Risso's Dolphin (Grampus griseus)

Risso's Dolphin
(Grampus griseus)
Photo: Scott Hill, NOAA Corps

Did You Know?

· 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.

Status
MMPA - Risso's dolphins, like all marine mammals, are protected under the MMPA.

Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Family: Delphinidae
Genus: Grampus
Species: griseus

Species Description
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. They have been reported with other species, 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.

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
For management purposes, Risso's dolphins inhabiting U.S. waters have been divided into four stocks: California/Oregon/Washington, Hawaiian, Northern Gulf of Mexico, and Western North Atlantic. The California/Oregon/Washington stock is estimated between 13,000-16,000 animals, the Hawaiian stock is estimated to be 1,500-2,500 animals, the Northern Gulf of Mexico stock is estimated to be 2,000 animals, and the Western North Atlantic stock is estimated to be 13,000-20,500 animals. There are approximately 175,000 animals in the eastern tropical Pacific, and 85,000 animals in the waters of Japan, western North Pacific, and the East China Sea. There are insufficient data for this species to determine the population trends.

Threats
Bycatch in fishing gear is the primary threat to Risso's dolphins. Several types of fishing gear, including gillnets, longlines, and trawls, have been documented to incidentally "take" this species. Historically, large numbers of Risso's dolphins were killed incidental to tuna purse seine fishing in the eastern tropical Pacific Ocean.

This species has been directly hunted for meat and oil in Indonesia, Japan (drive fishery), Caribbean (the Lesser Antilles), and the Solomon Islands. The populations in some of these areas where fisheries interactions and hunts occur may have declined in local abundance.

Small numbers of Risso's dolphins have been captured from the wild for the purpose of public display in aquariums and oceanariums.

Conservation Efforts
The International Union for Conservation of Nature and Natural Resources (IUCN's) Red List of Threatened Species considers this species "Data Deficient" due to insufficient information on population status and trends.

In 2005, NMFS 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 NMFS in 2006. A proposed rule to implement the pelagic longline take reduction plan was published on June 24, 2008.

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

Key Documents
(All documents are in PDF format.)

Title	Federal Register	Date
Stock Assessment Reports	n/a	various
Draft Pelagic Longline Take Reduction Plan	n/a	06/08/2006

More Information

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.