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.

Dolphins, along with whales and porpoises, are thought to be descendants of terrestrial mammals, most likely of the Artiodactyl order. Dolphins entered the water roughly fifty million years ago. Dolphins have a streamlined fusiform body, adapted for fast swimming. The basic colouration patterns are shades of gray with a light underside and a distinct dark cape on the back. It is often combined with lines and patches of different hue and contrast.

The head contains the melon, a round organ used for echolocation. In many species, the jaws are elongated, forming a distinct beak; for some species like the Bottlenose, there is a curved mouth that looks like a fixed smile.

The dolphin brain is large and has a highly structured cortex, which often is referred to in discussions about their advanced intelligence.

Senses

Most dolphins have acute eyesight, both in and out of the water, and their sense of hearing is superior to that of humans. Though they have a small ear opening on each side of their head, it is believed that hearing underwater is also if not exclusively done with the lower jaw which conducts the sound vibrations to the middle ear via a fat-filled cavity in the lower jaw bone. Hearing is also used for echolocation, which seems to be an ability all dolphins have. Their teeth are arranged in a way that works as an array or antenna to receive the incoming sound and make it easier for them to pinpoint the exact location of an object. The dolphin's sense of touch is also well-developed. However, dolphins lack an olfactory nerve and lobes and thus are believed to have no sense of smell, but they can taste and do show preferences for certain kinds of fish. Since dolphins spend most of their time below the surface normally, just tasting the water could act in a manner analogous to a sense of smell.

Common dolphin

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.

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.

Relative size

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.

Dusky dolphins

Appearance

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.

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.

Hector’s and Māui’s dolphins

The small Hector’s dolphin (Cephalorhynchus hectori), named after the New Zealand scientist James Hector, is also known by its Māori names tutumairekurai (special ocean dweller) and tūpoupou (to rise up). Restricted to New Zealand waters, it is found along parts of the South Island coast. The usual pod size is between 2 and 12, but sometimes larger. Māui’s dolphin (Cephalorhynchus hectori maui) is a northern subspecies. Also called the North Island Hector’s dolphin, Māui’s status as a separate subspecies was proposed by scientists in 2002.

Appearance

Both weigh around 50 kilograms and are 1.2–1.5 metres long, making them among the shortest of the world’s dolphins. They are black and grey with a mainly white belly, with a distinctive rounded black dorsal fin and a blunt rostrum (beak). To casual observers, Hector’s and Māui’s dolphins look alike. However, studies have shown that they have different sized beaks, the genital patches in the males are differently coloured, and Māui’s dolphins are significantly longer.

They feed on fish, squid and crustaceans. Females have one calf at intervals of 2–4 years or more. Their lifespan is about 20 years.

Distribution and population

The population of Hector’s dolphin was estimated at 7,270 in 2002, with the largest group (5,388) off the West Coast. The other main concentration is around Banks Peninsula, with smaller groups off Cloudy Bay in the Marlborough Sounds and in Te Waewae Bay, Southland. They are classified as a vulnerable species.

Māui’s dolphin is found off the North Island’s north-west coast, between Dargaville and New Plymouth. It once ranged as far south as Cook Strait and up to Ninety Mile Beach. The first survey in 1985 estimated a population of 134 individuals, but by the early 2000s there may have been fewer than 100. Categorised as endangered, it is one of the rarest marine dolphin subspecies in the world.

Genetic studies show there is no mixing between these populations.

Brain

The brain of a 38-kilogram Hector’s dolphin weighs 640 grams. This is 1.7% of its total body weight – one of the highest proportions in the animal kingdom. It is bettered only by the typical proportion for humans of 1.9 %. But to what extent a big brain means high intelligence is not well established.

Range

Like the other Cephalorhynchus dolphins (one off South Africa, two off South America and one off the Kerguelen Islands), Hector’s and Māui’s dolphins live inshore where they catch their preferred food and are less likely to be attacked by large sharks. The furthest away from the coast any have been recorded is 60 kilometres; mostly they stay within 10 kilometres.

Shorter length, shorter life

Hector's dolphins do not live as long as others: the smaller the species, the shorter the lifespan. Out of more than 80 Hector's which have been dissected – most of them caught in gill nets or through other fishing accidents – the oldest recorded ages have been 19 years for a female and 20 for a male. Some individuals may live longer than this, but the ages are comparable to those recorded for other Cephalorhynchus species. By contrast, larger dolphins such as the bottlenose live to between 25 and 50 years. A dolphin’s age is estimated from the layers in a cross-section of tooth.

The threat from nets

Fishing set nets are the major threats to Māui’s and Hector’s dolphins. A 1984–85 census showed there were about 740 dolphins in the Pegasus Bay–Canterbury Bight area, yet between 1984 and 1988 at least 230 were killed in nets there.

The dolphins cannot easily detect the nets, even when using echolocation (similar to sonar). This enables them to ‘see’ the hard parts of prey, or solid objects like rocks, but because nets are soft and flexible they do not bounce sounds back to the dolphin.

Echo world

Finding food and navigating under water by vision alone is tricky in murky water. Dolphins can use echolocation as well. They send out a stream of clicks and ‘read’ the echo that bounces back from hard surfaces, such as a rock wall or the taut swim bladder of a fish. This gives them information about size, density and distance.

Bottlenose dolphins

The bottlenose dolphin (Tursiops truncatus) has a long, blunt beak. The body is different shades of grey, darker above and lighter on the belly. Globally there is wide variation in size, from 2.4 to 4 metres, and in weight from 250 to 650 kilograms.

They live in all the temperate oceans of the world. Pod size can be from 2 to 60 or more. Fish and marine invertebrates are their main foods.

Females breed every 3–5 years, and calves suckle for approximately 2–3 years. They can live for 40 years or more.

Coastal and pelagic

Two global ecotypes (adapted to local ecological conditions) have been identified: pelagic (living mainly in the open ocean) and coastal. Elsewhere, the coastal dolphin is smaller than the pelagic, which can reach almost 4 metres. But New Zealand’s coastal dolphins are also generally about this size. Scientists are conducting a genetic comparison of coastal and pelagic bottlenose dolphins in different parts of the country to determine their genetic diversity.

In New Zealand there are three main coastal populations:

• about 450 individuals mainly in the Bay of Islands, but moving from Doubtless Bay in Northland down to Tauranga
• about 70 in Doubtful Sound, Fiordland
• a group in the Marlborough Sounds, some of which may stray down to Westport.

Depending on the time of year, the Northland bottlenose can occupy significantly different habitats. During summer and autumn the dolphins move to the outer waters of the Bay of Islands to fish in the warm East Auckland Current, which sweeps in from the Tasman Front. At other times they stay fairly close to shore, in water with a mean depth of 23 metres (compared to 80 metres for common dolphins). Bottlenose dolphins rarely mix with other species.

The Fiordland group

The Doubtful Sound bottlenoses occupy a vastly different habitat to bottlenoses elsewhere. Not only is it much cooler in the fiords than at sea, but there is also a 3–4-metre layer of fresh water above the sea water.

How do Fiordland bottlenoses cope in winter, when a thin layer of ice sometimes covers the inner fiords for weeks? They move to the warmer outer reaches of the fiords and the open ocean, and do not produce their offspring until mid-summer. Tropical-dwelling bottlenoses give birth all year round, but in Fiordland they have to time calving for an optimum start in life.

During autumn and winter they eat more in order to put on a thicker layer of protective blubber.