Sousa chinensis ssp. taiwanensis 

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Mammalia Cetartiodactyla Delphinidae

Scientific Name: Sousa chinensis ssp. taiwanensis (Osbeck, 1765)
Parent Species:
Common Name(s):
English Taiwanese Humpback Dolphin, Taiwanese White Dolphin
Sousa chinensis (Osbeck, 1765) (eastern Taiwan Strait subpopulation)
Taxonomic Source(s): Wang, J.Y., Yang, S.C. and Hung, S.K. 2015. Diagnosability and description of a new subspecies of Indo-Pacific humpback dolphin, Sousa chinensis (Osbeck, 1765) from the Taiwan Strait. Zoological Studies 54(1): 36.
Taxonomic Notes:

The Taiwanese Humpback Dolphin was discovered in 2002 (Wang et al. 2004a). It was initially considered to be an isolated, distinct and endemic subpopulation of Indo-Pacific Humpback Dolphins (Sousa chinensis) based on subtle but clear pigmentation differences compared to neighbouring provisional subpopulations along the coast of mainland China, specifically those of the western Taiwan Strait/Jiulong River Estuary (=Xiamen/Chinmen) and the Pearl River Estuary (=Hong Kong/Guangdong) (see Wang et al. 2008). The pigmentation differences of the Taiwanese Humpback Dolphin was later demonstrated to be diagnosable from the other provisional subpopulations above, which led to subspecies recognition for these dolphins, and was named Sousa chinensis taiwanensis (Wang et al. 2015); a more common, locally-used name for this subspecies is Taiwanese White Dolphin. Compared to humpback dolphins in the waters of Hong Kong, the Taiwanese subspecies also exhibit clear differences in social organization and structure (see Dungan et al. 2012, 2016).

Assessment Information [top]

Red List Category & Criteria: Critically Endangered C2a(i,ii); D ver 3.1
Year Published: 2018
Date Assessed: 2017-01-10
Assessor(s): Wang, J.Y. , Araujo-Wang, C.
Reviewer(s): Taylor, B.L. & Reeves, R.
Contributor(s): Perrin, W., Braulik, G.T., Dalebout, M., Jefferson, T.A., Karczmarski, L., Laidre, K., O’Corry-Crowe, G., Rojas-Bracho, L., Secchi, E., Slooten, E., Smith, B.D. & Zhou, K.
The total population (all ages) was estimated at about 100 individuals in the mid-2000s. More recent mark-recapture analysis of photo-identification data collected from 2007 to 2010 resulted in the largest annual estimate being 74 individuals in 2010 (CV=4%) (Wang et al. 2012). The extent of occurrence (EOO) is only a small stretch of coastal waters off western Taiwan (estimated to be ca 750 km²) with the core area being about 515 km² (Wang et al. 2007a, 2016). Given the number of development projects that are underway or proposed, and the fact that only minimal or no conservation measures are in place to reduce the probable impacts of the various threats (i.e., bycatch in net fisheries, severe reduction of freshwater flow to estuaries, land reclamation, noise and chemical pollution; see Wang et al. 2007b, Ross et al. 2010, Dungan et al. 2011), a continuing decline in the subspecies’ abundance is projected. Although there is no prospect of obtaining a long enough time series of data to show a decline over the last three generations (about 60 years; see Taylor et al. 2007), a decline almost certainly has occurred (at least since the beginning of Taiwan’s rapid industrialization about 30-40 years ago) and there is no reason to believe that the causes have stopped, or even slowed as recent PVA studies have shown (Araújo et al. 2014, Huang et al. 2014). Therefore, it is reasonable to project a continuing decline so this subspecies meets criterion C2a(i,ii) for Critically Endangered (total of fewer than 250 mature individuals, projected continuing decline, and at least 90% of mature individuals in a single subpopulation with fewer than 50 mature individuals in each subpopulation). With the most recent annual abundance estimates, this subspecies also meets criterion D for CR because the total number of mature individuals is fewer than 50 (regardless of the value used to estimate percent mature: 60% from Jefferson (2000) or 50% from Taylor et al. (2007)).
Previously published Red List assessments:

Geographic Range [top]

Range Description:

