The Hector’s Dolphin (Cephalorhynchus hectori) has two recognized subspecies: C. h. hectori found primarily around the South Island and C. h. maui found only on the west coast North Island (Baker et al. 2002). The current convention is to refer to the South Island (nominate) subspecies as Hector’s Dolphin and the North Island subspecies as Māui Dolphin (even though the species common name is also Hector’s Dolphin).

The Māui Dolphin (Cephalorhynchus hectori maui), a subspecies of Hector’s Dolphin and sister taxon to C. h. hectori, is considered to be Critically Endangered  (CR A2cd,+4cd; C2a(i,ii); D) due to a reduction in population size of 81% over three generations (40.5 years; Slooten et al. 2006, Taylor et al. 2007, Hamner et al. 2014a, Baker et al. 2016, Cooke et al. 2019, MacKenzie 2020, Constantine et al. 2021), using the precautionary value of a 4% per year decline. As the population size is estimated at 54 individuals aged one year or older in 2020 based on genotype capture-recapture (95% CI 48–66; Constantine et al. 2021), almost certainly fewer than 50 sexually mature individuals remain (criterion D). The subspecies also meets criterion C2a(ii) for Critically Endangered, as the single population contains 100% of the mature individuals, and whilst the principal cause of the historical decline (mainly bycatch in fisheries; Martien et al. 1999, Dawson et al. 2001) has been reduced with further extensions of the Marine Mammal Sanctuary and fishing restrictions under the Fisheries Act (Department of Conservation and Fisheries New Zealand 2021), there may still be some fisheries-related risk to the subspecies at the extremities of its potential range, particularly in the south or from unknown winter distribution. For criterion A2cd, the most recent estimated rate of decline is 3–4% per year between 2001 and 2016 (Cooke et al. 2019). Under criterion A4, future projections for the decline or recovery of the Māui Dolphin vary depending on changes in the fisheries and non-fisheries (disease) risk (Cooke et al. 2019). Greater uncertainty around non-fisheries rated mortality suggests the population may continue to decline.

The Māui Dolphin is endemic to Aotearoa/New Zealand waters (Dawson 2018, Baker et al. 2002). It is restricted to the west coast of the North Island, primarily between Whaingaroa/Raglan to the south and the Kaipara Harbour to the north (Oremus et al. 2012, Constantine et al. 2021) with no contemporary genetic evidence of the subspecies' presence in South Island waters (Pichler et al. 2001). The range of the Māui Dolphin has undergone a marked reduction as noted in surveys and stranding records, with the majority of sightings restricted to a small part of the former range—a ~40 km stretch of coastline typically within a few kilometres of the coast between Manukau Harbour and Port Waikato (Dawson et al. 2001, Slooten et al. 2006, DuFresne 2010, Oremus et al. 2012, Constantine et al. 2021). Some historical sightings along the west coast of the North Island may also have included South Island Hector’s Dolphins (Cephalorhynchus hectori hectori, generally referred to in New Zealand as Hector’s Dolphins) ranging northward, as shown in recent years based on genetic identification (Pichler et al. 1998, Hamner et al. 2014b, Baker et al. 2016, Constantine et al. 2021). The morphological similarities between the two subspecies mean genetic identification is the only way to determine subspecies identity with confidence. Since March 2000, the southernmost distribution of Māui Dolphins, confirmed via genetic identification, was near Whaingaroa/Raglan. Live animals sighted recently south of the current core remnant range of Māui Dolphins have not been sampled to determine subspecies (sightings summarized in Roberts et al. 2019a).

Cephalorhynchus hectori maui is distinct from the Hector’s Dolphin (Pichler et al. 1998, Baker et al. 2002, Hamner et al. 2012, Constantine et al. 2021). Every sampled individual (n=137) over a 21-year period (2001–2021) has been identified as the Māui subspecies based on nuclear markers and a single mtDNA lineage (‘G’) that has not been detected in any South Island Hector’s Dolphins sampled. The Māui Dolphin also has subtle morphological and coloration differences from the Hector’s Dolphin (subspecies), and these differences, taken together with the genetic evidence, supported the designation of the North Island subpopulation of Hector’s Dolphin as a distinct subspecies (Baker et al. 2002).

