Risso's dolphin

Habitat: Widespread, pelagic species preferring warm water, generally favouring continental slope waters.

Description: Body is stout, torpedo shaped, narrowing behind dorsal fin to narrow tail stock. Colouration dark to light grey on back and flanks. Older individuals are paler, sometimes white on head and anterior dorsal surface. Many conspicuous white scars on flanks of adults. Belly is white, enlarging to oval or anchor shaped patch on chest and chin. Head has a blunt, beakless snout and a slight melon. At close range, deep v-shaped crease down middle and front of head from blowhole to top lip may be visible. 2 - 7 (usually 4) peg-like teeth at tip of each lower jaw, often badly worn, sometimes lost; no teeth in upper jaw. Flippers long, narrow, pointed, usually dark. Dorsal fin tall, centrally placed, re-curved - taller and more erect in males - dark but may lighten with age, particularly along leading edge. Flukes are dark with median notch and concave trailing edge.

Size: 3.3 - 3.8 m - male slightly larger (male maximum 3.8 m; female maximum 3.3 m).

Weight: 350 - 400 kg.

Lifespan: Oldest individual estimated at > 29 years.

Distribution: Worldwide in tropical and temperate seas in both hemispheres. In eastern North Atlantic, it reaches the northern limits of its regular range in the Northern Isles of Scotland, south to Iberia and Mediterranean, also occurs around oceanic islands like the Azores, Canaries, and Cape Verde.

Diet and feeding: Mainly cephalopods, particularly octopus, cuttlefish, squids, and occasionally small fish. Mainly nocturnal feeding patterns at least in part of range.

Breeding: Poorly known. Off British Isles, mainly March - July, but may vary throughout range. Gestation lasts 13 - 14 months, lactation and calving intervals are unknown. Age at sexual maturity 3 - 4 years for both sexes. Suspected to hybridise with bottlenose dolphin in the wild, and a hybrid has been successfully produced in captivity.

Conservation status and population: Listed by IUCN as Data Deficient. No population estimates for any region in eastern North Atlantic; in the western parts, 29,000 estimated off eastern USA and 2700 in northern Gulf of Mexico. One study in north Minches, Scotland, identified at least 142 individuals.

Hunted in small numbers in several regions of the world, previously including Mediterranean and Lesser Antilles. Widely caught in fishing gear. Killed deliberately in Japan to reduce competition with fisheries; has been observed stealing from longlines. Recent annual takes estimated at 250 - 500. Bycatches off Sri Lanka thought to be unsustainable. Small numbers live-captured for dolphinaria.

Pollutant burdens poorly known. Levels of PCBs in two individuals stranded on Spanish coast very high, but lower in one stranded individual in Wales. Effects of disturbance from oil and gas exploration and shipping largely unknown.

Identification

Vocalisations: Variety of clicks, whistles, pulsed calls. Whistles are rarely heard, range over 2.5 - 20 kHz, usually 8 - 12 kHz, average duration 0.67 s. Clicks frequencies are from 0.2 - >100 kHz, peak frequency 65 kHz, lasting 40 - 100 s, with repetition rates of 4 - 200 /s. Click-bursts last 0.2 - 1.5 s. Fast sequences of pulses with repetition rates 187 - 3750 pulses /s resulting in harmonics.