The primary range of this subspecies consists of the coastal waters of western Taiwan (=eastern Taiwan Strait) from the estuaries of the Houlong and Jhonggang rivers (Miaoli County) in the north to Jiangjyun Harbour (Tainan City) in the south (see Figure 1 in the Supplementary Material). All sightings along western Taiwan have been within 3 km of shore with the exception of the intertidal/littoral waters of Changhua County (the central part of the distribution) where extensive oyster mariculture structures and associated activities have likely displaced dolphins physically (Wang et al. 2007a). The distribution is linear (i.e., similar to that of a riverine species). Most of the dolphins in this subspecies have been sighted in and around major estuaries of western Taiwan such as the Dadu River (Taichung City/Changhua County) (Wang et al. 2007a,b; Ross et al. 2010, 2012; Dares et al. 2014). Based on similar biophysical features, Ross et al. (2010) suggested that suitable habitat for this subspecies may extend from the Tsengwen River estuary of Tainan City to the waters influenced by the Danshui River estuary (the latter drains greater Taipei City and adjacent areas) and considered this entire region to be “priority habitat’ for this subspecies. Although a single dolphin was observed at the mouth of Fugang Harbour (Taitung County) where adjacent waters are deep and oceanic (i.e., clearly not the preferred habitat of any humpback dolphins), there is little doubt that it was a vagrant; this individual was observed on one day and was never seen again.

For further information about this species, see 133710_Sousa_chinensis_taiwanensis.pdf.
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Countries occurrence:
Taiwan, Province of China
FAO Marine Fishing Areas:
Pacific – northwest
Additional data:
Estimated area of occupancy (AOO) - km2:750Continuing decline in area of occupancy (AOO):Yes
Estimated extent of occurrence (EOO) - km2:
Continuing decline in extent of occurrence (EOO):Yes
Lower depth limit (metres):30
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Initial abundance of this subspecies was estimated to be 99 individuals (CV=51.6%) in the mid-2000s using line-transects (Wang et al. 2007a). However, recent estimates of total population using mark-recapture analysis of photo-identification data (Wang et al. 2012) resulted in a highest annual point estimate for the total population of 74 individuals with much higher precision (CV=4%). Using the proportion of mature individuals reported for the Pearl River Estuary subpopulation of 60% of the population (Jefferson 2000), there would be 44 mature individuals of Taiwanese Humpback Dolphins while using a default value of 50% mature individuals for this species presented by Taylor et al. (2007), there would only be only 37 mature individuals. With the exception of young animals that are newly recruited into the marked population, annual photo-identification monitoring of the subspecies has resulted in no new dolphins being added to a catalogue of recognizable individuals since 2010 (Wang et al. 2012) and this remains the same as of 2016 (J.Y. Wang unpub. data).

Population viability analysis (PVA) showed a high risk of population decline based on current levels of threats (Araújo et al. 2014, Huang et al. 2014). Although these two PVA studies were inconsistent with regards to the relative risks of different single threats, both demonstrated the same critical situation for the Taiwanese subspecies. Furthermore, the PBR (Potential Biological Removal) value of one individual every 7-7.6 years (see Slooten et al. 2013) indicated that even minimal additional human-caused mortality poses a great risk to the continued existence of this subspecies. The probability of extinction is disproportionately increased if adult females are removed (Araújo et al. 2014).
Current Population Trend:Decreasing
Additional data:
Number of mature individuals:37-44Continuing decline of mature individuals:Yes
Population severely fragmented:Yes
Extreme fluctuations in subpopulations:YesAll individuals in one subpopulation:Yes

Habitat and Ecology [top]

Habitat and Ecology:The Taiwanese dolphins appear to be year-round residents of the coastal waters of central western Taiwan (Wang and Yang 2011). Most of the sightings have been seen within 3 km of shore and in waters less than 20 m deep, but some have been seen crossing deeper (>30m), dredged, shipping channels leading to/from the ports of Taichung Harbour and Mailiao Industrial Park (Dares et al. 2014).

Schools of dolphins often patrol parallel to the coastline just off the surf zone and large sandbars (or over sandbars when tides permitted; the tidal flux in the region can be up to 6m). Estuaries are likely where most of the foraging occurs. Feeding behind active trawlers (as in Hong Kong and Australia) was not observed before about 2010. Since then, the frequency of observing both illegal trawling within dolphin habitat and dolphins following or being attracted to trawlers has increased (J.Y. Wang, unpub. data). Dolphins have also been observed to move along the length of set and drifting trammel or gill nets, possibly searching for injured or net-entangled fish. In general, they appear to be indifferent towards boats (at least to the research vessel reported by Wang et al. 2012) but tend to move away from larger or fast moving boats.

