Sousa chinensis (Osbeck, 1765)

English: Indo-Pacific-humpback dolphin, Chinese white dolphin
German: Chinesischer Weißer Delphin
Spanish: Delfín blanco de China
French: Dauphin blanc de Chine

Family Delphinidae


Sousa chinensis © Würtz-Artescienza (see "links")


1. Description

Humpback dolphins are medium sized and robust. Their melon is slightly depressed and slopes gradually to an indistinct junction with the long, narrow beak. The broad flippers are rounded at the tip and the flukes are broad and full, with a deep median caudal notch. The form of the dorsal fin varies geographically. Body length reaches 2.5-2.8m in different parts of the range. In South Africa, males may reach 2.7m and 260kg as opposed to the smaller females which only attain 2.4m and 170kg. Colour also varies greatly with age and location, in both the timing and extent in the loss of the grey background colour to white (pink when flushed; Ross, 2002).

For the first edition of this review, I followed Rice (1998) who separated the genus Sousa into three species: S. chinensis (eastern Indo-Pacific), S. plumbea (western Indo-Pacific) and S. teuszii (eastern Atlantic). However, a recent morphological study based on 222 dolphin skulls showed that only the distinctness of S. teuszii is clearcut (Jefferson and VanWaerebeek, 2004). Subsequent genetic analyses confirmed that all Indo-Pacific populations formed a robust, monophyletic clade (Frere et al. 2008), settling the dispute with respect to S. chinensis and S. plumbea, which seem to be a single species for which the name S. chinensis has priority (Ross, 2002).

Surprisingly, however, humpback dolphins from South Africa and China form a strongly-supported clade with the Atlantic S. teuszii, to the exclusion of animals from Australia. This results strongly suggest that Australian humpback dolphins are not S. chinensis but may represent a distinct species in their own right (Frere et al. 2008). back to the top of the page


2. Distribution

The Chinese white dolphin is discontinuously distributed in coastal waters of the Indian and Pacific Oceans. Known areas of occurrence from west to east are False Bay (18°30'E) in Cape Province, north along the coast of eastern Africa, including Madagascar, to the Red Sea as far north as Gulf of Suez, the Arabian Sea, and the Persian Gulf, thence east along the coasts of southern Asia at least as far as Vishakhapatam on the western Bay of Bengal. It is vagrant in the Ganges River 250 km from the sea and has also strayed into the Mediterranean Sea via the man-made Suez Canal (Rice, 1998). It occurs from the Bay of Bengal east to the coast of southern China, including Taiwan, from the Gulf of Tonkin to Jiangsu, entering the lower reaches of the Zhu Jiang (=-Canton River), the Jiulong Jiang (=-Amoy River), and the Mim Jiang (=-Foochow River), and ascending 1,200 km up the Chang Jiang (=-Yangtse River) as far as Wuhanthe Gulf of Thailand; the Strait of Malacca; the northwestern coast of Borneo from Sematan in Sarawak to Sandakan in Sabah (Rice, 1998). Following Rice (1998), this species account includes S. borneensis (Lydekker, 1901) and S. lentiginosa (Gray, 1866).

Distribution of Sousa chinensis: shallow coastal waters of the Indo-Pacific Ocean
(Reeves et al. 2008; © IUCN; enlarge map).

Finally, the distribution also includes the northwestern coast of Western Australia between North West Cape and Larrey Point; and the coast of eastern Australia from Cairns in Queenland to Wollonggong in New South Wales (Rice, 1998), but this may be another species or subspecies (see above).back to the top of the page


3. Population size

Recent information on population size is limited and comes from a few restricted locations. These reports present a picture of low if not declining population numbers everywhere. The sparse data available for selected areas indicate that humpback dolphins occur in discrete, geographically localized populations and are susceptible to anthropogenic threats. The summary spans the distributional area from West to East:

Eastern Africa: The minimum population size at Algoa Bay on the south Eastern Cape coast of South Africa was about 466 dolphins (Karczmarski et al. 1999).
Off the KwaZulu-Natal coast (South Africa) there is a total of perhaps 200 animals (Jefferson and Karczmarski, 2001), which was confirmed by Keith et al (2002) who reported a minimum of 181 individuals at Richards Bay.

