Hector's dolphin (Cephalorhynchus hectori) is the best-known of the four dolphins in the genus Cephalorhynchus and is the only endemic cetacean to New Zealand. At approximately 1.4 m in length, it is one of the smallest cetaceans.

Two subspecies occur: C. h. hectori, the more numerous subspecies, is found around the South Island, and the critically endangered Maui's dolphin (C. h. maui) is found off the northwest coast of the North Island. Maui's dolphin is one of the eight most endangered groups of cetaceans. The Hector's dolphin is also the world's smallest and rarest dolphin. A 2010/2011 survey of Maui's dolphin by the New Zealand Department of Conservation estimated only 55 adults remain.

Hector?s dolphin was named after Sir James Hector (1834?1907), who was the curator of the Colonial Museum in Wellington (now the Museum of New Zealand Te Papa Tongarewa). He examined the first specimen found of the dolphin. The species was scientifically described by Belgian zoologist Pierre-Joseph van Beneden in 1881.

M?ori names for Hector's and Maui's dolphin include tutumairekurai, tupoupou and popoto.

Hector?s dolphin is the smallest of the dolphins. Mature adults have a total length of 1.2?1.6 m (3 ft 11 in?5 ft 3 in) and weigh 40?60 kg (88?132 lb). The species is sexually dimorphic, with females being slightly longer and heavier than males. The body shape is stocky, with no discernible beak. The most distinctive feature is the rounded dorsal fin, with a convex trailing edge and undercut rear margin.

The overall appearance is pale grey, but closer inspection reveals a complex and elegant combination of colours. The back and sides are predominantly light grey, while the dorsal fin, flippers, and flukes are black. The eyes are surrounded by a black mask, which extends forward to the tip of the rostrum and back to the base of the flipper. A subtly shaded, crescent-shaped black band crosses the head just behind the blowhole. The throat and belly are creamy white, separated by dark-grey bands meeting between the flippers. A white stripe extends from the belly onto each flank below the dorsal fin.

At birth, Hector?s dolphin calves have a total length of 60?80 cm (24?31 in) and weigh 8?10 kg (18?22 lb). Their coloration is the same as adults, although the grey has a darker hue. Four to six vertical pale stripes, caused by fetal folds affecting the pigmentation, are present on the calf?s body until an age of about six months.
Population and distribution

Hector's dolphins are endemic to the coastal regions of New Zealand. The species has a fragmented distribution around the entire South Island, although only very occasional sightings occur in the deep waters of Fiordland. As of 2011, five areas are designed as marine mammal sanctuaries focusing on Hector's and Maui's dolphins around the nation.

The largest populations live on the east and west coasts of the South Island, most notably on Banks Peninsula and Te Waewae Bay while smaller local groups are scattered along entire South Island coasts such as at Cook Strait, Kaikoura, West Coast, Catlins (e.g. Porpoise Bay, Curio Bay), and Otago coasts (e.g.Karitane, Oamaru, Moeraki, Otago Harbour, Blueskin Bay). Maui?s dolphin lives on the west coast of the North Island between Maunganui Bluff and Whanganui. Occasionally, Hector's dolphins can reach the North Island up to Bay of Plenty or Hawke's Bay.

The latest estimate of South Island Hector's dolphins carried out by the Ministry for Primary Industries is around 15,000 individuals and almost twice the previous, published estimate. Problems with the survey methods and data analysis, identified in peer review, are to be discussed when the Hector's dolphin Threat Management Plan is reviewed in 2018. Until that review, it is reasonable to conclude that the current population is on the order of 10,000 Hector's dolphins. The current population is below 30% of the population estimated in 1970 which was about 50,000. The latest estimate of Maui's dolphin is 55 individuals (1 year and older). Additional population surveys have been carried out off the east coast in 2012 and 2013. The results of these surveys have not yet been published in a peer-reviewed scientific journal.

The species' range includes shallow waters down to 100 m (330 ft) deep, with very few sightings in deeper waters. The 100-m depth contour is a better predictor of offshore distribution, depending on water depth, they are found up to 20 nautical miles or more from the coast. In some areas, the seasonal difference in distribution is pronounced, with Hector's dolphins being sighted closer offshore and in shallower water in summer, and more spread out in winter. This is thought to be related to movements of their prey species. They tend to spend a lot of their time by small reef systems and surf zones of the South Island of New Zealand.

As Hector's dolphins group together, they are segregated by sex the majority of the time. Because the groups are either all male or female, the problems associated with reproduction harshly increase. Due to the difficulty in finding mates in addition to the small population of the species, the population only continues to decline. Studies have suggested Hector's dolphins have sex segregation because nursing or pregnant female dolphins are at greater risk from predators. Also, male dolphins show aggression when around other females or when seeking sexual activities. Sex segregation more likely occurs as a combination of these reasons.