Indo-Pacific Humpback Dolphins appear to be opportunistic feeders. They take a wide variety of nearshore, estuarine, and reef fishes. They may also eat cephalopods and crustaceans in some areas but such prey appear to be rare (Jefferson and Karczmarski 2001, Ross 2002, Ross et al. 1994). Little is known about the specific feeding habits of the Taiwanese subspecies but these dolphins have been observed feeding on croakers (Sciaenidae), mullets (Mugilidae), threadfins (Polynemidae), herring (Clupeidae) and congrid eel (Wang et al. 2007b, J. Y. Wang unpub. data).
For further information about this species, see 133710_Sousa_chinensis_taiwanensis.pdf.
A PDF viewer such as Adobe Reader is required.
Systems:Freshwater; Marine
Continuing decline in area, extent and/or quality of habitat:Yes

Use and Trade [top]

Use and Trade:

There is no information about direct usage of this subspecies in Taiwan. However, given that dolphins were consumed in large numbers in the past and there were no restrictions in hunting of cetaceans until the early 1990s, it is reasonable to assume that the subspecies was also taken intentionally for food. Takes would have been by harpoon or nets. An illegal market for dolphin meat for food still exists in Taiwan (Chang et al. 2014) but because of the high-profile conservation status of the Taiwanese Humpback Dolphin, fishermen are unlikely to catch this subspecies intentionally. However, bycaught individuals (especially grey young individuals, which may be mistaken for other dolphin species) may still be sold and consumed.

Threats [top]

Major Threat(s): Five major categories of threats to this subspecies have been recognized: fisheries interactions, habitat loss and degradation, reduction of freshwater flow to estuaries, air and water pollution and noise disturbance (see Wang et al. 2007b, Ross et al. 2010, Dungan et al. 2011).

This subspecies is not known to be hunted presently but is likely to have been hunted at least opportunistically in the past. Entanglements of humpback dolphins in gillnets have been recorded in coastal waters of the Indian Ocean (Ross et al. 1994, Jefferson and Karczmarski 2001). The greatest direct and immediate source of human-caused dolphin mortality for the Taiwanese subspecies is almost certainly incidental catches in fishing gear (Wang et al. 2007b, Slooten et al. 2013, Araújo et al. 2014). There are anecdotal records of incidental catches in trammel and other kinds of gill nets and trawls. Of the four known stranded individuals of this subspecies, two (one in 2009 and one in 2014) were almost certainly killed by net entanglement and a very large proportion of the known (photographically-identified) individuals bear serious scars, many of which have been attributed to fishing gear and shown to be unsustainable (Wang et al. 2017). A few known dolphins have also been observed and photographed carrying or seriously mutilated by fishing gear (Slooten et al. 2013, Wang et al. 2017, Wang and Araujo-Wang 2017). Thousands of trammel and gill nets operate in the coastal waters of western Taiwan and this clearly represents one of the most serious threats to this subspecies (Slooten et al. 2013, Araújo et al. 2014).

Habitat Degradation and Reduction (including loss of freshwater to estuaries)
Reduction of freshwater flow and other kinds of degradation of estuaries and adjacent coastal waters (e.g., land reclamation) area also impacting this subspecies, and there are continuing proposals for large-scale industrial development projects involving land reclamation (Wang et al. 2004b, 2007b, 2016). Besides the physical removal of habitat, activities associated with land reclamation, such as pile-driving, can cause major disturbance or even direct harm to the dolphins (see below).

Pollution (industrial, agricultural and residential discharge with minimal to no treatment) poses a risk to humpback dolphins via the consumption of marine prey species (Clarke et al. 2000, Parsons 2004). Spills of oil and other toxic substances by commercial ships could be catastrophic for a population so small and limited in its distribution. As with all mammals, this subspecies would also be at risk to air-borne pollutants that harm terrestrial mammals, including the local human population. Air quality is particularly poor in the coastal regions of central Taiwan where heavy industries (especially those that burn low quality fuels such as soft coal and petroleum coke) are abundant. The overlap of the effluents/discharges from heavy industries, agriculture, human residential areas, electronics manufacturing and dolphin habitat is great and very concerning. In a modelling study on the contaminant loads of Taiwanese Humpback Dolphin, 68% of the total population (including all males and first offspring) was predicted to have PCB loads above the threshold limit for immunotoxicity of 17 mg/kg LW (Riehl 2011).

