Dusky dolphins (Lagenorhynchus obscurus) are slightly smaller than common dolphins at 1.6–2.1 metres long and 50–90 kilograms in weight. They have tapered diagonal stripes along their side. Their main foods are krill, copepods and small fish. The average size of pods is 6–15, but groupings of several hundred or even thousands are often seen.
Breeding and lifespan
Breeding begins at 4–6 years, and after a gestation of about 11 months calving generally takes place in spring. The mother feeds her calf for 18 months. Dusky dolphins live for 30 years or more.
Distribution and population
Found in coastal and continental shelf areas around the southern hemisphere, dusky dolphins are the second most numerous species of dolphin around New Zealand. Their distribution is associated with the zone where subtropical and subantarctic waters converge, as it is at the boundary between warm and cool waters that food is most plentiful. They are most abundant from East Cape down to Kaikōura, and also occur as far east as the Chatham Islands. During winter a cool current which runs up as far as Gisborne encourages the dolphins north, but that is generally their natural northern limit.
In the 1970s duskies were reported at Taranaki and Tasman Bay; by the 1990s such sightings had stopped, although the dolphins are regularly seen in and outside Wellington Harbour and in the Marlborough Sounds. In summertime duskies venture further south along the West Coast and to Southland, Otago and Stewart Island; in 1973 a researcher estimated that about 5,000 surrounded Solander Island. Occasionally, enterprising pods brave the chill waters of the subantarctic Auckland Islands and Campbell Island.
There is a notable absence of dusky dolphins between the Conway River south of Kaikōura down to just north of the Otago Peninsula. The water in this area – including Pegasus Bay and the Canterbury Bight – is shallow, and does not attract duskies’ favoured prey.
Kaikōura is considered one of the best places in the world to see dusky dolphins in their natural environment. Each pod or group can number anything between 100 and 800; in autumn and winter this can increase into the thousands.
During November 1995 scientists at Kaikōura saw four orcas hunting dusky dolphins over 11 days. Most of the observed kills were adults, but the orcas also attacked younger dolphins. A dusky dolphin was flicked 10 metres out of the water and an orca leapt up to seize it in mid-air. This was not an isolated event. The dolphins rely on their skill at fleeing and hiding to avoid being killed.
Distinctive New Zealand duskies
Compared to dusky dolphins off the Argentine coast (where they are the most studied in the world), the New Zealand duskies off the Kaikōura coast are distinctive in their behaviour and feeding. DNA studies by United States scientist Frank Cipriano suggest that they may be a separate species.
Dusky dolphins do not develop pair bonds, but instead the males compete for female attention. Females may mate with a number of different males within just a few minutes.
During late spring and summer the Kaikōura duskies spend the mornings inshore resting and socialising, but by late afternoon they move between 6 and 15 kilometres offshore. At this time, too, they show off their full repertoire of leaps and somersaults. In winter they spend more time in deep water.
There may be another purpose to moving inshore during summer: since calves are born at that time of year, the dolphins may be taking shelter in shallow bays to avoid a surprise attack by a shark or killer whale.