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Risso’s Dolphin (Grampus griseus)
Risso’s dolphin (Grampus griseus) is the only species of dolphin in the genus Grampus.
Risso’s Dolphin
This dolphin is named after Antoine Risso, whose description formed the basis of the first public description of the animal, by Georges Cuvier, in 1812.
Another common name for the Risso’s dolphin is grampus (also the species’ genus), although this common name was more often used for the orca. The etymology of the word “grampus” is unclear. It may be an agglomeration of the Latin grandis piscis or French grand poisson, both meaning big fish. The specific epithet griseus refers to the mottled (almost scarred) grey colour of its body.
Risso’s dolphin has a relatively large anterior body and dorsal fin, while the posterior tapers to a relatively narrow tail. The bulbous head has a vertical crease in front.
Infants are dorsally grey to brown and ventrally cream-coloured, with a white anchor-shaped area between the pectorals and around the mouth. In older calves, the non-white areas darken to nearly black, and then lighten (except for the always dark dorsal fin). Linear scars mostly from social interaction eventually cover the bulk of the body. Older individuals appear mostly white. Most individuals have two to seven pairs of teeth, all in the lower jaw.
Length is typically 10 feet (3.0 m), although specimens may reach 13.12 feet (4.00 m). Like most dolphins, males are typically slightly larger than females. This species weighs 300–500 kilograms (660–1,100 lb), making it the largest species called “dolphin”.
They are found worldwide in temperate and tropical waters, usually in deep waters, but close to land. Their preferred environment is just off the continental shelf on steep banks, with water depths varying from 400–1,000 m (1,300–3,300 ft) and water temperatures at least 10 °C (50 °F) and preferably 15–20 °C (59–68 °F).
The population around the continental shelf of the United States is estimated 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 exists.
They feed almost exclusively on neritic and oceanic squid, mostly nocturnally. Predation does not appear significant. Mass strandings are infrequent.
These dolphins typically travel in groups of 10–51, but that may reach 400. Smaller, stable subgroups exist within larger groups. They also travel with other cetaceans. They harass and surf the bow waves of grey whales, as well as ocean swells.
Gestation requires an estimated 13–14 months, at intervals of over two years. Females mature sexually at ages 8–10, and males at age 10–12. The oldest specimen reached 39.6 years.
Risso’s dolphins have successfully been taken into captivity in Japan and the United States, although not with the regularity of bottlenose dolphins or orcas.
Under the name “grampus”, it was one of the royal fish which were traditionally the property of the English Crown.
This dolphin is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS), the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS), the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU) and the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU).
Risso’s dolphins are protected in the United States under the Marine Mammal Protection Act of 1992. The only country known to actively hunt Risso’s dolphin is Japan.
At least one case report of strandings in Japan’s Goto Islands has been associated with parasitic neuropathy of the eighth cranial nerve by a trematode in the genus Nasitrema.
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