Dolphins belong to the family Delphinidae. This family belongs to the order Cetacea, which includes all whales and dolphins. In New Zealand there are 10 species and one subspecies of dolphin, the most prevalent being the dusky and the common dolphin. Hector’s dolphin is the only species confined to New Zealand waters. The bottlenose, although well known, has a relatively small local population. Other dolphins are occasional visitors. The larger members of the Delphinidae family, such as orcas, are commonly called whales because of their size; five of these are also found in New Zealand.
One of the less common but more unusual-looking dolphins takes its name from a whale. Like its namesake, the southern right whale dolphin lacks a dorsal fin.
All dolphins are totally protected under the Marine Mammal Protection Act 1972, administered by the Department of Conservation. The most significant conservation issue is the accidental catching of Hector’s and Māui’s dolphins in fishing nets.
The short-beaked common dolphin (Delphinus delphis) is probably the most abundant in the world, although globally in decline. Most of the dolphins depicted in ancient Greek and Roman art are common dolphins.
This species has a tall dorsal fin and pale yellow side patches. It is found in coastal waters in New Zealand and elsewhere, as well as in deep waters of the Atlantic and Pacific. It is the most numerous dolphin around the New Zealand coast. It reaches 1.7–2.4 metres in length, and weighs 70–110 kilograms.
Squid and small schooling fish are its main food. Groups, or pods, range from several individuals to several thousand. Tourist operators searching for common dolphins are alerted to their presence by the mass feeding of gannets and terns on small schooling fish such as pilchards and saury. The birds take advantage of the bonanza when the dolphins herd the fish to the surface. In spring the dolphins and birds are sometimes joined by Bryde's and sei whales. As the whales cruise at speed through the massed fish, the dolphins ride their pressure waves.
In a survey carried out between 1962 and 1964, scientist David Gaskin discovered the largest concentrations in summer were from Kaikōura to Hawke Bay, although they were also abundant around the Hauraki Gulf and Northland. Winter sightings were confined mainly to Marlborough and points north.
An intensive survey off the West Coast in the summer of 1996–97 revealed them to be widespread and in good numbers. About a third of the groups, spotted in shallow waters, included calves; it is speculated that these nursery pods seek shelter from menacing deep-water sharks.
Females breed every two years, and calves suckle for six months. Their lifespan in the wild is unknown, but in 2004 two common dolphins at Marineland, Napier, were still alive 30 years after being captured.
While male Hector's dolphins weigh only 40 kilograms, their testes tip the scales at 1.2 kilos. A human male, on the other hand, is much heavier, but his testes weigh only 40 grams.
Mixing with other species
Off Kaikōura, common dolphins cavort with dusky dolphins, fishing together and forming close bonds. Sometimes individuals of the two species can be seen caressing each other, clapping jaws and mating. Hybrids can result – with a common dolphin’s beak and a dusky’s stripe below the dorsal fin – but it is uncertain whether these offspring can reproduce.
Around Northland common dolphins occupy a separate niche to the bottlenose dolphins, and the two are almost never sighted together, even if they are sometimes separated by only a few kilometres. Like the bottlenose, in spring and summer the common dolphin plies the warm waters of the East Auckland Current for food, moving closer to the Bay of Islands in autumn and winter. In contrast, common dolphins off Whitianga stay closer inshore during summer but move further out in winter.