In the Bazaruto Archipelago in Mozambique between the mainland and the Bazaruto islands, a 1992 survey counted 60 animals.
A mark-recapture analysis conducted between 1995 and 1997 in Maputo Bay suggested a population size of approximately 105 dolphins, but the precision of the estimate is low (30.5-150.9) (Guissamulo and Cockroft, 2004).

Around Mayotte in the northern Mozambique Channel, Kiszka et al (2007) observed 44 humpback dolphins.

Approximately 65 humpback dolphins were observed in five groups (mean group size of 13) off Anakao, Madagascar, during boat-based surveys conducted in 1999. These surveys and other sources of information indicate that humpback dolphins may largely be
restricted to the west coast (Razafindrakoto et a. 2004).
Off the south coast of Zanzibar, mark-recapture methods were used to estimate a population size of 71 (95% CI 48-94) humpback dolphins in a 26 km² study area in 2001 (Stensland 2004).

Persian Gulf: Estimates of cetacean abundance in the UAE differed significantly between 1986 and 1999 and indicate a population decline of 71%. In the region between Oman and Kuwait, the Indo-Pacific humpback dolphin is the second most commonly observed cetacean at sea (27%) (Preen, 2004).
In the immediate vicinity of Kuwait's Boubyan Island, a combined total of 524 individuals were estimated in 2004-2005 (Bishop and Alsaffar, 2008).

Indian-Humpback dolphin, plumbea-type, photographed off Giftun, Egypt, Red Sea,
Nov. 2009 © Rasmus Nielsen, Aarhus, DK

Indian Ocean: A rough population estimate for the Indus delta was 500 animals (Ross et al. 1994 and refs. therein). In nearshore waters of Bangladesh, 6 humpback dolphins were observed in a 2004 survey (Smith et al. 2008). Off the Mergui (Myeik) Archipelago of southern Myanmar 3 individuals were identified at sea (Smith and Tun, 2008)..

Chinese waters: There is only limited knowledge about distribution in the coastal waters of southern China, including Fujian, Taiwan and Guangxi provinces, where 5 important populations of S. chinensis can be identified, but populations appear to be in decline (Zhang and Tang, 2008).

In Hong Kong waters sightings occurred in all of the waters surrounding Lantau Island but were most common in the North Lantau area. Estimates in 1995-97 ranged from 88 dolphins in spring to 155 dolphins in autumn, with a year-round average of 109. Mark-recapture estimates of abundance suggest that between 208 and 246 different animals use the Hong Kong area (Jefferson and Leatherwood, 1997). Jefferson and Karczmarski (2001, and refs. there-in) concluded that >1,028 animals occur in Hong Kong waters and the adjacent Pearl River Estuary.

A 2005 preliminary estimate of the size of the Leizhou population is about 237 individuals, second only to the Pearl River estuary population, suggesting that Leizhou Bay has the potential to serve as a "humpback dolphin sanctuary" in Chinese waters (Zhou et al. 2007).

Liu and Huang (2000) recorded 392 individuals in Xiamen waters, with a negative population trend.
Off central western Taiwan there are few and sporadic records. Several groups of Indo-Pacific humpback dolphins were sighted in 2002 and overall 28 individuals observed (Wang et al. 2004)

Australia: Estimates for Moreton Bay in 1984-1986, and 1985-1987, respectively, were163 animals (95% confidence intervals 108-251), and 119 animals (95% confidence intervals 81-166); preliminary results for Cleveland Bay, in the Central Section of the Great
Barrier Reef, suggest a population less than 200 animals (Ross, 2006; Parra et al. 2004).back to the top of the page


4. Biology and Behaviour

Habitat: S. chinensis is rarely found more than a few kilometres from shore, preferring coasts with mangrove swamps, lagoons, and estuaries and areas with reefs, sandbanks, and mudbanks. Animals sometimes enter rivers, though rarely more than a few kilometres upstream and usually within the tidal range (Carwardine, 1995). They prefer water less than 25 m deep and, on more open coasts, are typically found in the surf zone (Ross, 2002).

