The dusky dolphin has a discontinuous semi-circumpolar range. The dolphins can be found off the coasts of South America, southwestern Africa, southern Australia and Tasmania, New Zealand, and some oceanic islands.
The dusky dolphin (Lagenorhynchus obscurus) is a dolphin found in coastal waters in the Southern Hemisphere. Its specific epithet is Latin for "dark" or "dim". It is very closely genetically related to the Pacific white-sided dolphin, but current scientific consensus holds they are distinct species. The dolphin's range is patchy, with major populations around South America, southwestern Africa, New Zealand, and various oceanic islands, with some sightings around southern Australia and Tasmania. The dusky dolphin prefers cool currents and inshore waters, but can also be found offshore. It feeds on a variety of fish and squid species and has flexible hunting tactics. The dusky dolphin is known for its remarkable acrobatics, having a number of aerial behaviors. The status of the dolphin is unknown, but it has been commonly caught in gill nets.
Almost no sexual dimorphism occurs in this species, although males have more curved dorsal fins with broader bases and greater surface areas. The back of the dolphin is dark grey or black, and the dorsal fin is distinctively two-toned; the leading edge matches the back in color, but the trailing edge is a much lighter greyish white. The dusky dolphins has a long, light-grey patch on its fore side leading to a short, dark-grey beak. The throat and belly are white, and the beak and lower jaw are dark grey. Two blazes of white color run back on the body from the dorsal fin to the tail. Right between the white areas remains a characteristic thorn-shaped patch of dark color, by which the species can easily be recognized. Aside from that, dusky dolphins may be confused with other members of their genus when observed at sea. It can be distinguished from the common dolphin, which has a more prominent and longer beak and yellow flank markings. The skull of a dusky dolphin has a longer and narrower rostrum than that of an hourglass dolphin or Peale's dolphin of similar age and size.
The dusky dolphin is small to medium in length compared with other species in the family. There is significant variation in size among the different population areas. The largest dusky dolphins have been encountered off the coast of Peru, where they are up to 210 cm (6 feet) in length and 100 kg (210 pounds) in mass. The size for dusky dolphins in New Zealand have been recorded to be a length range of 167–178 cm and a weight range of 69–78 kg for females and a length range of 165–175 cm and a weigh range of 70–85 kg for males.
Dusky dolphins prey consume a variety of fish and squid species. Common fish species eaten include anchovies, lantern fish, pilchards, sculpins, hakes, horse mackerel, hoki and red cod.They are generally coordinated hunters. They arrive at the hunting site individually, but form groups when in the layer. The dolphins use their echolocation to detect and isolate an individual prey. Groups of foraging dolphins tend to increase when the layer is near the surface and decrease when it descends.
Females with calves tend to gather in nursery groups in shallow water. Nursery groups likely provide mothers and calves more time to rest, which is important for both. While the behaviors of nursery groups vary by month, resting is the predominant behavior during most months. Adult males in these groups will aggressively herd and chase females. They can separate calves from their mothers and harass them, as well. Calves may also become even more vulnerable to predators as they become exhausted and disoriented. Mother dolphins may look after calves that are not their own.
Dusky dolphins live in a fission-fusion society, with most group size increases occurring during foraging and decreases in group sizes occurring during resting and traveling. These subgroups are composed of mating adults (mating groups), mothers with calves (nursery groups) and nonbreeding adults. Dusky dolphins have a promiscuous mating system in which both males and females mate with multiple partners. Mating groups are generally made of around 10 males and a single female. These mating groups can be found in both shallow and deep water but more often gather near shore.