Although the range of Māui Dolphins is currently limited to the west coast of the North Island, that range is not exclusive of Hector’s Dolphins. Since 2010, four living Hector’s Dolphins (C. h. hectori) have been identified based on genetic markers, in association with Māui Dolphins in the current core of the latter’s remnant range (see summary in Constantine et al. 2021). Two females, genetically assigned as originating from the west coast South Island subspecies of Hector’s Dolphin, were present off the west coast North Island in 2010 (Hamner et al. 2014b), one of which had also been sampled in 2011, 2015 and 2020 (Baker et al. 2016, Constantine et al. 2021). Neither of the two male Hector’s Dolphins, one sampled in 2015 and 2016 and one in 2021, could be assigned to a South Island subpopulation with confidence. This suggests that they migrated from an unsampled subpopulation of Hector’s Dolphins, possibly from the north of the South Island, or were the offspring of parents from different South Island subpopulations (most likely the east and west coast regions). Such subpopulation to subpopulation movements have been reported previously (Hamner et al. 2012). In addition, three dead Hector’s Dolphins have been found on the west coast North Island, one each from the Kapiti Coast northwest of Wellington, Ōpunake in southern Taranaki and Clarks Beach, Manukau Harbour (summary of recent living and dead Hector’s Dolphins found on the North Island can be found in Constantine et al. 2021). To date, there has been no evidence of interbreeding between the Māui and Hector’s subspecies, i.e., no individual has shown evidence of mixed subspecies ancestry in the comparison of mtDNA or population assignment, despite substantial sampling effort since 2010 (Baker et al. 2013; Hamner et al. 2014a,b; Baker et al. 2016, Constantine et al. 2021). However, interbreeding is believed to be possible and may be discovered in the future.

Based on the latest abundance estimate from 2020–2021 (Nc = 54, 95% CI 48–66), which included only individuals aged one year or older, fewer than 50 mature Māui Dolphins remain. This is consistent with a series of synoptic genotype capture-recapture estimates of abundance showing similarly high precision for 2010–2011 (Nc = 55, 95% CI 48–69; Hamner et al. 2014a) and for 2015–2016 (Nc = 63 95% CI 57–75; Baker et al. 2016). All three closed-population estimates apply to individuals that were alive in either of the two sample years, so they are likely to be biased upwards due to mortality. Cooke et al. (2018) used an open-population model to estimate an abundance of 57 (95% CI 44–75) Māui Dolphins in 2016, using the genotype data from Baker et al. (2016). The 2020–2021 genotype survey provided an effective population size (Ne) estimate of 35 (95% CI 21–67; Constantine et al. 2021). This was unchanged from the previous estimate for 2015–2016 (34, 95% CI 24–51; Baker et al. 2016), but lower than the estimates for 2010–2011 (68, 95% CI 34–293; Hamner et al. 2014a) and 2001–2007 (69, 95% CI 40–168; Baker et al. 2013). The smaller size of Ne relative to the abundance estimate (Nc) is consistent with the expectation that Ne only represents the individuals in the parental population that contributed successfully to offspring in the next generation. A reconstruction of the historical abundance of Māui Dolphins, prior to 1970 when monofilament setnets were introduced, suggests around 300 individuals, although this varies depending on assumptions about distance offshore and entanglement rates (MacKenzie 2020). The Mackenzie (2020) estimates are similar to previous estimates using different models (Martien et al. 1999, Burkhart and Slooten 2003, Davies et al. 2008).

An individual-based population model fitted to genotype capture-recapture of living and beach-cast or entangled dead Māui Dolphins estimated the population rate of decline as 3–4% per year between 2001 and 2016 (Cooke et al. 2019). This took into account the effects of mortality from fisheries bycatch (including data from Roberts et al. 2019a), unspecified anthropogenic threats and deaths attributable to toxoplasmosis (Roberts et al. 2019a, Roberts et al. 2021). With detailed genetic information from about 115 individual dolphins (Baker et al. 2013, Baker et al. 2016), this approach was suited to determining the estimated risk of different threats (Cooke et al. 2019).

