About the Species
Risso's dolphins, sometimes called gray dolphins, are found in the temperate and tropical zones of all the world’s oceans. These cetaceans generally prefer deeper offshore waters, especially near the continental shelf edge and slope, where they can dive to at least 1,000 feet and hold their breath for 30 minutes. They are also very active on the ocean surface.
Risso's dolphins are typically found in groups of between 10 and 30 animals, though they have been reported as solitary individuals, in pairs, or in loose aggregations in the hundreds or thousands. Occasionally, this species associates with other dolphins and whales. They are sometimes considered part of a subfamily referred to as “blackfish,” which also includes false killer whales, pygmy killer whales, melon-headed whales, long-finned pilot whales, and short-finned pilot whales.
Risso’s dolphins in the United States are not endangered or threatened. Like all marine mammals, they are protected under the Marine Mammal Protection Act. NOAA Fisheries and its partners are working to conserve Risso’s dolphins and further our understanding of this species through research and conservation activities.
NOAA Fisheries estimates population size for each stock of Risso’s dolphin in its stock assessment reports.
CITES Appendix II
- Throughout Its Range
SPAW Annex II
- Throughout the Wider Caribbean Region
- Throughout Its Range
Risso's dolphins have a robust body with a narrow tailstock. These medium-sized cetaceans can reach lengths of approximately 8.5 to 13 feet and weigh 660 to 1,100 pounds. Males and females are usually about the same size. They have a bulbous head with a vertical crease and an indistinguishable rostrum. They have a tall, curved, sickle-shaped dorsal fin located mid-way down their 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. Adult bodies are usually heavily scarred, with scratches caused from teeth raking between dolphins, as well as circular markings from prey (e.g., squid), cookie-cutter sharks, and lampreys. Mature adults swimming just under the water's surface usually appear white.
Risso’s dolphins have two to seven pairs of peg-like teeth in the front of their lower jaw to capture prey and usually none in their upper jaw. This low number of teeth is unusual when compared with other cetaceans.
Behavior and Diet
Risso's dolphins are typically found in groups that average between 10 and 30 animals, but they have been reported as solitary individuals, in pairs, or in loose aggregations of 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 if backlit (which is more distinct after long dives), and their head partially emerges at a 45° angle. Before diving, they usually take 10 to 12 breaths at 15- to 20-second intervals and will often display their tails (known as flukes). Risso’s dolphins are very active on the surface, often leaping out of the water, slapping their pectoral flippers or tails on the water's surface, and raising their heads vertically out of the water (known as spyhopping). They occasionally 'porpoise'—or move in and out of the water in a series of high-speed leaps—most often when being pursued or hunted by predators.
Risso's dolphins can dive to at least 1,000 feet and hold their breath for 30 minutes, but they usually make shorter dives of just a few 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. Most of their diet consists of squid, and they have been known to move into continental shelf waters when following their preferred prey.
Where They Live
Risso's dolphins have a cosmopolitan distribution, meaning they can be found worldwide in temperate, subtropical, and tropical oceans and seas from latitudes 64° North to 46° South. Their preferred habitats appear to be mid-temperate waters of the continental shelf and slope between 30° and 45° latitude. They prefer areas of deeper waters (3,300 feet) with steep bottom topography, but they are known to inhabit shallow coastal areas as well.
In the Northern Hemisphere, their range includes the Gulf of Alaska, Gulf of Mexico, Newfoundland, Azores, Norway, Japan, Russia, and the Red Sea. In addition, they are known to inhabit the Mediterranean Sea, are rare in the Black Sea, and do not appear to inhabit the Persian Gulf and some other very shallow, enclosed bodies of water. In the Southern Hemisphere, their range includes Argentina, Australia, Chile, South Africa, and New Zealand.
Little is known of their migration patterns or movements, but Risso’s dolphins may be affected by movements of spawning squid and oceanographic conditions.
Lifespan & Reproduction
Risso’s dolphins have an estimated lifespan of at least 35 years. Individuals become sexually mature when they reach a length of about 8.5 to 9 feet. Breeding and calving may occur year-round and the gestation period lasts approximately 13 to 14 months. The peak of the breeding and calving season may vary geographically (especially in the North Pacific), with most animal births occurring from summer to fall in Japanese waters and from fall to winter in California waters. Newborn calves are usually 3.5 to 5.5 feet in length and weigh about 45 pounds.
One of the main threats to Risso’s dolphins is becoming entangled or captured in commercial fishing gear such as gillnets, longlines, and trawls.
Risso’s dolphins are directly hunted for meat and oil in Indonesia, Japan, the Caribbean (the Lesser Antilles), Sri Lanka, and the Solomon Islands.