In Algoa Bay, South Africa, no apparent preference for clear or turbid water was observed, water depth probably being the main factor limiting their inshore distribution; the 25-m isobath seems to represent the critical depth. Within this confined, inshore distribution, dolphin activities concentrate in the vicinity of rocky reefs-their primary feeding grounds and "key habitat" (Karczmarksi et al. 2000). These habitat preferences were confirmed by aerial surveys of the Great Barrier Reef region, which showed that humpbacked dolphins occur mostly in waters close to the coast, although they also occur in offshore waters that are relatively sheltered, and close to reefs or islands (Corkeron et al. 1997).

Photo: Lindsay Porter/WWF

Behaviour: The species is usually quite difficult to approach and tends to avoid boats by diving and reappearing some distance away in a different direction. They rarely permit a close approach before diving, splitting up into small groups or single animals (Carwardine,1995; Ross et al.1994).

Schooling: Humpback dolphins form small schools throughout their distribution, ranging from one to about 25 dolphins off South Africa and the northern Indian Ocean (Ross et al. 1994 and references therein). In Maputo Bay, Mozambique, estimated group size was 14.9 individuals and was the largest reported for the eastern Africa region. There was no change with month, season, daylight, or tidal state (Guissamulo and Cockroft, 2004). Off Kuwait, however, group size in 159 sightings was low; 40% of sightings were comprised of a single individual and 26% consisted of pairs. Pods of 10 or more members were observed on 13 occasions (Bishop and Alsaffar, 2008).

Off southern China, schools usually contain three to five animals. In Moreton Bay, Queensland, mean group size was 2.4 animals (range 1-9, n =9). S. chinensis associates with bottle-nose dolphins and, to a lesser extent, with finless porpoises and spinner dolphins (Ross et al. 1994).

Reproduction: Some calves may be born throughout the year, but spring or summer calving peaks are the norm. Gestation lasts 10-12 months, and age at sexual maturity is 10 years in females and 12-13 years in males (Jefferson and Karczmarski, 2001. Some females may cycle outside of the apparent summer breeding season, perhaps indicating a secondary winter season. Circumstantial evidence suggests a minimum of a 3-year calving interval. Maternal care lasts at least 3-4 years, but female-calf separation is seemingly not related to the female's next pregnancy (Karczmarski, 1999).

Food: According to Ross (2002), food consists mainly of fish and cephalopods, dolphins temporarily beaching to retrieve bonefish washed onto exposed sandbanks. Fish species comprise sardines, mackerel, mullet and other near-shore fishes. Off southern Africa humpback dolphins seem to feed on or close to reefs along rocky coastal areas in preference to areas with sandy bottoms (Reyes, 1991 and refs. therein; Ross et al. 1994 and refs. therein).

All 503 prey items in the stomachs of 17 dolphins captured in shark nets off Natal, South Africa, were fish. Numerically, the major prey species were Thryssa vitrirostris (46.4%), Trickiurus lepturus (9.2%), Pomadasys olivaceum (8.6%), Otolithes ruber (7.2%), and Diplodus sargus (3.6%). The remaining 24% comprised a further 28 prey species. Nearly 61% of all fish were littoral or estuarine species, and a further 25% were demersal species primarily associated with reefs (Ross et al. 1994 and references therein).

Humpback dolphins in China feed on several species of demersal and estuarine fishes, with little evidence of predation on cephalopods or crustaceans (Jefferson and Hung, 2004). Stomachs of two dolphins netted off the northern Queensland coast contained fish remains, and in one case, some crustacean fragments. In Moreton Bay, southeastern Australia, humpback dolphins feed with bottlenose dolphins on trawl discards (Ross et al. 1994 and refs. therein). back to the top of the page


5. Migration

Indo-Pacific Sousa are not known to be migratory (Ross 2002), although numbers of animals increase seasonally in South Africa. Some seasonal inshore-offshore and longshore movements are recorded for West African Sousa and these most likely cross international boundaries. In Richards Bay, South Africa, photo-identification suggests that some humpback dolphins display long-distance movement patterns (up to 150 km), while other individuals display long-term residency within the area (Keith et al. 2002).

