Hector's dolphin

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. 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

thumb|250px|left|Hector's dolphin has a unique rounded dorsal fin.

Hector's dolphin is the smallest dolphin species. Mature adults have a total length of and weigh . The species is sexually dimorphic, with females being about 5–7% longer than males. The throat and belly are white, while the rest of the dolphin is grey. 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. 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%.

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.

Ecology

Habitat

The species' range includes murky coastal waters out to depth, though almost all sightings are in waters shallower than . 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. 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). Unconfirmed predators of Hector's dolphins include orca (Orcinus orca), mako sharks (Isurus) and copper sharks (Carcharhinus brachyurus).

Behaviour

thumb|250px|left|[[Cetacean surfacing behaviour|Group jumping]]

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,

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. 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.

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). 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). Māui dolphin are typically found on the west coast of the North Island between Maunganui Bluff and Whanganui.

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). 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.

Previously, the deep waters of the Cook Strait were considered to be an effective barrier to mixing between the South Island Hector's and North Island Māui sub-species for around 15,000 and 16,000 years. This is coeval with the separation of the North and South Islands of New Zealand at the end of the last ice age. To date, there is no evidence of interbreeding between South Island Hector's dolphin and Māui dolphin, but it is likely they could given their close genetic composition.

Threats

thumb|250px|right|Skeleton in the collection of the [[Pisa Charterhouse]]

thumb|In [[Otago, New Zealand]]

Fishing

Hector's and Māui dolphin deaths occur as a direct result of commercial and recreational fishing due to entanglement or capture in gillnets or trawls. The species' reliance on shallow areas for foraging brings it in frequent contact with intensive fishing operations. Death is ultimately caused by suffocation, although injury and sub-lethal effects can also result from the mechanical abrasion of fins resulting from entanglement. Since the 1970s, gillnets have been made from lightweight monofilament, which is difficult for dolphins to detect. Hector's dolphins are actively attracted to trawling vessels and can frequently be seen following trawlers and diving down to the net, which could result in the unwanted bycatch.

Deaths in fishing nets were previously considered to be the most serious threat (responsible for more than 95% of the human-caused deaths in Māui dolphins), with currently lower level threats including tourism, disease, and marine mining. Research of decreases in mitochondrial DNA diversity among hector's dolphin populations has suggested that the number of gill-net entanglement deaths likely far surpasses that reported by fisheries.

The latest government-approved estimates of annual deaths in commercial gillnets (for the period from 2014/15 to 2016/17) was 19–93 South Island Hector's dolphins and 0.0–0.3 Māui dolphins annually. Five marine mammal sanctuaries were designated in 2008 to manage nonfishing-related threats to Hector's and Māui dolphins. Their regulations include restrictions on mining and seismic acoustic surveys. Further restrictions were introduced into Taranaki waters in 2012 and 2013 to protect Māui dolphins.

The Banks Peninsula Marine Mammal Sanctuary was expanded in 2020, with restrictions introduced on seismic surveying and seabed mining. The sanctuary stretches from the Jed River south to the Waitaki River, and extends 20 nautical miles out to sea, a total area of about 14,310 km<sup>2</sup>.

The Scientific Committee of the International Whaling Commission has recommended extending protection for Māui dolphin further south to Whanganui and further offshore to 20 nautical miles from the coastline. The IUCN has recommended protecting Hector's and Māui dolphins from gill-net and trawl fisheries, from the shoreline to the 100 m depth contour. Sustainable fishing practices could also reduce the impacts of fishing on Hector's dolphins and other species.

Infectious diseases

The unicellular parasiteToxoplasma gondii is considered to be the main non-fishery cause of death. A 2013 study found that seven of 28 beachcast or bycaught Hector's and Māui dolphins died as a result of toxoplasmosis, which had necrotising and haemorrhagic lesions in the lung (n = 7), lymph nodes (n = 6), liver (n = 4) and adrenals (n = 3). The same study found that approximately two-thirds of dolphins had previously been infected with the toxoplasma parasite. An update to this study found that toxoplasmosis had killed nine out of 38 post-weaning age Hector's and Māui dolphins found washed up or floating at-sea, and that were not too autolised to determine a cause of death. Of these nine, six were reproductive females, tentatively indicating that this demographic may be more susceptible to infection. Brucellosis has been determined from necropsies to have killed both Hector's and Māui dolphins and to have caused reproductive disease, indicating that it may affect the reproductive success of both sub-species.

Potential interbreeding between Hector's and Māui dolphins could increase the numbers of dolphins in the Māui range and reduce the risk of inbreeding depression, but such interbreeding could eventually result in a hybridisation of the Māui back into the Hector's species and lead to a reclassification of Māui as again the North Island Hector's. Hybridisation in this manner threatens the Otago black stilt and the Chatham Islands' Forbes parakeet and has eliminated the South Island brown teal as a subspecies. Researchers have also identified potential interbreeding as threatening the Māui with hybrid breakdown and outbreeding depression.

Additional threats to Hector's dolphins include oil spills, gas drilling, and noise pollution.

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External links

Specimen MNZ MM001915, collected Kaikoura, New Zealand, no date data
NZ Dept. of Conservation – Hector's dolphin information
NABU International www.hectorsdolphins.com
Three decades on the tail of Hector's dolphins