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Risso's Dolphin

Risso’s Dolphin
Grampus griseus

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.

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.

Range
Pantropical/Temperate
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 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 U.S. 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.

Habit
Tropical/Temperate – Risso’s dolphins are pelagic, 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.

Diet 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.