A high level of seasonal immigration of humpback dolphins into, and emigration from, the Algoa Bay region in summer has been reported (Karczmarski 1999, 2000). There is evidence for summer influxes of humpback dolphins into eastern Maputo Bay, and there are considerable numbers of apparently transient individuals. A substantial proportion of humpback dolphins (13.5%) display high site fidelity to eastern Maputo Bay and could be long-term residents (Guissamulo and Cockroft, 2004). Migration of the species along the coast is related to the movements of the fishes on which they feed. In other areas, movements are poorly understood (Reyes, 1991 and refs. therein).

The Pearl River, southern China, influences the hydrography of the region, notably with regard to turbidity, salinity, pH, tides, currents and temperature of the waters of Hong Kong and Lingding Bay. Consequently the dramatic increase in its freshwater output during the summer also changes fish distribution, which in turn influences the abundance distribution of Hong Kong's Pacific humpback cetaceans (Parsons 2002a). Dolphins in Hong Kong and the Pearl River Estuary have individual ranges averaging 99.5 km², which is only a small portion of the population's range (Jefferson and Hung, 2004). Seasonal changes in their abundance were significantly correlated with water temperature (positively) and salinity (negatively) (Parsons 1998b).

Humpback dolphins appear to be present throughout the year off southern China and northern Queensland (Ross et al. 1994). However, stranding rates differ between various seasons (with peaks during the summer monsoon), which seems to indicate variable dolphin densities and possibly seasonally differing habitats (Parsons, 1998a).back to the top of the page


6. Threats

Direct catch: Small numbers have been taken for food and oil in the Red Sea, Arabian Sea and Persian Gulf, and meat is consumed on the southwest coast of India (Calicut). This may still be practiced today (Reyes, 1991 and refs. therein; Jefferson et al. 1993). They are also among the dolphin species intentionally targeted for their meat in south-western Madagascar. From interview surveys, 22 humpback dolphins had been recorded as directly hunted (Razafindrakoto et a. 2004). Ross (2006) reported that live capture may occur in Queensland, and North South Wales, Australia, with permits granted for up to 12 per year at present.

By-catches: The inshore distribution of these dolphins makes them very susceptible to many human activities in the coastal zone, particularly those relating to fishing. Fishing nets, including seine nets and especially gill nets set for sharks and other large fish, pose the greatest threat to humpback dolphins throughout much of their distribution. Entanglements in gillnets are reported from Zanzibar, Djibouti, the Arabian Gulf, the Indus delta, the south-west coast of India (Ross et al. 1994 and references therein; Lal Mohan,1988), Pakistan, Sri Lanka, Iraq, Kuwait (Reyes, 1991, and refs. therein), Madagascar (Razafindrakoto et a. 2004), Bangladesh (Smith et al. 2008), Myanmar (Smith and Tun, 2008) and China (Chen et al. 2005) Fisheries interactions in China are made responsible for the decline of the population (Wang and Han, 2007).

On Unguja island of Zanzibar, the level of reported incidental catches in artisanal gillnet fisheries in 1999 was 5 Indo-Pacific humpback dolphins, extrapolated to approx. 10 in the whole fleet (Amir et al. 2002). As a follow-up to that survey, from January 2000 to August 2003, incidental catches of dolphins in drift- and bottom set gillnets collected from 12 fish landing sites were recorded. In total, 11 Indo-Pacific humpback dolphins were caught.
Most of the bycatches (71%) were in nets set off the north coast of Unguja Island. This estimate may be high enough as to have a significant negative impact on local populations (Amir et al. 2005).

Dolphins are also caught in shark nets set to protect bathing beaches along the Natal coast, South Africa. At least 67 humpback dolphins were caught in the Natal nets between 1980 and 1989, or about 7-8 animals per year (Ross, 2002). Anti-shark nets are a source of ongoing incidental mortality in South Africa (Reyes, 1991 and refs. therein; Atkins et al. 2004).

