Dusky dolphin  (Lagenorhynchus obscurus)

The dusky dolphin is a medium-sized dolphin, which is easily recognised by the evenly sloping head from the blowhole to the tip of the snout, which lacks a beak. As the common name suggests, the back and tail of this species are a dusky bluish-black, with a dark band running diagonally across the flanks to the tail. The underside is white and the tips of the snout and lower jaw are dark. A grey area runs from the eye to the flipper, and two cream stripes extend from the tail to the blunt dorsal fin (2).

Whilst found almost everywhere in the southern hemisphere, particularly high concentrations of dusky dolphins are found around New Zealand, Tasmania, southern Australia and south-western Africa. Populations are also found around the western coast of South America, but appear to be separated both geographically and biologically by a gap spanning 1000 km of Chilean coastline. Individuals to the north of this separation are larger than those to the south, and are sometimes considered separate subspecies (1).

The dusky dolphin occupies warm to cool temperate waters and coastal regions (2).

Feeding in large schools of 300 – 1000 individuals, these dolphins were thought to be very abundant, but declining numbers in by-catch and during hunting suggest that dusky dolphins are no longer as plentiful as was imagined (1). These extremely numerous aggregations are temporary collections of the more normal 6 – 15 strong pods gathering together for tactical hunting of their anchovy, squid and shrimp prey. The prey are herded together by groups of cooperating dolphins, who leap and slam their bodies on the surface of the water to push the prey closer together as well as to notify other dolphins to join the feeding (4).

Said to be one of the most acrobatic and social of all dolphins, the dusky dolphin is attracted to boats to bow-ride at speeds of up to 20 knots, and will spend afternoons playing, leaping, and mating. They communicate regularly with whistles, squeaks and clicks (2).

Mating occurs in spring, and is somewhat promiscuous within each school, but no pair bonds are formed. Gestation lasts up to 11 months and single calves are born between June and August and measure less than 60 cm (2) (3). Calves are weaned at 18 months (2). Females give birth every two to three years (3).

The extent of the threats to this species is not fully understood, but major losses are recorded as a result of gillnet entanglement and illegal harpooning (1). Dolphin meat is in demand in Peru and blubber is used as shark bait (1).

The direct fishery of all small cetaceans is prohibited but the demand is high enough for this ban to be poorly enforced and therefore ineffective (2).

For further information on this species see Animal Diversity Web:
http://animaldiversity.ummz.umich.edu/site/accounts/information/Lagenorhynchus_obscurus.html

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk