Risso's Dolphin

Classification

Class: Mammalia
Order: Cetacea
Suborder: Odontoceti
Family: Delphinidae
Genus: Grampus
Species: griseus
Common Names: Risso's Dolphin; Grey Dolphin; White-head Grampus; Grey Grampus; Grampus
Irish Name: An deilf liath
 

Key Identification Features

Max. adult body length/weight: 3.8m/500kg
Average adult body length/weight: 3.2m / 400kg
Average length/weight of calf at birth: 1.5m / Unknown
Blow: Generally only seen at close range
Head: The head is blunt and bulbous, sloping steeply to the mouth, with no beak. Risso's dolphin has an odd, deep V-shaped crease, which extends from the blowhole to the tip of the rostrum (to the upper lip) bisecting the forehead. This crease is visible at close range and is unique to this species. The eye is prominent. The mouth slants upwards towards the eyes.
Dorsal Fin: The dorsal fin is prominent – tall and dark, located mid-way along the back. Due to its size (up to 50cm high) can often lead to confusion with killer whale or bottlenose dolphin. The tip may be rounded or pointed with a concave trailing edge.
Pectoral Fins: Pectoral fins are long, dark and sickle shaped.
Colouration: Risso's dolphins can show immense variation in colouration. At birth the calf is uniformly grey (overall colour ranging from light to dark grey) with seven pale vertical stripes. They soon lose this juvenile colouring becoming a dark grey, turning to a chocolate brown as a juvenile. As they age the adult Risso's becomes paler and paler with body colour fading to a pale grey or even becoming as white as a Beluga, particularly around the head. White scarring as found on the body may provide identification features for individuals.
Markings: There is a whitish anchor shaped patch on the chest and the underside of the belly is also typically white, although this may vary. They have a distinctively battered appearance with extensive body scarring caused by the teeth of other Risso's dolphins and circular marks on the body caused by one of their main prey items - squid.
 

Field Identification

The blunt forehead, lack of a distinct beak, pale body colouring and extensive body scarring are characteristics that should be looked for. The tall dorsal fin may lead to confusion with Killer Whale or Bottlenose Dolphin. These dolphins are relatively slow swimmers and are often seen spread out in a long line when hunting as a group.
 

Species Similar in Appearance

May be confused with killer whale or bottlenose dolphins. The best way to tell them apart is through the body colouration; Killer Whales have an overall black body colour with a distinctive white eye patch and a greyish saddle behind the dorsal fin. The Bottlenose Dolphin has a distinctive beak and is typically more active.
 

Behaviour

Risso's dolphins are often seen travelling and surfacing slowly, and can be encountered either singly or in groups of up to ten at a time. Although not generally highly active, young animals are known to breach quite frequently, whilst older animals tend to do a half breach and then slap the head on the side of the water surface. Risso's dolphins rarely bow-ride, but often may approach a vessel and swim alongside or in its wake. They sometimes surface with only their tail or head showing above the water, and are also known to spyhop high in the water with their head, upper body and flippers exposed, remaining in this position for 3 seconds or more, as if surveying the surroundings. When travelling (at a speed of about 5 knots), they are reported to show the top of the head and dorsal fin as they blow. When diving, they often raise their tail flukes out of the water and descend vertically, and after surfacing, remain motionless on the surface of the water for several seconds. They sometimes swim by porpoising and may surface at an angle of 45° to breathe. Typically dive for 1 to 2 minutes, and then take up to a dozen breaths at 15 to 20 second intervals before diving again. They can dive for up to 30 minutes, reflecting their preference for deep water prey species.
 

Status and Distribution

Risso's dolphins are fairly abundant with a world-wide distribution in tropical and temperate seas, but do not generally penetrate far into high latitudes.

In the North Atlantic, they have been reported from Newfoundland in the west and the Shetland Islands off Scotland in the east, south to the Lesser Antilles in the Caribbean and the Azores and Mediterranean. They are sighted regularly around the Irish coast, with sightings and strandings concentrated on the south-west and west coast of Ireland, with occasional sightings in the Irish Sea.

In the south Atlantic, they have been sighted as far south as Argentina and South Africa. In the Pacific, they are found as far north as Alaska and the Bering Sea, off British Columbia and the Kurile Islands, and south to Central Chile and Cape Horn.

They are present in the Red Sea, are distributed throughout the Indian Ocean and as far south as New Zealand and Australia. They may also be seen in the South China Sea, Philippine Sea, off the Solomon Islands and New Guinea.
 

Where and When Best Seen in Ireland

Risso's Dolphins appear to prefer deep offshore waters but on occasion can be seen close inshore around the Irish coast. They are found year round in Irish waters, perhaps moving inshore in the summer months. The headlands on the south and west coast are good viewing areas, but a lot of “sea watching” hours may have to be invested before ticking off this species. Dursey Island, Co Cork, Saltees Islands, Co Wexford and the Blasket Islands in Co Kerry are considered to be potential Risso's hotspots, but they are also seen with some regularity between Arklow and Kilcoole along the Co Wicklow coastline, although this is certainly explained by increased observer effort in these areas.
 

Food and Feeding

Risso's dolphins feed mainly on squid, cuttlefish and octopus, and small quantities of fish. They hunt in deep and shallow waters, often foraging co-operatively.
 

Reproduction and Life Cycle

Max. life expectancy:Unknown.
Average life expectancy: 20 years.
Mating Occurs: June - September.
Season of birth: March – June.
Gestation: Unknown.

Risso's dolphins breed in Irish waters, one was born in Blacksod Bay in Co. Mayo in the 1930's, whilst a young calf showing colouration typical of a young Risso's stranded at Rosbrin Cove (Co. Cork) in 1990.
 

Social Structure

Schools as large as several hundred may form temporarily but smaller schools of 3-10 animals are more common in Irish waters.
 

Conservation Issues

Reliable estimates of number of Risso's dolphins in Irish waters, and the world, are unknown. Risso's dolphins are hunted in small numbers world-wide, including off Southern Europe, off Sri Lanka, the Philippines, the Lesser Antilles, South America, the US, Japan, Indonesia, the South Pacific and China. They are also widely caught incidentally in fishing nets, and have been killed deliberately in Japan to try to reduce competition with fisheries. Crude estimates of population size and catches in Sri Lanka indicate that the exploitation rate there is probably unsustainable.

Contamination of the oceans by toxic man-made chemicals and other pollutants may have an impact on Risso's dolphins, as has been suggested for other species. The effects of oil and gas exploration, other industrial activities, development of offshore wind turbines and shipping in the Risso's dolphin's habitat along the Irish coastline, are largely unknown and warrant further study.
 

Stranded Animals

The general stout form of this dolphin aids in identification of stranded animals. The body is robust in front of the fin, becoming slender and narrow at the tail stock. The flippers are long and sickle shape with pointed tips and are dark in colour. The tail flukes are curved with a slight central notch and are also dark in colour. The vertical groove on the front of the melon is also diagnostic.

The Risso's Dolphin has between 4 – 14 (usually 8) peg like, strong oval teeth located at the front of the lower jaw. They typically have three to four teeth in each lower jaw with no teeth in the upper jaw, but occasionally one or two tiny teeth may be hidden in the upper jaw.
 

ID image

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