Originally named ‘Fitzroy’s dolphin’ by Charles Darwin after the Captain of the Beagle (5), the dusky dolphin is a relatively small, compact dolphin, 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)(6).
The dusky dolphin is a highly social species, sometimes found in herds of over 1,000 individuals, although groups of 20 to 500 are more common (3). Large groups often come together to cooperatively hunt prey, which is quite varied and includes anchovy, squid and shrimp. The species may also feed at night. The dusky dolphin frequently associates with other cetaceans(2)(3)(6), and is said to be one of the most acrobatic of all dolphins, readily approaching boats to bow-ride, and often leaping high out of the water and tumbling in the air (3).
Mating is believed to occur in spring, with a single calf born after a gestation period of 11 months (6) and measuring around 55 to 70 centimetres at birth (3)(6). Births usually peak in summer (November to February) around New Zealand and Argentina, and in winter (August to October) around Peru (2). Calves are weaned at around 18 months (6).
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 1,000 kilometres 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 is classified as Data Deficient (DD) on the IUCN Red List (1), and is listed on Appendix II of CITES (4). All cetaceans are listed on Annex A of EU Council Regulation 338/97; they are therefore treated by the EU as if they are included in CITES Appendix I, so that commercial trade is prohibited.
The extent of the threats to this species are not fully understood, but major losses are recorded as a result of gillnet entanglement and illegal harpooning (1)(3)(6). Dolphin meat is in demand in Peru and blubber is used as shark bait (1). Although the dusky dolphin is believed to be relatively abundant throughout its range, very little survey work has been conducted and no population estimates are available (2)(3)(6).
Dolphin hunting was banned in Peru in 1996 (2), and the species is also offered some protection from international trade under its listing on Appendix II of the Convention on International Trade in Endangered Species (CITES) (4). Commercial dolphin-watching is now a major industry in New Zealand, but so far no large-scale or long-term adverse effects on the dolphins have been recorded (2). Conservation measures recommended for the dusky dolphin include intensive photo-identification, genetic and survey studies to determine population numbers, and further investigation into the levels of by-catch in gillnets (1).
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