Page 3 – 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 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.
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. An aerial survey in 2004 estimated the number as 111. 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.
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.
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.
Researchers believe that in the past, ancestral Cephalorhynchus dolphins left an evolutionary dispersal point – assumed to be off South Africa, home today of their cousin, the Heaviside’s dolphin. Following the sea current known as the West Wind Drift, a founding population arrived in New Zealand and evolved into a separate species with a stay-at-home lifestyle. Others established populations west and east of South America, and became respectively the Chilean dolphin and Commerson’s dolphin.
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 – some of them caught in fishing nets – 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 is the most significant known threat to Māui’s and Hector’s dolphins. Since 1988 there have been 161 reported deaths of the South Island east coast population. Of the 52 mortalities attributable to a specific cause, 25% of reported mortalities were caused by fish nets.
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.
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.
In 1988 the Department of Conservation created a 1,140-square-kilometre Marine Mammal Sanctuary around Banks Peninsula. Commercial netters are banned all year round, while amateurs cannot set nets between November and the end of February. To protect Māui’s dolphin, set nets cannot be used within 4 kilometres of the shore between Dargaville and north of New Plymouth. The Manukau Harbour–Waikato River area is especially lethal: in the early 2000s, six Māui's dolphins died there after becoming entangled.
Hector's dolphins are still caught off the Canterbury coast, outside the sanctuary. One solution is acoustic alarms, or pingers, which scare off a dolphin as it approaches a net. The pingers are placed at 100-metre intervals along a net, and are activated by a battery lasting 30 days.