Risso's dolphin  (Grampus griseus)

Classified as Least Concern (LC) on the IUCN Red List (1) and listed on Appendix II of CITES (3).

One of the most enigmatic cetaceans, this little-known dolphin has an unusual appearance. Unlike many other dolphins, Risso’s dolphin lacks a beak, and the bulbous head rises almost vertically from the upper jaw and blunt snout. The body is robust and powerful, and tapers towards a relatively narrow tail, and a distinct crease runs along the top of the melon (4) (5). The sickle-shaped dorsal fin is somewhat short for the body length, and the flippers are long and pointed (5) (6). Risso’s dolphin may also be recognised from the extensive lines of white scar tissue that stretch down the sides of the body. These are thought to be caused by bites from other Risso’s dolphins during playing or fighting, although some of the scars may be the result of squid bites (4) (5) (7). The calves are born with a grey skin that becomes chocolate brown with age, before they eventually take on the adult colouring of a slaty or black upperside, tinged with blue or purple, and a whiter underside, with darker flippers and tail (4) (5). The sexes are similar in appearance, although the dorsal fin may be taller and more erect on the male (8).

Risso’s dolphin is widely distributed throughout the world’s temperate and tropical waters, ranging from Newfoundland, the Gulf of Alaska and the North Sea, south towards South America, Southern Africa, Australia and New Zealand (1) (5). The species’ range appears to be limited by water temperature, with this dolphin most abundant in waters between 15 and 20 degrees Celsius, and rarely occurring in waters below 10 degrees Celsius (5) (9). Risso’s dolphin is also found in a number of partially enclosed waters, including the Red Sea, Gulf of Mexico and Mediterranean Sea, and appears to migrate northwards during winter, with some individuals sighted as far north as the Gulf of Alaska and the Shetland Islands (1) (4).

You can view distribution information for this species at the National Biodiversity Network Gateway.

Risso’s dolphin shows a preference for deep, shelf-edge waters of 400 to 1,000 metres depth in offshore areas, although it may also inhabit shallower coastal areas, particularly in northern Europe and around oceanic islands (1) (2) (5).

Due to its offshore habits, relatively little is known about the biology of Risso’s dolphin (5). However, in common with most other dolphins, this species is highly sociable, and typically travels in groups of 10 to 50, averaging around 30, although huge gatherings of up to 4,000 animals have been seen on occasions where food is in abundance (4) (5) (9). Many groups consist of same sex and similar aged individuals, and Risso’s dolphin often mixes with groups of other cetaceans while foraging (5) (9). It feeds almost exclusively on squid at night time, when its prey migrates towards the water’s surface (2) (5) (10). 

The reproduction of Risso’s dolphin is poorly understood. However, due to its almost cosmopolitan distribution, it is likely that breeding and calving occurs year-round, although it at least peaks between December and April off South Africa (2) (4). A single calf is born after a gestation period of some 13 to 14 months, and it remains with the female and continues to feed on energy-rich milk for several months (2) (8). The seasonal movements of this species are equally unclear, but it appears as though some populations in more stable environments stay in the same place all year, while others show northern movements during summer and southern movements during winter (9). 

While at the water’s surface, Risso’s dolphin displays an impressive variety of acrobatic acts, including high speed jumps out off the water, known as breaching, violent tail slapping and bobbing the head. Although these behaviours are not fully understood, they may be used in communication (2). Similarly, this species produces a number of sounds, including characteristic signature ‘whistles’, and many of these vocalisations are important for detecting prey through echolocation (8) (11). 

The widespread distribution and high abundance in certain areas of Risso’s dolphin means that this species is not currently at risk of extinction. However, in parts of its range this species is threatened by overfishing and incidental bycatch. In Sri Lanka, it is the second most commonly caught cetacean, with around 1,300 animals killed each year. This is of some concern as the population in that region is numbered somewhere between 5,500 and 13,000 and this intensity of fishing is likely unsustainable. In Japan, Risso’s dolphin is also fished for meat and some 250 to 500 are killed annually, while it is also killed in mixed cetacean fisheries in both the Caribbean and Indonesia (1). This dolphin may also be directly persecuted by artisanal fishermen in retribution for taking fish from longlines, and incidentally captured in large-mesh driftnets in oceanic fisheries for large pelagic fish (1) (2). These threats are compounded by the species’ slow reproduction rate, which can hinder its ability to recover from a decline (6).

Like many other deep-diving cetaceans, Risso’s dolphin may also be threatened by anthropogenic sounds that resonate through the oceans. This may be produced by navy sonar or seismic exploration, and can kill dolphins by causing gas-bubble disease (12). Climate change also has the potential to threaten this dolphin, as rising sea levels and increasing sea surface temperatures could alter the abundance and distribution of its prey (13).

Risso’s dolphin is protected by law in parts of its range, including in UK waters where, under the Wildlife and Countryside Act 1981, it is illegal to intentionally kill, injure, or harass any cetacean. Several other European countries have also agreed to set up protected areas, promote research and monitor and control pollution where cetaceans are found, as part of the Agreement on the Conservation of Small Cetaceans in the Baltic and North Seas (14). With very little known about this enigmatic dolphin, further research is required into its ecology, threats and population so that conservation priorities may be identified (1).

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View information on this species at the UNEP World Conservation Monitoring Centre.

There may be further information about this species available via the National Biodiversity Network Gateway.

ARKive is supported by OTEP, a joint programme of funding from the UK FCO and DFID which provides support to address priority environmental issues in the Overseas Territories, and Defra

The UK Biodiversity Action Plan for this species is available at UK BAP.

To learn more about the conservation of whales and dolphins see:

• Whale and Dolphin Conservation Society:
  http://www.wdcs.org