Dolphins are also caught in shark nets set to protect bathing beaches along the coast off Queensland and New South Wales (Reyes, 1991; Parra et al. 2004). Accurate catch data for humpback dolphins in the Australian nets are unavailable, though six of 10 dolphins examined by Heinsohn et al. (1980, in Ross et al. 1994) were taken from shark nets. Some specimens were taken in an offshore driftnet fishery operating off northern Australia (Reyes, 1991, and references therein).

Mass strandings: Between 23 August and 30 October 1986, over 500 dead dolphins were found on the western shores of the Persian Gulf, primarily those of Saudi Arabia and Qatar. At least 140 of these were humpback dolphins (Ross et al. 1994). The cause of this mortality, which included three other odontocete species, dugongs, sea turtles and fish, was not estab-lished conclusively.

Habitat degradation: Increased use of sensitive habitats also poses a threat to humpback populations. Pilleri and Pilleri (1979, in Ross et al. 1994) pointed to the reduction in prime habitat for these dolphins in the Indus delta through construction of harbor facilities, drainage and destruction of mangroves, pollution and boat traffic which disturbs their habitat. Dolphins are no longer present in the lower reaches of rivers because of the construction of dams, silting of river mouths and increasing pollution (Reyes, 1991 and refs. therein).

The disposal of contaminated mud arising from Hong Kong's dredging and reclamation projects poses a risk to the Indo-Pacific humpback dolphin via reduced abundance of prey species (Clarke et al. 2000). Acoustic disturbance results from industrial activity underwater, such as pile-driving during land-reclamation as in the construction of Hong Kong Kai Tak airport. Würsig et al. (2000) reported on the successful development of an air bubble curtain to reduce underwater noise of percussive piling.

Hong Kong is one of the busiest ports in the world with approximately half a million oceanic and river-going vessels per year and thirty high-speed and hydrofoil ferries daily passing through the area of greatest humpback dolphin abundance (Parsons, 1997a). Boat traffic seems to interfere with acoustic communication between the animals (Parijs et al. 2001). Between 1993 and 1998 four Pacific humpback dolphin strandings were diagnosed to have been caused by boat strikes (Parsons and Jefferson, 2000). This represents 14% of all humpback dolphin strandings during this period (Parsons and Jefferson, 2000; Parsons, 2004). Lu and Fang (2008) reported on a stranded animal which had ingested parts of a gillnet.

Pollution: Off the South African coast, organochlorine levels are the highest found in any marine mammal. These levels may affect the reproductive efficiency of males and be lethal to neonates of females pregnant for the first time (Reyes, 1991 and refs. therein).

Hong Kong's population of Indo-Pacific hump-backed dolphins inhabits an area where a high volume of sewage waste discharge and the close proximity of contaminated mud pits mean a considerable potential for trace metal contamination (Parsons 1998c). Mercury concentrations in dolphin tissues were an order of magnitude higher than in prey items and could be considered potentially health threatening (max: 906 µg/kg dry wt.) (Clarke et al. 2000). The concentrations of organochlorines were significantly higher than those found in various seals collected from other parts of the world. Correlations between the concentrations of tris-chlorophenyl compounds with other persistent organochlorines such as HCHs, CHLs, DDTs and PCBs were significant, suggesting their bio-accumulation (Minh et al. 1999).

Concentrations of the flame-retardant hexabromocyclododecane (HBCDs) in animals collected from the South China Sea (31-380ng/g lipid) were higher by one order of magnitude than in finless porpoises (4.7-55ng/g lipid), which was attributed to habitat differences (Isobe et al. 2007). The concentrations of yet another flame retardant, polybrominated diphenyl ethers (PBDEs), ranged from a low value of 6.0 ng/g lipid wt. in spinner dolphins (Stenella longirostris) from India to a high value of 6000 ng/g lipid wt. in Indo-Pacific humpback dolphins from Hong Kong (Kajiwara et al. 2006). The pesticides PCB and DDT also showed high concentrations in animals collected from Hong Kong waters, and the ratio of DDT to its breakdown products (and other information) suggests that there may be a recent or nearby source of DDT discharging into the dolphins' ecosystem (Ramu et al. 2005; Jefferson et al. 2006).

