Mammals · Dolphins

Risso's Dolphin

Grampus griseus (G. Cuvier, 1812)

syn. Delphinus (Grampus) griseus, Delphinus aires, Delphinus griseus, Delphinus orca, Delphinus risso, Delphinus rissoanus +15 more

3 - 4 m300 - 500 Kg1-600 mCITES IILeast Concern

821

Risso's Dolphin (Grampus griseus) is a unique marine mammal and the only member of its genus. It is closely related to various other dolphins and whales such as pilot whales and false killer whales. Characterized by their distinctive white scars, Risso's dolphins typically reach about 3 meters (10 feet) in length. They inhabit a wide range of marine environments, preferring areas along continental shelves. This preference is largely due to their diet, which is primarily composed of cephalopods like squid. These dolphins are impressive divers, able to reach depths of up to 600 meters (nearly 2,000 feet) in pursuit of their prey.

Socially, Risso's dolphins travel in pods of 10 to 50 individuals, where strong bonds are formed. At times, they may gather in larger "super-pods" that number in the thousands. The composition of these groups often varies by age and sex, with long-term associations observed particularly among adult males, while younger dolphins frequently change groups.

Risso's dolphins are identifiable by their solid build and a large dorsal fin that tapers towards their tail. They have a bulbous head marked by a vertical crease. Younger dolphins exhibit a grey to brown coloration with a lighter underside and distinctive white areas. As they age, their nonwhite areas become darker and eventually lighten again due to scarring. These scars, which are a result of social interactions and competition, are more prevalent in older individuals and may serve to deter competition among males.

Their range covers the 🇮🇳 Indian, Pacific, and Atlantic Oceans, as well as seas such as the Baltic and Mediterranean. Risso's dolphins prefer waters with depths of 400 to 1,000 meters and temperatures between 10°C and 20°C. They rely on echolocation to hunt, often planning dives based on prey distribution, as observed in studies from the Azores. Social behaviors at the surface include interactive displays like tail slaps and spy-hopping, potentially for communication or mating purposes.

These dolphins face threats similar to many marine species, such as pollution from noise and plastics, as well as entanglement in fishing gear. While protected under certain laws like those in the 🇺🇸 United States, they are still hunted in some parts of the world.

Named after Antoine Risso, the species was first scientifically described by Georges Cuvier in 1812. The original specimen is preserved in France, and the naming has undergone revisions since its initial designation.

Reproduction in Risso's dolphins involves a gestation period of 13 to 14 months. Females reach sexual maturity between 8 and 10 years, while males mature at around 10 to 12 years. These dolphins can live up to 39.6 years. Occasionally, they have been known to hybridize with bottlenose dolphins, though this is rare and lacks specific evolutionary benefits.

Why it's threatened

Biological resource use

Intentional use: (subsistence/small scale) [harvest] · Unintentional effects: (subsistence/small scale) [harvest]

Human intrusions & disturbance

Recreational activities · War, civil unrest & military exercises

Pollution

Noise pollution

Climate change & severe weather

Habitat shifting & alteration

In Sri Lanka, Risso's Dolphins were, at least during the 1980s, the third most commonly taken cetacean in fisheries, providing meat for human consumption and fish bait (Leatherwood and Reeves 1989, Leatherwood 1994). Around 15% of the estimated 9,000 cetaceans landed annually in Sri Lanka between 1984-1986 were Risso’s Dolphins, most of them having been caught in gillnets or harpooned (Leatherwood 1994). In Japan, Risso's Dolphins are taken regularly by hand harpoon and driving, in set nets, and as a limited catch in the small-type whaling industry, with reported catches in recent years ranging from about 250–500 (Kasuya 2018). In 2007/2008, 246 Risso’s Dolphins were reported to have been taken by the harpoon fishery operating off Hokkaido, Sanriku, Boso, and Kii (Kazuya 2018). They are also a major target of artisanal hunting and are taken incidentally in gillnets and other fishing gear in the Philippines and China (Dolar 1994, Dolar et al. 1994, Perrin et al. 2005). Off eastern Taiwan, Risso’s Dolphins are taken opportunistically by harpoon and they are caught incidentally in large-mesh driftnets for large pelagic fish (Perrin et al. 2005). In 2012, several groups of Risso’s Dolphins were intentionally killed in the Faroe Islands (Bloch et al. 2012) where Long-finned Pilot Whales (Globicephala melas) and Atlantic White-sided Dolphins (Lagenorhynchus acutus) are the primary targets of the drive hunt for cetaceans.

Bycatch occasionally occurs in pelagic longline fisheries in the southwestern Indian Ocean and along the U.S. Atlantic coast, particularly since Risso’s Dolphins regularly depredate squid used as bait (Garrison 2007, Kiszka 2015). Between 2011 and 2012, several Risso’s Dolphin deaths were recorded in the pelagic longline fishery based in La Reunion (Sabarros et al. 2013). In Tanzania, bycatch has been recorded in drift gillnet fisheries targeting tuna, billfish, sharks, and other large pelagic fish off Zanzibar and Pemba (J. Kiszka, unpublished data). Bycatch in gillnets has also been documented in Ghana, the Azores, Peru, and the Solomon Islands (Bearzi et al. 2011, Debrah et al. 2010, Reeves et al. 2013). Bycatch rates of Risso’s Dolphins in the Swordfish and Thresher Shark gillnet fishery off California are considered relatively low based on observer data (Carretta and Enriquez 2012).

Like beaked whales (Ziphiidae), which are also deep-divers that prey on cephalopods, Risso’s Dolphins are likely susceptible to disturbance by loud anthropogenic sound, such as that generated by military sonar and seismic surveys (Cox et al. 2006, Modest 2015, Southall et al. 2016). They are also exposed to disturbance by whale-watching boats (Visser et al. 2011).

Risso’s Dolphins in the Mediterranean carry substantial contaminant burdens (Kim et al. 1996, Marsili and Focardi 1997).

Threat classification from the IUCN Red List.

Quick facts

GroupDolphins

Max size3 - 4 m

Max weight300 - 500 Kg

Depth1-600 m

Conservation status

Least Concern

The species is doing well and widespread. Not currently at risk of extinction.

International trade · CITES

Appendix II