Underwater noise threatens Risso's dolphin populations, interrupting their normal behavior and driving them away from areas important to their survival. Increasing evidence suggests that exposure to intense underwater sound in some settings may cause this species to strand and ultimately die.
Contaminants enter ocean waters from many sources, including oil and gas development, wastewater discharges, urban runoff, and other industrial processes. Once in the environment, these contaminants move up the food chain and accumulate in predators at the top, such as Risso’s dolphins. Because of their long lifespan and blubber stores, Risso’s dolphins accumulate contaminants including trace metals and organochlorines like PCBs and DDTs in their bodies, threatening their immune and reproductive systems. Coastal populations generally are more susceptible.
In the Spotlight
Like all marine mammals, the Risso’s dolphin is protected under the Marine Mammal Protection Act. NOAA Fisheries is working to conserve this species to ensure populations remain stable.
Reducing Entanglement in Fishing Gear
Risso’s dolphins are caught as bycatch in fishing gear, leading to serious injuries and death. In 2005, NOAA Fisheries convened the Atlantic Pelagic Longline Take Reduction Team to address bycatch of Risso's dolphins in the mid-Atlantic region of the Atlantic pelagic longline fishery. The team submitted its recommendations for reducing bycatch to NOAA in 2006. NOAA Fisheries published a proposed rule to implement the Pelagic Longline Take Reduction Plan on June 24, 2008, and NOAA Fisheries published a final rule (74 FR 23349) on May 19, 2009.
Overseeing Marine Mammal Health and Stranding Response
We work with volunteer networks in all coastal states to respond to marine mammal strandings including all dolphins and porpoises. When stranded animals are found alive, NOAA Fisheries and our partners assess the animal’s health and determine the best course of action. When stranded animals are found dead, our scientists work to understand and investigate the cause of death. Although the cause often remains unknown, scientists can sometimes attribute strandings to disease, harmful algal blooms, vessel strikes, fishing gear entanglements, pollution exposure, and underwater noise. Some strandings can serve as indicators of ocean health, giving insight into larger environmental issues that may also have implications for human health and welfare.
Marine Mammal Unusual Mortality Events
Risso’s dolphins have been part of a declared unusual mortality event in the past. Under the Marine Mammal Protection Act, an unusual mortality event is defined as "a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response." To understand the health of marine mammal populations, scientists study unusual mortality events.
Addressing Ocean Noise
Underwater noise threatens whale and dolphin populations, interrupting their normal behavior and driving them away from areas important to their survival. Increasing evidence suggests that exposure to intense underwater sound in some settings may cause Risso’s some dolphins to strand and ultimately die. NOAA Fisheries is investigating all aspects of acoustic communication and hearing in marine animals, as well as the effects of sound on dolphin behavior and hearing. In 2016, we issued marine mammal acoustic technical guidance for assessing the effects of anthropogenic (human-caused) sound on marine mammal hearing.
Risso’s dolphins are protected under the Marine Mammal Protection Act (MMPA).
In 1999, the United States signed on as a Party to the Agreement on the International Dolphin Conservation Program. In addition to other requirements, the AIDCP mandates the establishment of an international tuna tracking program for tuna caught in the eastern tropical Pacific Ocean. The program helps minimize dolphin deaths during fishing for tuna destined for canning.
The International Dolphin Conservation Program Act (PDF, 19 pages) amended the MMPA to make the objectives and requirements of the AIDCP legally effective in the United States.
Key Actions and Documents
NOAA Fisheries conducts various research activities on the biology, behavior, and ecology of Risso’s dolphins. The results of this research are used to inform management decisions for this species.
Determining the size of Risso’s dolphin populations helps resource managers determine the success of NOAA Fisheries’ conservation measures. Our scientists collect and present these data in annual stock assessment reports.
NOAA Fisheries conducts research cruises to collect information on dolphin stocks, such as habit preferences and feeding ecology. For example, NOAA Fisheries has used oceanic research vessels to perform surveys of Risso’s dolphin movements and distribution. Information from this research can be used in management actions to protect these animals.
Scientists have used small aircraft to observe and record Risso’s dolphin numbers and distribution. Under the Marine Mammal Protection Act, NOAA’s Southwest Fisheries Science Center conducts population estimates every 2 to 5 years to monitor the health, status, and trends of the population in its region. By comparing numbers collected over multiple years, scientists can spot trends, such as whether the population is increasing, decreasing, or remaining stable.
Recent Science Blogs
Injury Determinations for Marine Mammals Observed Interacting With Hawaii and American Samoa Longline Fisheries During 2015–2016
Marine mammal interactions (i.e., hookings and entanglements) with the Hawaii and American Samoa…