Risso's Dolphin

Description

Risso’s dolphin is a large, robust dolphin species that is immediately recognizable due to its unique appearance. Adults measure 8.5 to 13 feet in length and weigh between 650 and 1,100 pounds. The most distinctive characteristic of Risso’s dolphins is their extensive scarring: the body is covered in linear white scratches and scars caused by the teeth of other Risso’s dolphins during social interactions and by the beaks of squid during feeding. Young calves are born dark gray or olive-brown, but as they age and accumulate more scars, they progressively lighten, and older adults can appear almost entirely white, particularly on the head and anterior body. The head is blunt and bulbous with a distinctive vertical crease down the front of the melon, and unlike most dolphins, Risso’s dolphins lack a prominent beak. The dorsal fin is tall, erect, and pointed, one of the tallest relative to body size among dolphins. Their flippers are long and pointed. Risso’s dolphins have no teeth in the upper jaw and only 2 to 7 pairs of teeth in the lower jaw, reflecting their specialization as squid predators.

Habitat & Range

Risso’s dolphins are found in temperate and tropical waters worldwide, generally preferring deep oceanic waters along the continental shelf edge and over submarine canyons and escarpments where their squid prey is concentrated. In the eastern North Pacific, they range from British Columbia to Baja California and beyond. The Southern California Bight is a particularly important area for Risso’s dolphins, and they are regularly encountered on whale-watch excursions from Orange County harbors, especially in areas where the seafloor drops off into deeper water. The California Current stock is estimated at approximately 6,000 individuals. Risso’s dolphins tend to favor water depths of 1,300 to 3,300 feet and are often found along the edge of the continental shelf where upwelling and topographic features concentrate deep-water prey near the surface. They may shift their distribution seasonally in response to water temperature changes and prey movements, with some evidence suggesting they move closer to shore during periods of cool water and higher productivity.

Diet

Risso’s dolphins are specialist feeders with a diet consisting almost exclusively of cephalopods, particularly squid. Their preferred prey species include market squid, jumbo squid, and various deep-water squid species that inhabit the mesopelagic zone. Their reduced dentition (few or no teeth in the upper jaw, limited teeth in the lower jaw) is an adaptation to a squid-based diet, as they swallow their prey whole using suction rather than grasping it with teeth. Risso’s dolphins are primarily nocturnal feeders, diving at night when squid migrate toward the surface as part of the daily vertical migration of the deep scattering layer. They can dive to depths of over 300 meters and hold their breath for up to 30 minutes, though most feeding dives are shallower and shorter. An adult Risso’s dolphin may consume approximately 20 to 30 pounds of squid per day. The circular and linear scars found on squid-feeding cetaceans, caused by the sharp suckers and beaks of squid, are particularly abundant on Risso’s dolphins due to their specialization on cephalopod prey.

Behavior

Risso’s dolphins are moderately social, typically found in groups of 10 to 50 individuals, though aggregations of several hundred have been observed in areas of high prey concentration. They are generally slower and less acrobatic than many other dolphin species, though they do occasionally breach, spy-hop, and slap their tails and flippers on the surface. A particularly distinctive behavior of Risso’s dolphins is “headstanding,” where the animal positions itself vertically in the water with the tail flukes waving above the surface. They also engage in extensive social rubbing and raking, which is the primary source of the white scarring that characterizes the species. Risso’s dolphins have been observed associating with other cetacean species, including Pacific white-sided dolphins, common dolphins, and pilot whales. They are deep divers relative to their size, spending significant time foraging at depth, particularly at night. Females give birth to a single calf after a gestation period of approximately 13 to 14 months. The calving interval is estimated at two to three years. Risso’s dolphins are believed to have a lifespan of at least 35 years.

Conservation Status

Risso’s dolphins are listed as Least Concern on the IUCN Red List, though population data is limited for many regions. In the United States, they are protected under the Marine Mammal Protection Act. The eastern North Pacific population appears to be stable, but Risso’s dolphins face several threats. They are taken in drive fisheries in Japan, where several hundred may be killed annually. In some regions, they are victims of bycatch in gillnets, purse seines, and other fishing operations. In California waters, the drift gillnet fishery for swordfish and thresher shark has historically been a source of incidental mortality for Risso’s dolphins, prompting regulatory actions to reduce bycatch. Other threats include ocean pollution, marine debris ingestion, and underwater noise from shipping, sonar, and seismic surveys, which may be particularly disruptive for a species that relies heavily on echolocation for nocturnal deep-water foraging. The effects of climate change on squid populations and distribution patterns could have significant implications for Risso’s dolphins, given their high degree of dietary specialization.