Parsons (1997) estimated that a humpback dolphin in Hong Kong waters ingests up to 70,500 faecal coliforms/day (minimum) from sewage-contaminated seawater. To put this in context, a one-off ingestion rate of 200-300 coliforms is considered unacceptable for humans (Parsons 2004). Hong Kong discharges over two billion litres of sewage into its coastal waters every day. The Taiwanese Humpback Dolphin live in the coastal and estuarine waters of western Taiwan where there is minimal treatment for residential, industrial, agriculture and other sewage. Therefore, as in Hong Kong, a high rate of ingestion of contaminants of many kinds is to be expected, with associated concerns for the health of dolphins living in the coastal and estuarine waters of western Taiwan. Furthermore, Yang et al. (2013) reported various epidermal lesions on 37% of photographically-identified individuals in this subspecies, which is suggestive that some level of immune deficiency may already be occurring.

Anthropogenic sources of noise have also been raised as a concern for the Taiwanese dolphins. Even though not likely at the levels of noise found in the waters of Hong Kong, the impacts and disturbance of noise on this small population may be quite substantial. In a drive for green energy initiatives in Taiwan, a proposal to erect a massive ‘offshore’ wind farm (~1,000 units) in and adjacent to the habitat of the Taiwanese Humpback Dolphins is moving forward rapidly. Without adequate mitigation of the potentially damaging construction noise (especially driving piles), the impacts on this declining population can be devastating. In addition, the recent permitting of boat-based tourism targeting the Taiwanese Humpback Dolphins by the local government (an action that was against clear, strong recommendations against allowing such activities by an international panel of expert scientists - see below) is a new threat that adds yet another stressor on these dolphins. Not only does this activity increase noise disturbance but also the risk of vessel collision for the dolphins.

Conservation Actions [top]

Conservation Actions: Sousa spp. are listed in Appendix I of CITES. Sousa spp. are also listed in the highest category of protection under Taiwan’s Wildlife Conservation Act.

Efforts are being made to characterize this subspecies and the threats it faces, and to integrate relevant information into Taiwan’s environmental impact assessment and mitigation processes (Wang et al. 2007b). In 2011, the massive Kuokuang petrochemical plant that was proposed for the waters in the centre of the Taiwanese Humpback Dolphin habitat was halted due to growing concerns about this project’s large impact on these dolphins, which would have severely compromised their future existence. In 2014, Major Wildlife Habitat (covering ~80% of the confirmed habitat of this subspecies) was proposed but implementation has been severely delayed (In Taiwan’s conservation act, Major Wildlife Habitat represents areas of importance to certain species and designation carries restrictions to reduce the impacts of human activities). The cancellation of the petrochemical plant and the proposed Major Wildlife Habitat are clearly positive steps. However, there has been no progress on the reduction of any of the existing major threats that were identified a decade ago. Currently, there is a push by the local government for “green” energy, which includes the erection of 500-800 offshore wind turbines to achieve 4,000 MW generating capability. Some of the sites for these turbines are within the habitat of the Taiwanese Humpback Dolphins so there is great concern about the impacts of construction and physical displacement from the limited remaining habitat.

In January 2008, an international group of cetacean and other marine scientists making up the Eastern Taiwan Strait Sousa Technical Advisory Working Group was established to provide expert advice and guidance on the conservation of the ETS population (now known as the Taiwanese subspecies).

The National Marine Fisheries Service (USA) is currently reviewing a petition (received on 9 March 2016) to list this subspecies as threatened or endangered under the US Endangered Species Act.

Amended [top]

Amended reason: This amended version of the 2017 assessment was created to correct the Assessor and Contributor fields.

Citation: Wang, J.Y. , Araujo-Wang, C. 2018. Sousa chinensis ssp. taiwanensis (amended version of 2017 assessment). The IUCN Red List of Threatened Species 2018: e.T133710A122515524. . Downloaded on 14 August 2018.
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