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Risso’s Dolphin (Grampus griseus)

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Risso’s dolphins, sometimes called “gray dolphins,” have a robust body with a narrow tailstock. These medium sized cetaceans can reach lengths of approximately 8.5-13 feet (2.6-4 m) and weigh 660-1,100 pounds (300-500 kg). Males and females are usually about the same size. They have a bulbous head with a vertical crease, and an indistinguishable beak. They have a tall, “falcate”, sickle-shaped dorsal fin located mid-way down the back. Calves have a dark cape and saddle, with little or no scarring on their body. As Risso’s dolphins age, their coloration lightens from black, dark gray or brown to pale gray or almost white. Their bodies are usually heavily scarred, with scratches from teeth raking between dolphins, as well as circular markings from their prey (e.g., squid), cookie-cutter sharks (Isistius brasiliensis), and lampreys. Mature adults swimming just under the water’s surface appear white.

Risso’s dolphins are found in groups of 5-50 animals, but groups typically average between 10-30 animals. They have been reported as solitary individuals, pairs, or in loose aggregations in the hundreds and thousands. Occasionally this species associates with other dolphins and whales, such as bottlenose dolphins, gray whales, northern right whale dolphins, and Pacific white-sided dolphins. When at the surface, they have a small inconspicuous blow (the blow is more distinct after long dives) and their head partially emerges at a 45°angle. Before diving, they usually take 10-12 breaths at 15-20 second intervals, and will often display their flukes. This species is often very active on the surface, engaging in behavior such as “breaching”, “flipper-slapping”, “lobtailing”, and “spyhopping”; but is usually only observed “porpoising” when being pursued or hunted by predators.

Risso’s dolphins are part of the group of delphinids of the subfamily Globicephalinae, that also includes false killer whales, pygmy killer whales, melon-headed whales, long-finned pilot whales, and short-finned pilot whales. This subfamily is sometimes referred to as “blackfish.”

Risso’s dolphins are capable of diving to at least 1,000 feet (300 m) and holding their breath for 30 minutes, but usually make shorter dives of 1-2 minutes. They feed on fish (e.g., anchovies), krill, and cephalopods (e.g., squid, octopus and cuttlefish) mainly at night when their prey is closer to the surface. The majority of their diet consists of squid, and they have been known to move into continental shelf waters when following their preferred prey. They use the 2-7 pairs of peg-like teeth in their lower jaw to capture prey. Their dentition is considered abnormal because of the low number of teeth overall, and the lack of teeth in the upper jaw.

There is not much known about the reproduction of Risso’s dolphins. Individuals become sexually mature when they reach a length of about 8.5-9 feet (2.6-2.8 m). Breeding and calving may occur year-round and the gestation period is approximately 13-14 months. The peak of the breeding and calving season may vary geographically (especially in the North Pacific), with most animal births occurring during summer to fall in Japanese waters and from fall to winter in California waters. Newborn calves are usually 3.5-5.5 feet (1.1-1.7 m) in length and weigh about 45 pounds (20 kg). They have an estimated lifespan of at least 35 years.

This species is classified as LEAST CONCERN according to the IUCN's Red List.
This species is classified as LEAST CONCERN according to the IUCN’s Red List.

Status

MMPA – Risso’s dolphins, like all marine mammals, are protected under the MMPA.
CITES Appendix II – throughout its range

Species Description

Weight:
660-1,100 pounds (300-500 kg)
Length:
8.5-13 feet (2.6-4 m)
Appearance:
robust gray/whitish-gray, body with a narrow tailstock
Lifespan:
35 years
Diet:
squid primarily, but also fish like anchovies, krill, and other cephalopods like octopus and cuttlefish
Behavior:
usually found in groups of 10-30; very active on the surface

RELATIVE SPECIES: False Killer whale looks similar, but the Rissos dolphin is the only one in its genus.

OTHER NAMES: Gray Dolphins

NEIGHBORING SPECIES: Killer Whales, False killer whale, bottlenose dolphin

PREDATORS: Humans

THREATS: Killed in drive hunts in Japan for meat. Often killed in fishing gear.

