Risso’s dolphins are robust blunt-headed animals, lacking distinct beaks. The flippers are long, pointed, and recurved; the dorsal fin is tall and falcate. Risso’s dolphins have mouthlines that slope upward. Newborns are 1.2-1.5 m long and adults range up to 3.8 m long. This a large delphinid, with weights of up to 400 kg recorded, although the maximum may be near 500 kg.
One of the most distinctive features of the Risso’s dolphin is a vertical crease on the front of the melon. At sea, the best identification characteristic is coloration and scarring. Adults range from dark gray to nearly white, but are typically covered with white scratches, spots, and blotches. The chest has a whitish anchor-shaped patch and the appendages tend to be darker than the rest of the body. Young animals range from light gray to dark brownish-gray and are relatively unmarked. The teeth are also unique; there are 2-7 pairs in the front of the lower jaw, usually none in the upper jaw. Some or all of the teeth may be worn-down in adults, or missing altogether.
Can be Confused With
Risso’s dolphins are generally easy to identify when seen at close range; however, from a distance they may be confused with other large delphinids with a tall dorsal fin (such as bottlenose dolphins, false killer whales, and killer whales). When visible, the light, extensively-scarred bodies and squarish heads of Risso’s dolphins make them unmistakable.
This is a widely-distributed species, inhabiting deep waters of the outer continental shelf and slope from the tropics through the temperate regions in both hemispheres. They may be associated with strong bathymetric features or oceanic fronts. The species range is pantropical/temperate, roughly 60°N to 60°S, in waters where sea surface temperature is greater than 10°C.
Atlantic Ocean – In the western Atlantic, the Risso’s dolphin ranges from Newfoundland; along the east coast of Canada and the United States; in the Gulf of Mexico; and on rare occasions south along the coasts of Central and South America to Cabo de Hornes in Chile. In the western Atlantic, Risso’s dolphins can be found from the British Isles and North Sea; south along the coast of Europe; in the Mediterranean; near the Canary Islands; and in the waters along Cape Province in South Africa.
Pacific Ocean – In the western Pacific, Risso’s dolphins are found in the Sea of Okhotsk, near the Kuril Islands, Kamchatka Peninsula, and Japan; south to Geographe Bay and Sydney in Australia; and the North Island of New Zealand. In the eastern Pacific, they are found from the Gulf of Alaska; in the waters off British Columbia and the western United States; off Baja California and in the Gulf of California; in the eastern tropical Pacific; in Hawaiian waters in rare instances; and south along the coast of South America all the way to Chile.
Indian Ocean – The Risso’s dolphin’s range includes the Indian Ocean from Africa to Indonesia, with sightings concentrated near the Natal coast of Africa, the Bay of Bengal including the Gulf of Aden and the Gulf of Oman; and the waters near Sri Lanka.
Ecology and Behavior
Risso’s dolphins habitat are tropical/temperate pelagic waters, often associated with sea mounts, distinct bottom topography, or upwelling currents. These large dolphins are often seen surfacing slowly, although they can be energetic, sometimes breaching or porpoising, and occasionally bowriding. Herds tend to be small to moderate in size (< 100), but groups of up to 4,000 have been reported. Risso’s dolphins commonly associate with other species of cetaceans. Hybrids between this species and bottlenose dolphins have been recorded, both in captivity and possibly in the wild. In at least the North Atlantic, there appears to be a summer calving peak.
Feeding and Prey
Risso’s dolphins’ diet are dominated by squid > other invertebrates.
Risso’s dolphins feed on crustaceans and cephalopods, but seem to prefer squid. Squid bites may be the cause of some of the scars found on the bodies of these animals. They appear to feed mainly at night by seizing prey.
Threats and Status
Main threats include:
• Direct take
• Fisheries bycatch
• Entanglement in debris/fishing gear
The IUCN lists the Risso’s dolphin as “Data Deficient”, in that too little is known about the species to assess its conservation status. Occasional direct killing of Risso’s dolphins has occurred. This is generally as a result of the dolphins stealing fish from longlines, or in multispecies small cetacean fisheries, such as those that occur in Sri Lanka, the Caribbean, and Indonesia. There are also records of incidental catches in several fisheries, in particular driftnet and longline fisheries. Population estimates for the Risso’s dolphin have been made for some stocks in the United States the California/Oregon/Washington stock is estimated to include 13,079 animals; the Gulf of Mexico stock is estimated at 2,199 animals; and the western North Atlantic stock is thought to have 22,916 animals.
Baird, R.W. 2002. Risso’s dolphin Grampus griseus. pp. 1037-1039 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press.
Baird, R.W. and P.J. Stacey. 1991. Status of Risso’s dolphin, Grampus griseus, in Canada. Canadian Field-Naturalist 105:233-242.
Kruse, S., D.K. Caldwell and M.C. Caldwell. 1999. Risso’s dolphin Grampus griseus (G. Cuvier, 1812). pp. 183-212 in S.H. Ridgway and R. Harrison, eds. Handbook of marine mammals, Vol. 6: The second book of dolphins and the porpoises. Academic Press.
Kruse, S., S. Leatherwood, W.P. Prematunga, C. Mendes and A. Gamage. 1991. Records of Risso’s dolphins, Grampus griseus, in the Indian Ocean, 1891-1986. UNEP Marine Mammal Technical Report 3:67-78.
Kruse, S.L. 1989. Aspects of the biology, ecology, and behavior of Risso’s dolphins (Grampus griseus) off the California coast. M.Sc. thesis, University of California, Santa Cruz.
Shane, S.H. 1995. Behavior patterns of pilot whales and Risso’s dolphins off Santa Catalina Island, California. Aquatic Mammals 21:195-198.