Maui's dolphin or popoto (Cephalorhynchus hectori maui) is the world's rarest and smallest known subspecies of dolphin.^[1] They are a sub-species of the Hector's dolphin. Maui's dolphin are only found off the west coast of New Zealand's North Island, and are the country's only endemic subspecies of cetacean.^[2] As of 2012, it is estimated that there are 55 Maui's dolphins at least one year old in the world.^[3] Maui's dolphins are generally found close to shore in groups or pods of several dolphin. They are often seen in water less than 20 metres deep, but may also range further offshore. Most of their time is spent feeding.

Etymology[edit]

The word "Maui" from the Maui's dolphin's name comes from te Ika-a-Māui, the Māori word for New Zealand's North Island. However, the Māori word for the dolphin itself is popoto.^[4]

Genetics[edit]

In 2002, Maui's dolphins were classified as a sub-species of Hector's dolphin. Previously, they had been known as the North Island Hector's Dolphin. Dr Alan Baker found that genetic and skeletal differences in the Maui's dolphins made them distinct from others in the Hector's species.^[5] These significant differences over a small geographical distance have not been found in any other studies of marine mammals.^[6] There are 22 different mitochondrial DNA identification haplotypes in the Hector's species, the Maui's ‘G' haplotype being one of them.^[7]

In 2002, it was not known that Hector's dolphins were capable of swimming across Cook Strait and co-existing with Maui's dolphins. Instead it was understood that the deep waters of the Strait had been an effective barrier between South Island Hector's and its North Island Maui's subspecies for between 15,000 and 16,000 years.^[7] It was therefore surprising to researchers that the 2012 Auckland University/Department of Conservation boat survey tissue sampling of Maui's in core range, which included historical samples, revealed some Hector's dolphins living with Maui's in this range area.^[8]

There is no evidence so far that the Hector's and Maui's interbreed,^[8] but, given their close genetic composition, it is likely that they could. Interbreeding may increase the numbers of dolphins in the Maui's range and reduce the risk of inbreeding depression. But such interbreeding could eventually result in a hybridisation of the Maui's back into the Hector's species and lead to a reclassification of Maui's as again the North Island Hector's. Hybridisation in this manner threatens the Otago black stilt^[9] and the Chatham Islands' Forbes parakeet^[10] and has eliminated the South Island brown teal as a subspecies.^[11] Researchers have also identified potential interbreeding as threatening the Maui's with hybrid breakdown and outbreeding depression.^[8]

Physical description[edit]

Range of Maui's dolphin (blue) in New Zealand's North Island, with the area covered by the net ban marked in red.

Having distinctive grey, white and black markings and a short snout, they are most easily recognized by their round dorsal fin. Maui's dolphins are generally found close to shore in groups or pods of several dolphins. They have a solidly built body with a gently sloping snout and a unique rounded dorsal fin. (Maui's and Hector's are the only dolphins with a well-rounded black dorsal fin.) Females grow to 1.7 metres long and weigh up to 50 kg. Males are slightly smaller and lighter. The dolphins are known to live up to 20 years.

Population and distribution[edit]

Maui's dolphins are classed by the Department of Conservation in the New Zealand Threat Classification System as "Nationally Critical".^[12]

Maui's are only found off the west coast of the central and upper North Island of New Zealand, approximately between Dargaville and New Plymouth.^[13] Since 2001 the known range of the Maui's has been between the Kaipara Harbour south to Raglan Harbour.^[14] Most sightings of the dolphins are made in their core range between Manukau Harbour and Port Waikato, a 22 nautical mile distance.^[15] There are previous records of dolphin sightings off the east coast of the North Island, which could indicate there was a much larger population or another subspecies of Hector's has since become extinct.^[6]

Its estimated that 55 adult individuals of the species currently remain in the wild. This is a marked decrease from a 2004 survey that found the population to be around 100 dolphins. A survey of Maui's dolphins in 1985 estimated their numbers to be at 134.^[5] The data from the 2012 report is not directly comparable with earlier aerial surveys because of the different methods used, but the reports highlight that the population is very small and are indicative of a recent decline.^[14]

A likely Maui's dolphin (possibly a Hector's dolphin) was caught in a set net off Cape Egmont on 2 January 2012. A Hector's dolphin was found washed up on the Opunake beach on 26 April 2012.^[16]

The last confirmed Maui's presence off Taranaki was at Oakura Beach on 6 December 1988. Other historical presence further south has been confirmed by DNA analysis, dating back to Wellington Harbour in 1873.^[14]

Ecology and behaviour[edit]

Vocalizations and echolocation[edit]

Maui's dolphins use echolocation to navigate, communicate and find their food. High-frequency ultrasonic clicks reflect back to the dolphin any objects found in the water.^[17]

Foraging and predation[edit]

Maui's dolphins feed on small fish, squid and ocean floor dwelling species like flatfish and cod.^[18] Maui's dolphins spend much of their time making dives to find fish on the sea floor. They also find fish and squid in mid water and at times feed near the surface.^[17]

Social behaviour and reproduction[edit]

Female Maui's dolphins are not sexually mature until they are 7 - 9 years of age. They then produce one calf every 2 - 4 years.^[19]

They have been observed playing (e.g. with seaweed), chasing other dolphins, blowing bubbles, and play fighting.^[20]

Very little is known about the Maui's dolphin reproductive physiology.

Threats[edit]

Fishing[edit]

Gill net fishing has had an adverse effect on the Maui's dolphin population.

Some groups in the fishing industry are against increased bans on set nets into waters further offshore and into harborus, and say there are other factors responsible for the decline in population, including disease, pollution, mining and natural predation. ^[21]

Disease[edit]

In 2006, Brucella was identified in a dead Maui's dolphin and DOC says this disease could have serious ramifications for the small Maui's population. Brucella is a pathogen of terrestrial mammals that can cause late pregnancy abortion, and has been seen in a range of cetacean species elsewhere.^[22]

Recent post mortems on Hector's and Maui's have shown a 61% infection rate of the parasite Toxoplasma. Two of the three Maui's examined were killed by toxoplasmosis. Toxoplasmosis is known to reduce fertility in livestock, with cats playing a key role in its spread. It is not known how toxoplasmosis spread to Maui's and Hector's dolphins.^[14]

Set net ban[edit]

Currently a trawling and set net ban stretches from Maunganui Bluff (north of Auckland) to Pariokariwa Point (north Taranaki), out to seven nautical miles from shore.^[23] Harbours along this stretch of coast do not have a set net ban.

After what MPI believed at the time in January 2012 was the capture of a Maui's dolphin off Taranaki, in June 2012 the New Zealand government announced an interim set net ban extension south around the Taranaki coast to Hawera and out to two nautical miles from shore,^[24][25] and placed observers to look for Maui's dolphins on all vessels setting nets out to seven nautical miles.

References[edit]