Cornwall Mammal Group
Recognition:
In April 2004 two Risso’s dolphins were reported dead to the Cornwall Wildlife Trust Marine Strandings Network – one at Trevone, near Padstow, and another at Trevaunance Cove, St Agnes, a week later. Both animals showed evidence of bycatch and may have been accidentally caught together in one incident. In February 2013, a juvenile male Risso's dolphin stranded at Lamorna Cove. Narrow rope marks in the skin suggested it may have been bycaught. In August 2008 a newborn Risso’s dolphin was found dead on the Isles of Scilly. A post mortem examination was performed by the CWTMSN. It was found to be possibly the first recorded case of an attack on this species by bottlenose dolphins, as evidenced by the numerous rake marks across its body and associated internal injuries.
Risso’s dolphin is among the largest of the dolphin species, adults growing up to 3.8 m. They are mostly grey but are usually paler or white underneath which may help to conceal them from predators. As they get older, males in particular will gain an increasing number of scars from battles with other males to establish dominance within a group, giving them access to the females
for mating. The scars are made by their teeth as they ‘rake’ each other across the body during the fights, causing a series of long thin slices that, once healed over, are white. Older animals therefore can appear much lighter in colour, and this feature is of great help when identifying the species. Other notable features are the long sickle-shaped pectoral fins, with the dorsal fin, also sickle-shaped, standing tall from its central position on the back. Risso’s dolphins have a blunt head with no protruding beak, a crease runs from the blowhole on top of the head down to the upper lip. Inside the mouth there are only between 4 and 14 teeth, all of which are in the lower jaw. The upper jaw and palate are hard and flat, adapted for its main prey of squid. Risso’s dolphins are generally active at the surface with reaching (jumping partially or entirely out of the water), lobtailing (throwing the tail above the surface) and spyhopping (lifting the head and upper body above the surface, sometimes held in position for several seconds) regularly recorded. They tend to shy away from boats and rarely ride the bow wave, preferring instead to swim alongside nearby if they do interact.
Risso’s dolphins are a deep-water species, preferring the offshore areas near the
continental shelf. They are also sometimes seen close to the coast, particularly where
land quickly gives way to deep water. It is not known whether this is part of a
seasonal movement or migration, but may be linked to prey movements. Little is
known about the life cycle of Risso’s dolphins, although it is thought that they have a
gestation period of 13 to 14 months, and in the North Atlantic and Mediterranean
there may be a peak in calving between December and June. Calves measure 1.3 to
1.7 m at birth and adults can live for approximately 30 years. They travel in groups,
or pods, usually with up to 30 members, but have also been observed in pairs or alone.
Photographic studies using the distinctive scar patterns of the animals have shown
that groups are stable over extended periods of a year or more. Rare gatherings of
hundreds of animals have also been reported. They are a social species, and in many
locations have been seen interacting with other cetacean species including pilot
whales and bottlenose dolphins. Squid is the key prey of Risso’s dolphin, hence their
preference for deep water. They will also take shallow water species such as cuttlefish,
octopus, and fish, which may be why they sometimes come inshore.
Larger predators such as orca and sharks may prey on Risso’s dolphins in some parts
of the world, especially young or weak individuals that would be more vulnerable and
less able to escape. Otherwise it is humans that pose the greatest risk from entanglement in fishing nets, pollution which can degrade their habitats, and overfishing which may reduce the amount of prey available to them. Drive hunts in Japan and the Solomon Islands force entire pods into shallow water where they are killed, and as a result there have probably been local declines in the area of this, and several other, cetacean species affected by these large-scale operations. Harpooning also takes places in Indonesia, the Lesser Antilles and Japan, further endangering this
particular population.
Did you know?:
Risso’s dolphin is also known as the grey dolphin, white head Grampus, grey Grampus and Grampus. Some of these are of course similar to the scientific name Grampus griseus, which could translate as ‘large’ or ‘fat mottled fish’. Grampus is an old name that has been used in the past by the Cornish people to refer to Risso’s dolphin.
© PGH Evans - Sea Watch Foundation
Distribution:
In Cornwall the best area to see Risso’s dolphins seem to be around the Land’s End peninsula, although they are a relatively rare sight for local seawatchers. Around Britain and Ireland they are mainly recorded from the Hebrides, western coasts and in the Irish Sea around the Isle of Man. They occur in many areas of the world: the Atlantic, Pacific and Indian Oceans, but not in the cold waters around the Arctic and Antarctic regions.
Records:
2007-2012: 146
2002-2007: 142
Pre-2001: 430
Total: 718
Survey Methods:
Both land and boat-based sea watching surveys have recorded Risso’s dolphins in many locations worldwide. In the UK, Marine life and Organisation Cetacea (ORCA) run surveys from ships of opportunity such as ferries, while land-based projects such as Sea Watch South West has also reported the species on occasion.
Conservation:
Not enough is known about Risso’s dolphin to make an estimate of the global population, but on a smaller scale some populations are thought to number around 175,000 in the tropical East Pacific, 85,000 in the western North Pacific and East China Sea, and about 30,000 each for the east and west coasts of the USA. In the UK they are protected against intentional harm and disturbance under the Wildlife and Countryside Act 1981. Listed as ‘least
concern’ in the IUCN Red List.