Genotype capture-recapture has shown that some individuals are older than 20 years (Constantine et al. 2021), which is consistent with the revision of the maximum age of Hector’s Dolphins from 20 years (Slooten and Lad 1991) to at least mid-20s (Gormley et al. 2009). Individual home ranges are around 35 km in nearshore waters during summer but individuals can range up to around 80 km alongshore, although they may be distributed further offshore in winter (DeFresne 2010, Oremus et al. 2012, Constantine et al. 2021). To date there is no evidence of population sub-structure (whereas there is such structure in the much more abundant Hector’s Dolphin subspecies; Hamner et al. 2012).

The habits and biology of Māui Dolphins are similar to those of Hector's Dolphins around the South Island (recent reviews in Dawson 2018, Constantine 2019). Most of what is known about the distribution of Māui Dolphins is from the summer months when they come closer inshore and are often found in turbid waters (Derville et al. 2016, Roberts et al. 2019a). Whilst their current core remnant range is between Port Waikato and the Manukau Harbour, public reports are often received from just north of the Manukau Harbour and south off Whaingaroa/Raglan (summarized in Roberts et al. 2019a). Dedicated surveys frequently find small numbers of dolphins just south of the Kaipara Harbour entrance, approximately 75 km north of the Manukau Harbour, but these animals are not isolated and there is known movement between these locations (Oremus et al. 2013, Hamner et al. 2014a, Baker et al. 2016, Constantine et al. 2021).

Māui Dolphins have a broad diet similar to Hector’s Dolphins, eating a variety of small (typically <10 cm) benthic and pelagic species (Miller et al. 2013, Ogilvy et al. 2022). Productivity is considerably lower along the west coast North Island contemporary range of the subspecies, in comparison to the South Island (Roberts et al. 2019a). Determining indices of prey availability is important under climate change scenarios, as years with warmer temperatures may cause shifts in prey availability or preferences (Ogilvy et al. 2022), and levels of turbidity. With a reduction in abundance, social Allee effects may be constraining group size, distribution and sex-composition (Oremus et al. 2013), notably with a lack of female-dominated nursery groups as seen in Hector’s Dolphins (Webster et al. 2009). Climate change and social Allee effects may lead to further reduction in the core remnant range of Māui Dolphins; in 2020-2021 the majority of re-sightings were <10 km apart, even though dolphins were also seen near the Kaipara Harbour entrance to the north (Constantine et al. 2021).

The life history is considered similar for both subspecies with an estimated average age of maturity at 6–7 years old (Slooten and Lad 1991, Edwards et al. 2018) and one calf born every 2–3 years (Slooten and Lad 1991, Edwards et al. 2018). They have a generation length of 13.5 years (Taylor et al. 2007). The maximum lifespan of Māui Dolphins is over 20 years (Constantine et al. 2021) and it is possible that some individuals live more than 25 years as observed in Hector’s Dolphins (Gormley 2009).

Māui Dolphins face pressures from human activities. Entanglement in commercial and recreational gillnets was primarily responsible for the reduction in range and decline in abundance (Dawson et al. 2001). This threat was reduced with the introduction of the Marine Mammal Sanctuary in 2008 and subsequent area closures to fishing activity (Cook et al. 2019). Winter distribution of Māui Dolphins remains unknown, but the winter distribution of South Island the Hector’s Dolphin varies with sub-populations extending alongshore and/or further offshore than in summer (reviewed in Bräger and Bräger 2018, Constantine 2019). Hector’s Dolphins are also known to associate with trawlers and several instances of multiple dolphins dying in a single trawl have been recorded (Rayment and Webster 2009, see the DOC incident database for details www.doc.govt.nz/our-work/hectors-and-Māui-dolphin-incident-database). Therefore, uncertainty remains around the risk to Māui Dolphins from commercial trawl fishing offshore, which would be at the extremes of their range in summer, but unknown in winter. Therefore, this bycatch remains a potential threat for the subspecies.

