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(Lacepède, 1804) - Commerson's dolphin

Distinctive Characteristics

The stocky Commerson's dolphin is similar in body shape to porpoises (phocoenids), as are other species of the genus. The head is blunt, with little or no beak and a relatively straight mouthline. The dorsal fin is moderately low and rounded, rising at a shallow angle from the back; the flippers and flukes have rounded tips.

The colour pattern is strikingly contrasted black and white (dark grey and light grey in the Kerguelen Islands). There is a white band that completely encircles the body dorsally from just behind the blowhole to in front of the dorsal fin, and ventrally from behind the flippers to behind the genital area. There is a large white patch on the throat, and a black oval to heart-shaped patch around the genitals that varies in shape between males and females (females generally have the heart-shaped patch pointing forward, males pointing backward). The rest of the animal is black, including the top of the head, flippers, dorsal fin, and flukes. Newborn animals have a muted pattern of mostly grey tones, as do many small cetaceans.

There are 28 to 35 small, pointed teeth in each tooth row.

The Kerguelen Islands population is apparently distinct. These animals are larger than South American ones, and have shades of grey replacing black and white in the colour pattern.

Can be confused with

The only other black and white small cetacean likely to be confused with this species is the spectacled porpoise, but the dorsal-fin shape and colour pattern differences should make these 2 easily discernable. In some areas, Commerson's dolphins may also be confused with black dolphins, which are mostly dark grey.

Size

Length at birth ranges from 65 to 75 cm. Off South America, the adult size of Commerson's dolphin is up to 1.5 m and 66 kg; those off the Kerguelen Islands reach 1.75 m and 86 kg. Females are slightly larger than males in this species.

Geographical Distribution

There are at least 2 disjunct populations of Commerson's dolphins, those off South America and the Falkland Islands, and those off the Kerguelen Islands. There are unsubstantiated reports of this species at South Georgia, but these are rejected by recent workers. Commerson's dolphins appear to prefer relatively shallow coastal waters, but some populations move slightly offshore in winter.

Biology and Behaviour

Small groups of less than 10 individuals are the norm for this species, although they do sometimes aggregate into groups of over 100. These are quick, active animals. They are known to ride bow waves and to engage in various types of leaps. Commerson's dolphins often swim upside down.

The breeding season is in the southern spring and summer, September to February.

Feeding is on various species of fish, squid, and shrimp. Commerson's dolphins appear to be opportunistic, feeding primarily near the bottom.

Exploitation

Gillnet incidental catches probably represent the greatest threat to Commerson's dolphin populations, but they are also taken directly for crab bait in southern Chile. The effects of these kills on the populations involved are not known. Some Commerson's dolphins have been captured live in recent years, and the species appears to have done relatively well in captivity.

IUCN Status

Insufficiently known.

Commerson's dolphin (Cephalorhynchus commersonii)