Hong Kong discharges over 2,000 million litres of sewage into its coastal waters every day. Bacteria can gain egress into the mammalian body by a variety of routes, and Parsons (1997b) estimated that a Hong Kong humpback dolphin's minimum daily intake of sewage bacteria through ingesting contaminated seawater alone could be up to 70,500 faecal coliforms/day. By comparison a one-off ingestion rate of 200-300 coliforms is considered to be unacceptable for humans (Parsons, 2002b).

According to Parsons (2002b) it is also extremely likely that many areas populated by humpback dolphins are highly contaminated with butyltin. For example, humpback dolphins inhabit the waters of several coastal ports in Asia that host a large volume of shipping and, therefore, potential butyltin pollution, e.g., Shanghai, Bombay, Singapore and Hong Kong. However, next to nothing is known about levels or effects of BT contamination on these cetaceans, and analysis of BT contamination in the tissues of humpback dolphins in areas of high shipping traffic should be a priority

Noise pollution: Würsig and Greene (2002) reported on heavy noise pollution in Hong Kong harbour, potentially masking echolocating sounds and acoustic communication in humpback dolphins. The noise is related to heavy vessel traffic.

Tourism: Karczmarski et al. (1997, 1998) reported that the behaviour of Indian humpback dolphins in Algoa Bay, South Africa was not affected by the presence of bathers or surfboats. However, powerboats did cause changes in behaviour, and when these vessels were present avoidance reactions were observed by the dolphins in 95.3% of occasions (Karczmarski et al. 1998). The response to boat traffic involved the animals taking a long dive, changing their direction and swimming away perpendicular to the route of the boat (Karczmarski et al. 1997).
In Kizimkazi (Zanzibar) marine mammals were previously used as bait for sharks. However, in the mid 1990's the local fishermen realised that their touristic value far exceeded their value as shark bait. As many as 2,000 tourists visit the dolphin site at Kizimkaki per month. Dolphin-tourism is currently becoming a popular economic activity. Successful management of the dolphin-tourist trade will ensure continued visitors to the villages where dolphins are present and thus add income to these villages while contributing to management and conservation (Ali and Jiddavi, 1999).back to the top of the page


7. Remarks

Range states (Reeves et al. 2008) :
Australia; Bahrain; Bangladesh; Brunei Darussalam; Cambodia; China; Comoros; Djibouti; Ecuador; Egypt; Ethiopia; Hong Kong; India; Indonesia; Iran, Islamic Republic of; Iraq; Israel; Kenya; Kuwait; Macao; Madagascar; Malaysia; Mozambique; Myanmar; Oman; Pakistan; Papua New Guinea; Philippines; Qatar; Saudi Arabia; Singapore; Somalia; South Africa; Sri Lanka; Taiwan, Province of China; Tanzania, United Republic of; Thailand; Timor-Leste; United Arab Emirates; Viet Nam; Yemen.

Sousa chinensis is listed in Appendix II of CMS. Sousa sp. is listed in Appendix I of CITES. The IUCN considers S. chinensis as "Near Threatened". This is based on the fact that although the population may consist of more than 10,000 individuals, it is under substantial threat by fisheries activities and habitat degradation. It is possible that the reductions in population size have been large and pervasive enough to cause a net reduction for the entire species of at least 30% over a period of 3 generations. The species may however be re-classified as "Vulnerable" in the near future, if population size was estimated to lie below 10,000 individuals, with a discontinuous distributional range (Reeves et al. 2008).

More research on biology, taxonomy, stock identity and movements is needed. Assessment of ecological impact should be requested of development projects through the range. Compilation of Information on direct takes and incidental mortality should be encouraged.

See also general recommendations on Southeast Asian stocks in Perrin et al. (1996) in Appendix 2. Recommendations for further research are given by Jefferson (2000).back to the top of the page


8. Sources

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© Boris Culik (2010) Odontocetes. The toothed whales: "Sousa chinensis". UNEP/CMS Secretariat, Bonn, Germany.http://www.cms.int/small-cetaceans
© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.

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