Risso’s Dolphin (Grampus griseus)
The Risso’s Dolphin (Grampus griseus) is the only species of dolphin in the genus Grampus. Another common name for Risso’s Dolphin is the ‘Grampus’ (also the species genus) – although historically, it was the common name used to describe the Orca (Killer Whale). The specific name griseus refers to the mottled (almost scarred) grey colour of the dolphins body.
Risso’s Dolphins are found worldwide in temperate and tropical waters, usually in deep waters rather than close to land. As well as the tropical parts of the Indian, Pacific and Atlantic Oceans, Rissos are also found in the Mediterranean and Red Seas, though are absent from the Black Sea. Their preferred environment is just off the continental shelf on steep banks, with water depths varying from 400 – 1000 metres and water temperature at least 10° Centigrade and preferably in excess of 15° Centigrade.
The population of Risso’s Dolphins around the continental shelf of the United States has been recorded to be in excess of 60,000. In the Pacific a census recorded 175,000 individuals in eastern tropical waters and 85,000 in the west. No global estimate of population exists.
Risso’s Dolphin Characteristics
Risso’s Dolphins can quite easily be identified, particularly when they mature. This is because they become scarred and battered which is caused by other Risso’s Dolphins teeth. Risso’s have teeth only in the front of their lower jaw which are used when playing or fighting.
Risso’s Dolphins length is typically 10 feet (3 metres), although some have been recorded to measure up to 12.5 feet (3.8 metres). Like most dolphins, males are typically slightly larger than females. Weight averages about 650 pounds (300 kilograms) and large individuals may weigh up to 1100 pounds (500 kilograms).
Risso’s Dolphins are grey all over when they are born. As they mature, their colouring becomes more chocolate brown or pale grey with paler undersides. Their flippers and tail remain darker. Risso’s Dolphins have broad flukes that have pointed tips. They have a very tall dorsal fin which can measure around 50 centimetres in length. The tip of this dorsal fin may be pointed or curved. They have a single blowhole.
Instead of the usual dolphin beak, Risso’s have a blunt snout and a rounded and bulging forehead that slopes steeply towards their mouths. Their mouths curve upwards, giving the appearance of a ‘smile’ which is a common feature on all dolphins.
Risso’s Dolphin Behaviour
Risso’s Dolphins do not often ‘bow ride’ in front of boats like some other dolphins, but may swim beside or in the bubbly wake that a boat leaves. They generally swim in groups of between 3 and 50 animals. These groups spread out in a long line when hunting for food.
Risso’s are often seen breaching, clearing the water and slapping their heads, tails or sides on the surface.
Some groups are shy, however, some allow humans to approach close to them. Young dolphins are very energetic and engage in breaching (leaping out of the water), spy hopping (lifting their head above the surface of the water to have a look around), slapping their flippers against the surface of the water and surfing on the waves.
Risso’s Dolphins are often seen in the company of other whales and dolphins.
Risso’s Dolphin Diet
The diet of the Risso’s dolphin consists of mainly cephalopods (especially squid), crustaceans and occasionally small fish.
Risso’s Dolphin Communication
Risso’s Dolphins are highly social and there often seems to be much communication going on between animals in a group with much dramatic splashing at the surface.
Risso’s Dolphin Reproduction
Risso’s dolphins are believed to have a life span of at least 20 years.
Risso’s Dolphin Predators
Risso’s dolphins are probably preyed upon by killer whales, sharks and possibly false killer whales (although no incidents of attacks on Risso’s dolphins have actually been observed).
Risso’s Dolphin Conservation
Bycatch is among the worst threats to these animals, meaning that they are caught up in fishing nets and then drown. In some regions, they are intentionally killed for food or killed by fishermen who consider them to be competition. As a deep-diving species, they are also threatened by noise pollution, caused mainly by ship traffic and military sonar. This species is also heavily burdened by toxins such as mercury and chlorides, particularly in the Mediterranean Sea, but also in other regions.
Protection within the framework of international conventions The North and Baltic Sea populations are listed in Appendix II of the Bonn Convention for on the Conservation of Migratory Species of Wild Animals (CMS). In the Mediterranean Sea, the animals are protected by the regional ACCOBAMS and in the North and Baltic Sea by the ASCOBANS Agreements.
The World Conservation Union (IUCN) lists the animals in the category ‘data deficient’, which means that there is not enough information available to say to what degree the Risso’s dolphin is threatened.