Page 4 – 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.
These fiord-dwellers are a small group – 70-strong and practically closed: during seven years of observation (sometimes for more than 100 days a year) researchers never saw any individuals leaving the group, or newcomers entering it. As a result, the dolphins have developed extremely strong and long-lasting bonds.
Such behaviour – unique among bottlenose dolphins – is influenced by the ecology of the fiords. Food is not only scarce, it is also difficult to find. Survival depends on transferring information from one member to the other; cooperation and stable relationships are the key to continued existence.
Males tend to be more boisterous, competing for access to females by leaping high into the air. The Fiordland dolphins are larger than their tropical counterparts, following the general rule that the cooler the climate the bigger the animal. During autumn and winter they eat more in order to put on a thicker layer of protective blubber.