Hector's dolphin (Cephalorhynchus hectori) is one of six dolphin species belonging to the genus Cephalorhynchus. Hector's dolphin is the only cetacean endemic to New Zealand, and comprises two subspecies: C. h. hectori, the more numerous subspecies, also referred to as South Island Hector's dolphin, and C. h. maui, the critically endangered Māui dolphin that is found off the West Coast of New Zealand's North Island.
Distinctive features of the species include a rounded dorsal fin, grey, white, and black coloration, and a stocky body shape. Their lifespan is typically around 20 years, and the species favours shallow coastal waters around the South Island, where they live in small groups and use echolocation clicks to communicate.
Threats to the species include bycatch in gillnets and trawls, disease, genetic issues, and additional anthropogenic hazards. Action to conserve the species has been taken through the establishment of marine protected areas and other recent protections.
Etymology Hector's dolphin was named after New Zealand scientist and explorer James Hector (1834–1907), who became the director of the Colonial Museum in Wellington (now the Museum of New Zealand Te Papa Tongarewa), where he oversaw the publication of annual geological exploration reports. The species was scientifically described by Belgian zoologist Pierre-Joseph van Beneden in 1881. Māori names for Hector's dolphin include Tutumairekurai, Tuupoupou, Pehipehi, Hopuhopu, Upokohue, and Waiaua.
Description
Hector's dolphin is the smallest dolphin species. Mature adults have a total length of and weigh .Slooten, E. and Dawson, S.M. 1994. Hector's dolphin Cephalorhynchus hectori. Pp. 311–333 in: Handbook of Marine Mammals. Volume 5 (Delphinidae and Phocoenidae) (S.H. Ridgway and R. Harrison eds). Academic Press. New York, U.S.A. The species is sexually dimorphic, with females being about 5–7% longer than males. The body shape is stocky, with a large fluke relative to their body size and no discernible beak. The most distinctive feature is the rounded dorsal fin, with a convex trailing edge and undercut rear margin. Māui dolphins tend to be longer than South Island Hector's dolphins.
The overall coloration of Hector's dolphins is a mixture of grey, white, and black. Their eyes are surrounded by a black mask, and a thin black line extends from the head to the blowhole. The throat and belly are white, while the rest of the dolphin is grey.
At birth, Hector's dolphin calves have a total length of and weigh .Slooten, E. 1991. Age, growth and reproduction in Hector's dolphins. Canadian Journal of Zoology 69: 1689–1700. Their coloration is the almost same as adults, although the grey has a darker hue. Newborn Hector's dolphins have distinct fetal fold marks on their flanks that cause a change in the coloration pattern of the skin. These changes are visible for approximately six months and consist of four to six vertical light grey stripes against darker grey skin.
Life history
Data from field studies, beached individuals, and dolphins caught in fishing nets have provided information on their life history and reproductive parameters. Photo-ID based observations at Banks Peninsula from 1984 to 2006 show that individuals can reach at least 22 years of age. Males attain sexual maturity between 6 and 9 years old and females begin calving between 7 and 9 years old. Females will continue to calve every 2–3 years, resulting in a maximum of 4–7 calves in one female's lifetime. Calving occurs during the spring and summer.Slooten, E. (1991). Age, growth, and reproduction in Hector's dolphins. Can J. Zool. 69(6): 1689–1700. Calves are assumed to be weaned at around one year of age, and the mortality rate in the first 6 months has been estimated to be around 36%.Bräger, S. H.-J. (1998). Behavioural ecology and population structure of Hector's dolphin (Cephalorhynchus hectori) (Thesis, Doctor of Philosophy). University of Otago.
These combined characteristics mean that, like many other cetaceans, Hector's dolphins are only capable of slow population growth. Previously their maximum population growth had been estimated at 1.8–4.9% per year. This estimate was later updated to 3–7% per year based on updated demographic information and a life history invariant observed across all vertebrates. Due to the short lifespan and slow population increase rate of the species, Hector's dolphin populations are vulnerable and at risk of decline.
Ecology
Habitat The species' range includes murky coastal waters out to depth, though almost all sightings are in waters shallower than . The species is most often observed about 1000 m from the shoreline. Hector's dolphins display a seasonal inshore-offshore movement; favouring shallow coastal waters during spring and summer, and moving offshore into deeper waters during autumn and winter. The species tends to display more dispersal during the winter season. They have also been shown to return to the same location during consecutive summers, displaying high foraging site fidelity. The inshore-offshore movement of Hector's dolphins are thought to relate to seasonal patterns of turbidity and the inshore movements of prey species during spring and summer.
Diet Hector's dolphins are generalist feeders, with prey selection based on size (mostly under 10 cm in length) rather than species, although spiny species appear to be avoided. 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. The stomach contents of dissected dolphins include a mixture of surface-schooling fish, midwater fish, squid, and a variety of benthic species. The main prey species in terms of mass contribution is red cod, and other important prey include flatfish (Peltorhamphus), ahuru (Auchenoceros punctatus), New Zealand sprat (Sprattus muelleri), New Zealand arrow squid (Nototodarus sloanii), and juvenile giant stargazer (Kathetostoma giganteum).