DIET: Mostly squid

MANNER OF FEEDING: Forage at night when their prey travels to the surface.

BEHAVIOR: Groups of 12-40 are usually seen. Rarely groups of hundreds or thousands are seen. Often seen in miked schools of other toothed whales and dolphins. Breaching and tail slapping is common, as well as flipper slapping. Does not usually bow ride. Can hold breath for 30 minutes.

REPRODUCTION: Calving occurs year round. Gestation last 13-14 months.

LIFE SPAN: 35 Years

Habitat

Risso’s dolphins are found in temperate, subtropical and tropical waters of 50-86°F (10-30°C) that are generally greater than 3,300 feet (1,000 m) and seaward of the continental shelf and slopes. They are more common in waters of 59-68°F (15-20°C) and may be limited by water temperature. In the northern Gulf of Mexico, they may prefer habitats on the continental slope where the bottom topography is steeper. In the waters off northern Europe, they are known to inhabit shallower coastal areas.

Distribution

Risso’s dolphins have a cosmopolitan distribution in oceans and seas throughout the world from latitudes 60°N to 60°S. In the Northern Hemisphere, their range includes the Gulf of Alaska, Gulf of Mexico, Newfoundland, Norway, Persian Gulf and Red Sea. They are known to inhabit the Mediterranean and Black Sea. In the Southern Hemisphere, their range includes Argentina, Australia, Chile, South Africa, and New Zealand. Little or nothing is known of their migration patterns or movements, but they may be affected by movements of spawning squid and oceanographic conditions.

Population Trends

The most recent stock assessment reports with population estimates are available on our website.

Threats

  • bycatch in fishing gear, including gillnets, longlines, and trawls
  • directly hunted for meat and oil in Indonesia, Japan, Caribbean (the Lesser Antilles), and the Solomon Islands
  • capture from the wild for the purpose of public display in aquariums and oceanariums, in small numbers

Historically:

  • incidental take in tuna purse seine fishing in the eastern tropical Pacific Ocean

Conservation Efforts

In 2005, we convened the Atlantic Pelagic Longline Take Reduction Team to address bycatch of both short-finned and long-finned pilot whales, as well as Risso’s dolphins, in the mid-Atlantic region of the Atlantic pelagic longline fishery. The Team submitted their recommendations for reducing bycatch to us in 2006. A proposed rule to implement the Pelagic Longline Take Reduction Plan was published on June 24, 2008. We published a final rule to implement the Plan (74 FR 23349) on May 19, 2009.

Regulatory Overview

This species is protected under the Marine Mammal Protection Act of 1972 as amended.

Taxonomy

Kingdom: Animalia
Phylum: Chordata
Class: Mammali
Order: Cetace
Family: Delphinidae
Genus: Grampus
Species: griseus

References:

  • Reeves, R. R., P. A. Folkens, et al. (2002). Guide to Marine Mammals of the World. New York, Alfred A. Knopf. p. 422-425.
  • Jefferson, T. A, M. A. Webber, and R. L. Pitman. (2008). Marine Mammals of the World, A Comprehensive Guide to their Identification. Amsterdam, Elsevier. p. 213-215.
  • Shirihai, H. and B. Jarrett (2006). Whales, Dolphins and Other Marine Mammals of the World. Princeton, Princeton University Press. p. 94-96.

Cool/Gross/Weird:

  • Risso’s dolphins scientific name (Grampus griseus) is derived from the word griseus for “grizzled, mottled with gray.” The term Grampus may come from a combination of the Latin words granis “large or great,” and piscis for “fish.”
  • Risso’s dolphins are named for Mr. Risso, who in 1812 was the first person to describe the specimen type to Georges Cuvier (Reeves et al. 2002).
  • Risso’s dolphins do not have any teeth in their upper jaw.
  • Although they don’t bow ride, one individual called “Pelorus Jack” often escorted vessels crossing the Cook Strait in New Zealand for more than 20 years.
  • They have no teeth in their upper jaw.
  • It is part of a group including pilot whales, false killer whales and melon-headed whales all refered to as the “blackfish”.

-TSF-

 

 

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