Lagenorhynchus obscurus
dusky dolphin

Dusky dolphins have a circumpolar distribution in Southern Hemisphere. They can be found near the coasts of South America, South Africa, Kerguelen Island, South Australia and New Zealand.

Dusky dolphins are usually found in warm to cool temperate waters and at coastal region.

The dusky dolphin is a medium-sized dolphin, ranging from 1.8m to 2m in length. It has virtually no beak, as the head slopes evenly down from the blowhole to the tip of the snout. The tip of the dorsal fin is rather blunt and is not markedly hooked. A dusky dolphin has a bluish-black tail and back. A dark band runs diagonally across the flanks from below the dorsal fin towards the vent and along the tailstock. The underside of the body is white, and whitish-grey color extends over the flanks. The tips of the snout and lower jaw are dark. A grey area extends from the eye down to the flipper. Two diagonal whitish streaks run forward from tail up past the base of dorsal fin (Baker 1990).

There are 24-36 pairs of small, pointed teeth about 3mm in diameter in each jaws. The upper jaw ususally has 2 less teeth than the lower (Dawson 1985).

The mating pattern of these dolphins is promiscuous, adult males competing for mating access to females (Evans 1987). Pair bonds do not appear to be formed, but social cohesion within the school seems strong (Dawson 1985).

Mating of dusky dolphins usually takes place during spring. Gestation lasts for 11 months, and the peak months of birth for dusky dolphins are June to August. Dusky dolphins produce one young per birth; the weight of the young is about 5 kg (Nowak 1991). The lactation period lasts for 18 months (Evans 1987). There is no information on the sexual maturity and the life span of dusky dolphins.

Dusky dolphins are often seen in groups of 6-15, occasionally up to 300 for feeding aggregations. Groups are a mixture of ages and include members of both sexes (Evans 1987). Groups are usually stable for at least several days (Herman 1980). Dusky dolphins are among the most altruistic of the dolphins. They have been reported to assist other dolphin species in distress, and to aid humans (Dawson 1985).

They spend night-time in small schools of 6-15 animals no more than a kilometer offshore. During this period, dusky dolphins are at rest with only slow movements. In the morning, they move into deeper water, about 2-10 km from shore. At this time, they search for food in groups, swimming in a line abreast with each animal 10m from the next. Then they may aggregate up to 300 for co-operative herding. By mid-afternoon, feeding may be concentrated in one area, and dusky dolphins start to interact socially in play and sexual activity. In the evening, the large school of dusky dolphins splits up into smaller groups and returns inshore (Evans 1987).

Dusky dolphins are extremely fond of playing and leaping; they often leap in schools. Dusky dolphins are especially attracted to boats. They are fast swimmers, reaching speeds up to 20 knots.

Dusky dolphins produce a variety of whistles, squeaks, squeals and clicks. These sounds are loud and directional (Dawson 1985). The sound of a dusky dolphin reentering the water after a leap carries at least 500m but less than 1km underwater. In air, the sound can be heard as far away as 3-km distance. Dusky dolphins often leap before and after feeding. The leap has been hypothesized to function to recruit other dolphins to assist in feeding activities (Herman 1980).

The main prey species of dusky dolphins include anchovy (the southern form), squid and schooling shrimps. Dusky dolphins feed both at bottom and at the surface (Dawson 1985). They generally feed with cooperative herding of large schools of small fish by aggregations of up to 300 dolphins.

The fresh meat of dusky dolphins can be sold in the markets of Lima for U.S. $1-1.25 per kg. Nearly 10000 individuals are killed each year (Grzimek 1975). Dusky dolphin is an interest to tourists. For instance, at Kaikoura, an intensive dolphin-watching industry is centered on taking tourists to see and to swim with these animals.

Large catches off western South America have caused serious concern. Thousands of dusky dolphins are killed annaually by the small-cetacean fishery in Peru. Although in 1990 the Peruvian government banned the direct fishery for small cetaceans, the ban has not been very effective (Reeves and Leatherwood 1994).

Killer whales are the major predators of dusky dolphins. Dusky dolphins are spectacular acrobats, often turning on displays of jumping and chargin line-abreast near ships (Baker 1990).

Dusky dolphins have been recorded swimming with schools of Common Dolphins, and even mating with them (Dawson 1985).