beak

Risso's Dolphin

Latin: Grampus griseus
Gaelic: Deilf-risso

Physical Description

The Risso's dolphin has a robust, stocky body and a tall, falcate (curved) dorsal fin. The melon (forehead) is blunt and bulbous with a unique V-shaped crease running from the upper lip to the blowhole. This species has no prominent beak and just two to seven pairs of teeth in the lower jaw. Adult Risso’s dolphins measure between 2.6 to 3.8 metres in length and can live for more than 30 years. The colour pattern varies greatly between individuals, and with age. Calves are born grey, but turn darker grey to dark brown as they become juveniles. As they age, the skin tone lightens to silvery-grey in some cases and the body is increasingly covered with scratches and scars inflicted by other Risso’s dolphins and prey species such as squid.

Habitat and Distribution

Risso’s dolphins are widely distributed throughout most oceans and seas between 60° North and 55° South. The north of Scotland represents the northern limit for this species. In the Hebrides, Risso's dolphins tend to inhabit deeper water, which is home to their preferred prey of squid, octopus and cuttlefish. They can occasionally be seen in coastal areas.

Behaviour

In the Hebrides, Risso's dolphins are usually seen singly or in groups of up to 20 animals, although in other areas they are reported in large groups of several hundred individuals. Social behaviour is gregarious and sometimes rough, possibly accounting for some of the scars and tooth rake marks seen in adult animals; observed behaviours include breaching, tail slapping, spy-hopping, splashing and sometimes striking one another. Risso's dolphins are commonly seen travelling and surfacing slowly and will rarely approach vessels or bow-ride.

Food and Foraging

The diet of the Risso's dolphin consists mainly of squid, with some octopus and cuttlefish, and it has been suggested that they feed at night-time when their preferred prey migrate towards the surface. They are able to dive for about 30 minutes to depths in excess of 1000 metres, and sometimes forage cooperatively. Their soft-bodied prey is caught with teeth in the lower jaw and swallowed whole. Scars from such encounters are visible on the skin surface.

Status and Conservation

Many squid eating marine animals, including turtles and sea birds, swallow plastic bags that they mistake for their prey. Once ingested, plastic may accumulate in the stomach of the animal causing starvation and eventual death. It is likely that Risso’s dolphins commonly encounter plastic bags in the ocean and may be affected by this. Risso’s dolphins are also subject to incidental capture in fishing nets causing drowning, may be disturbed by noise produced by offshore oil and gas exploration, and are exposed to marine pollutants including organochlorines (pesticides). Risso’s dolphins are protected under UK and EU law, principally under Schedule 5 of the Wildlife and Countryside Act 1981, the Nature Conservation (Scotland) Act 2004 and by the 1992 EU Habitats and Species Directive.