Predators The remains of Hector's dolphins have been found in the stomachs of broadnose sevengill sharks (Notorynchus cepedianus, considered to be their main predator), great white sharks (Carcharodon carcharias) and blue sharks (Prionace glauca). Unconfirmed predators of Hector's dolphins include orca (Orcinus orca), mako sharks (Isurus) and copper sharks (Carcharhinus brachyurus).
Behaviour
Group dynamics Hector's dolphins have been found to show a high level of fluidity with weak inter-individual associations, meaning they do not form strong bonds with other individuals. Three types of small preferential groups have been found: nursery groups; immature and subadult groups; and adult male/female groups. They display a sex-age population group composition, meaning they group by biological sex and age, and have a mean group size of 3.8 individuals, preferring to form sex-segregated groups of less than 5 individuals. Groups of greater than 5 individuals are formed much less frequently, are usually composed of members of both sexes, and have been shown to form only to forage or participate in sexual behaviour. Nursery groups can also be observed and are usually composed of less than 7 mothers and their young, with no adult males present.Webster, T.A.; Dawson, S.M.; Slooten, E. (2009). Evidence of Sex Segregation in Hector's Dolphin (Cehalorhynchus hectori). Aquatic Mammals. Vol. 35, Iss. 2.: 212–219.
Sexual behaviour Males of the species have extremely large testes in proportion to body size, with the highest relative weight in one study being 2.9% of body weight. Large testes in combination with males' smaller overall body size suggests a promiscuous mating system. This type of reproductive system would involve a male attempting to fertilise as many females as possible and little male-male aggression. The amount of sexual behaviour per individual in the species is observed most when small single sex groups form large mixed sex groups. Sexual behaviour in the species is usually non-aggressive.
Echolocation 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.Kyhn, L.A.; Tougaard, J.; Jensen, F.; Wahlberg, M.; Stone, G.; Yoshinaga, A.; Beedholm, K.; Madsen, P.T. 2009: Feeding at a high pitch: source parameters of narrow band, high-frequency clicks from echolocating off-shore hourglass dolphins and coastal Hector's dolphins. Journal of the Acoustical Society of America 125(3): 1783–1791. The species has a very simple repertoire with few types of clicks, as well as little audible signals in addition to these. More complex clicks could be observed in large groups.Dawson, S.M. (1991) Clicks and Communication: The Behavioural and Social Contexts of Hector's Dolphin Vocalizations. Ethology. Vol 88, Iss. 4.
Distribution and population size Hector's dolphins are endemic to the coastal regions of New Zealand. The South Island Hector's dolphin is most abundant in discontinuous regions of high turbidity around the South Island. They are most abundant off the East Coast and West Coast, most notably around Banks Peninsula, with smaller, more isolated populations off the North Coast and South Coast (notably at Te Waewae Bay).Slooten, E., Dawson, S.M. and Rayment, W.J. 2004. Aerial surveys for coastal dolphins: abundance of Hector's dolphins off the South Island West Coast, New Zealand. Marine Mammal Science 20:477–490. Around Banks Peninsula, some areas are used much more heavily than others; these areas continue to be preferred over decades. Smaller populations are scattered around the South Island, including: Cook Strait, Kaikōura, Catlins (e.g., Porpoise Bay, Curio Bay), and Otago coasts (e.g.Karitane, Oamaru, Moeraki, Otago Harbour, and Blueskin Bay).Slooten L.. Benjamins S.. Turek J.. 2011. Potential impacts of Project Next Generation on Hector's'dolphins and other marine mammals . Otago University. Retrieved 4 November 2014 Māui dolphin are typically found on the west coast of the North Island between Maunganui Bluff and Whanganui.Slooten, E., Dawson, S.M., Rayment, W. and Childerhouse, S. 2006. "A new abundance estimate for Maui's dolphin: What does it mean for managing this critically endangered species?". Biological Conservation 128: 576–581.
An aerial survey of South Island Hector's dolphin abundance—which was commissioned by the Ministry for Primary Industries, carried out by the Cawthron Institute, and endorsed by the International Whaling Commission—estimated a total population size of 14,849 dolphins (95% confidence interval = 11,923–18,492).Report of the Scientific Committee, 2017. Journal of Cetacean Research and Management 18 This was almost twice the previous published estimate from earlier surveys (7,300; 95% CI 5,303–9,966). This difference was primarily due to a much larger estimated population along the east coast, which was also distributed further offshore than previously thought.
The latest population estimate of the Māui dolphin was published in 2023, and indicated just 48 individuals.
Mixing of subspecies In 2012, a genetic analysis of tissue samples from dolphins in the core Maui dolphin range, including historical samples, revealed the presence of at least three South Island Hector's dolphins off the West Coast of the North Island (two of them alive), along with another five South Island Hector's dolphins sampled between Wellington and Oakura from 1967 to 2012.