Extreme isolation occurs in Hector's dolphins over the summer due to high site fidelity, meaning certain pods only stay in specific shore locations. This results in hindered gene flow between populations and ultimately leads to gene isolation.
Ecology and life history
Data from field studies, stranded individuals, and dolphins caught in fishing nets have provided information on their life history and reproductive parameters. Photo-ID research at Banks Peninsula, and other locations around the South and North Island since 1984 has shown that individuals reach around 23 years of age. Males attain sexual maturity between five and nine years of age, and females have their first calf between seven and nine years old. The calving interval is two to four years.

These life-history characteristics mean that Hector?s dolphins, like many other small cetaceans, have a low potential for population growth. Maximum population growth rate has been estimated to be 1.8-4.9% per year, although the lower end of this range is probably more realistic.
Foraging and predation

Hector's dolphins live in groups of two to eight individuals. They feed at the ocean surface and sea floor, with their diets including ahuru, yellow-eyed mullet, kahawai, arrow squid, herring, and red cod.

Hector?s dolphins are generalist feeders, with prey selection based on size rather than species. Typically, they feed on smaller prey which tend to measure under 10 cm. in length. Stomach contents of dissected dolphins have included surface-schooling fish, midwater fish, and squid, and a wide variety of benthic species. The largest prey item recovered from a Hector?s dolphin stomach was an undigested red cod weighing 500 g with a standard length of 35 cm. Many Hector's dolphins can be observed following fishing trawlers as a result of the amount of disturbed or escaped flatfish and red cod on which the species typically feed. However, this activity can result in the unwanted bycatching of the species.

Similar to the hourglass dolphin, Hector's dolphins use high-frequency echolocation clicks. However, the Hector's dolphin produces lower source-level clicks than hourglass dolphins due to their crowded environment. This means they can only spot prey at half the distance compared to an hourglass dolphin.

Natural predators of Hector?s dolphins include sharks and killer whales (orca). Remains of Hector's have been found in sevengill and blue shark stomachs.

Although the biggest threat to this endangered species is inshore fishing, they also suffer from an infectious agent, Toxoplasma gondii. A study of 28 captured Hector's dolphins showed seven of them died due to toxoplasmosis. These dolphins were examined, and hemorrhagic lesions in the lungs, liver, lymph nodes, and adrenals were found.

Conservation
Dolphin deaths in bottom-set gillnets and trawl fisheries have been responsible for substantial population declines in the last four decades. Gillnets are made from lightweight monofilament that is difficult for dolphins to detect, especially when they are distracted (e.g. chasing fish) or moving around without using echolocation. Hector's and Maui's dolphins swim into the nets, get caught, and drown - or more accurately, suffocate (breathing is active in dolphins). Hector's dolphins are actively attracted to trawling vessels and can frequently be seen following trawlers and diving down to the net. Occasional mistakes can lead to injury or death.

The nationwide estimate for bycatch in commercial gillnets is 110-150 dolphins per year which is far in excess of the sustainable level of human impact. Deaths in fishing nets are the most serious threat (responsible for more than 95% of the human-caused deaths in Maui's dolphins), with currently lower level threats including tourism, disease, and marine mining.

The New Zealand government proposed a set of plans to avoid further extinction of Hector's dolphin, including protecting/closing areas where dolphins are normally found and changing fishing methods to avoid catching dolphins. The plan involves three steps; The first is to change voluntary codes involving practice and monitoring for fisherman. The second is to close inshore areas from fishing. The third plan is formed around the potential biological removal (PBR) concept which tells how much change would need to be done to protect the dolphins and how much of the dolphins' extinction is caused by humans. PBR testing was done in eight bodies of water surrounding the South Island of New Zealand. These test unfortunately resulted in telling the time it would take to determine the populations of dolphins in these waters is much longer than expected and cannot be done until the populations are depleted, but affirms more protection needs to occur in these waters.

The first marine protected area (MPA) for Hector's dolphin was designated in 1988 at Banks Peninsula, where commercial gillnetting was effectively prohibited out to 4 nmi (7.4 km; 4.6 mi) offshore and recreational gillnetting was subject to seasonal restrictions. A second MPA was designated on the west coast of the North Island in 2003. Populations continued to decline due to bycatch outside the MPAs.

Additional protection was introduced in 2008, banning gillnetting within 4 nmi of the majority of the South Island?s east and south coasts, out to 2 nmi (3.7 km) offshore off the South Island?s west coast and extending the gillnet ban on the North Island?s west coast to 7 nmi (13 km; 8.1 mi) offshore. Also, restrictions were placed on trawling in some of these areas. For further details on these regulations, see the Ministry of Fisheries website. Five marine mammal sanctuaries were designated in 2008 to manage nonfishing-related threats to Hector?s and Maui?s dolphins. Their regulations include restrictions on mining and seismic acoustic surveys. Further restrictions were introduced into Taranaki waters in 2012 and 2013 to protect Maui's dolphins.

The Scientific Committee of the International Whaling Commission has recommended extending protection for Maui's dolphin further south to Whanganui and further offshore to 20 nmi from the coastline. The IUCN has recommended protecting Hector's and Maui's dolphins from gillnet and trawl fisheries, from the shoreline to the 100 m depth contour.