The threat of diseases such as brucellosis (Buckle et al. 2017) and toxoplasmosis (a poorly understood condition caused by the cat-borne parasite Toxoplasma gondii; Roe et al. 2013, Roberts et al. 2021) is of concern. The limited availability of fresh carcasses (i.e. carcasses that are not in an advanced state of decomposition) and the lack of any health assessment of living dolphins creates uncertainty around the extent of the disease threat. However, Brucella has been isolated in Māui Dolphins and caused the death of adult and young individuals (Buckle et al. 2017). Prior to 2007, tests for T. gondii were not conducted, but since that time, the active disease toxoplasmosis has caused the death of two Māui Dolphins and a third individual was infected (Roberts et al. 2021). Both Hector’s Dolphin subspecies carry the disease, and a recent analysis of Māui and Hector’s Dolphin carcasses found that 61% of the animals for which a PCR test for the disease was possible, were infected with T. gondii (Roberts et al. 2021). Given that so few Māui Dolphin carcasses are recovered, and that some of these are in a condition where the cause of death cannot be determined, uncertainty remains around the extent of risk to the population. However, there is agreement that T. gondii should be considered an anthropogenic threat as it is spread by cats, an introduced taxon in New Zealand, and consequently actions need to be taken to manage this threat to the dolphins.

Cephalorhynchus hectori is listed in Appendix II of CITES. This implicitly covers both subspecies.

The Māui Dolphin is restricted to waters of Aotearoa/ New Zealand; therefore national conservation measures are mentioned here. The New Zealand Marine Mammals Protection Act 1978 (MMPA) prohibits the deliberate killing or injury of marine mammals. Bycatch in fishing gear is not illegal but can be regulated. The Māui Dolphin is listed as Nationally Critical under the New Zealand Threatened Species Listing (Baker et al. 2019).

The Potential Biological Removal (PBR) from the population, based on the 2010–2011 abundance estimate, was calculated as one dolphin every 10 to 23 years (Hamner et al. 2014a). More recently a spatially explicit fisheries risk assessment (SEFRA) approach has been used to estimate the Population Sustainability Threshold (PST), and to determine mortality risk (Roberts et al. 2019a). Restrictions on commercial and recreational fishing activity under the Fisheries Act (primarily due to government-imposed area closures) in the current range of the subspecies have reduced the risk of bycatch and are likely to have slowed the decline of the Māui Dolphin population (Cooke et al. 2019, Roberts et al. 2019b). However, considerable uncertainty remains around all threats, as well as around the dolphins’ winter and offshore distribution, and there are challenges with determining how to account for cryptic mortality (given an estimate that only around 10% of carcasses are recovered and reported; Cooke et al. 2019).

The West Coast North Island Marine Mammal Sanctuary was established in 2008 and expanded in 2020 by the Ministers of Conservation and Fisheries as part of the Threat Management Plan (TMP) Review (Department of Conservation and Fisheries New Zealand 2021). The revision of the TMP was triggered by the 2012 net entanglement of a dolphin by a commercial fisherman off Taranaki. As a tissue sample of the dolphin was not obtained due to the bycatch landing rules at the time, it is unknown whether it was a Māui or Hector’s Dolphin. However, Taranaki is well south of the current southern limit of the subspecies. Intensive onboard observer effort was undertaken in the Taranaki region where commercial fishing still occurred but yielded no sightings of either subspecies, despite occasional sightings reported by members of the public (summarized in Roberts et al. 2019a). The Marine Mammal Sanctuary now covers 20,574 km² of coastal ocean along the west coast of the North Island, managing seismic survey activity alongside the Fisheries Act which manages commercial and recreational fishing activity out to 22.2 km (see Department of Conservation and Fisheries New Zealand 2021 for details of the restrictions). In addition, a Research Strategy has been developed to improve the understanding of threats to both subspecies and to improve conservation outcomes (https://www.doc.govt.nz/our-work/protecting-species/protecting-marine-species/our-work-with-maui-dolphin/maui-dolphin-research/).

A Toxoplasmosis Action Plan (https://www.doc.govt.nz/nature/pests-and-threats/diseases/toxoplasmosis-and-hectors-and-maui-dolphin/toxoplasmosis-action-plan/) has been proposed to mitigate the risk of this disease to the species. Public engagement and further research will be needed to deal with the uncertainty around this threat and funding has been allocated to support the research. The impacts of climate change on prey availability and distribution, turbidity and ocean temperature will need to be considered in future management plans. However, the main impact will likely be on the distribution of Māui Dolphins’ preferred prey which probably shifts offshore, although it appears the dolphins may change their diet depending on prey availability (Ogilvy et al. 2022).