Sousa chinensis (eastern Taiwan Strait subpopulation)

The eastern Taiwan Strait (ETS) subpopulation of Indo-Pacific humpbacked dolphins was only recently discovered (Wang et al. 2004a).  Dolphins from this subpopulation have pigmentation that differs consistently from that of nearby subpopulations along the coast of mainland China (specifically those of western Taiwan Strait/Jiulong River Estuary (= Xiamen/Chinmen) and the Pearl River Estuary (=Hong Kong/Guangdong)) (Wang et al., in review, but also see Jefferson 2000, Jefferson and Hung 2004, Wang et al. 2007b). 

The primary range of this subpopulation consists of coastal western Taiwan from the estuaries of the Houlong and Jhonggang rivers (Miaoli County) in the north to Waishanding Zhou (a large sandbar off Chiayi County) in the south (see Figure 1 in the attached PDF).  However, one sighting of about 20 dolphins has been confirmed from the inshore waters of Tainan County and a dolphin, almost certainly a “stray,” 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 this species).  All sightings have been within 3 km off shore with the exception of the mud flats/littoral zone in the Changhua County, the central part of the distribution, where extensive oyster mariculture structures and associated activities likely exclude dolphins physically (Wang et al. 2007b).  The distribution is linear, i.e. similar to that of a riverine species.  Most of the dolphins in this subpopulation have been sighted in and around the two main estuaries of western Taiwan (Dadu and Joushuei rivers of Taichung, Changhua and Yunlin counties) (Wang et al. 2007a, b).

For further information about this species, see 133710.pdf.

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The subpopulation was estimated to number 99 individuals (CV=51.6%) in the mid-2000s (Wang et al. 2007a).  By analogy with the Pearl River Estuary subpopulation of S. chinensis, mature individuals constitute about 60% of this subpopulation (Jefferson 2000), or about 60. Using a default value of 50% percent for this species (Taylor et al. 2007), however, would suggest only about 50 mature individuals. Almost all individually recognizable dolphins were novel in 2002 but by 2004, most had been photographed in previous years (Wang et al. 2007a); the catalogue of recognizable dolphins numbered fewer than 30 at that time (J.Y. Wang pers. comm., December 2007).

The ETS dolphins appear to be year-round residents of the coastal waters of central western Taiwan where dedicated surveys have resulted in sightings from April to August (Wang et al. 2007a).  Opportunistic sightings have been made in other months; as of December 2007, the only months with no confirmed sightings were January, February and March, when conditions and opportunities for observations are poor (J.Y. Wang pers. comm., 13 December 2007).  In late winter and early spring, grey mullet (Mugil cephalus) fishermen report seeing humpback dolphins near their nets (trammel and gill nets that are commonly used as encircling nets as well).  Recreational shore fishermen report that the dolphins are seen most commonly in the winter months in the Dadu River estuary. Although reports by fishermen need to be viewed skeptically because of the possibility of misidentification, other species of dolphins are generally not present in the near-shore waters of western Taiwan so the chances of confusion are relatively small in this instance (Wang et al. 2007b). All sightings have been in waters less than 25 m deep, most in less than 15 m and within 3 km of shore.  The few measurements of sea surface temperatures at sightings have varied from about 24 to 30^oC (Wang et al. 2007a).

Schools of dolphins often patrol parallel to the coastline just off the surf zone and large sandbars.  Estuaries are likely where most of the foraging occurs.  Feeding behind active trawlers (as in Hong Kong and Australia) has not been observed but dolphins move along the length of set trammel or gill nets, possibly searching for injured or net-entangled fish.  In general, they appear to be indifferent towards boats (at least the research vessels that have been used to study them – a fishing boat and a large raft made of plastic tubing).

Indo-Pacific humpback dolphins appear to be opportunistic feeders.  They take a wide variety of nearshore, estuarine, and reef fishes.  They also eat cephalopods in some areas, although crustaceans appear to be rare in the diet (Jefferson and Karczmarski 2001, Ross 2002, Ross et al. 1994). Little is known about the specific feeding habits of the ETS subpopulation but these dolphins have been observed feeding on croakers (Sciaenidae), mullets (Mugilidae), threadfins (Polynemidae) and herring (Clupeidae) (Wang et al. 2007b).

Fisheries

This population 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 ETS subpopulation is probably incidental catches in fishing gear (Wang et al. 2007b).  There are anecdotal records of incidental catches in trammel and other kinds of gill nets and trawls.  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 subpopulation.

Habitat Degradation and Reduction

Reduction of freshwater flow and other kinds of degradation of estuaries and adjacent coastal waters (e.g. land reclamation) are almost certainly having an impact on this dolphin population, and there are continuing proposals for large-scale industrial development projects involving land reclamation (e.g., offshore wind farms, steel factory of the Formosa Plastic Group, Chinese Petroleum Company’s petrochemical factory within the animals’ restricted habitat) (Wang et al. 2004b, 2007b).  Besides the physical removal of habitat, activities associated with land reclamation, such as pile-driving, can cause disturbance or even direct harm to the dolphins.

Pollution

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.

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 ETS humpback dolphins live in the coastal and estuarine waters of western Taiwan where there is minimal treatment for residential 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. In January 2008, an international group of 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.

Sousa spp. are listed in Appendix I of CITES.

Efforts are being made to characterize this dolphin population and the threats it faces, and to integrate relevant information into Taiwan’s environmental impact assessment and mitigation processes (Wang et al. 2007b).