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Description |
Geographic Range [top]
Range Description: Chinensis-type
Indo-Pacific humpback dolphins of the chinensis-type are found in shallow, coastal waters from the east and west coasts of northern Australia and from southern China in the east, throughout the Indo-Malay Archipelago, and westward around the coastal rim of the Bay of Bengal to at least the Orissa coast of eastern India (Ross et al. 1994; Jefferson and Karczmarski 2001; Sutaria and Jefferson 2004). They regularly occur in some enclosed seas, such as the Gulf of Thailand. Their distribution appears to be limited to waters of the continental shelf, and the only places where they range far offshore are those where the water remains shallow (<100 m).
Plumbea-type
The plumbea-type is found in a narrow strip of coastal waters from southwestern tip of South Africa eastward around the rim of the Indian Ocean to the southeastern coast of India (Jefferson and Karczmarski 2001; Ross 2002; IWC 2003). It occurs off Madagascar, Mayotte and the Comoro Islands and around the Arabian Peninsula from the Red Sea into the Arabian (Persian) Gulf and east to Pakistan. There is an extralimital record from Israel in the Mediterranean Sea (apparently a stray that moved through the Suez Canal from the Red Sea – Kerem et al. 2001). In the region between northeastern India and Myanmar (Burma) plumbea-type and chinensis-type dolphins are partially sympatric.
Countries occurrence:
Native:
Australia; Bahrain; Bangladesh; Brunei Darussalam; Cambodia; China; Comoros; Djibouti; 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
FAO Marine Fishing Areas:
Native:
Indian Ocean – western; Indian Ocean – eastern; Pacific – northwest; Pacific – southwest; Pacific – western central
Additional data:
Range Map: Click here to open the map viewer and explore range.
Population [top]
Population: Chinensis-type
Studies have been carried out in only a few parts of the chinensis-type’s range, and there is no overall estimate of total population size. Certain subpopulations are thought to be depleted, mostly by habitat destruction/degradation and bycatch in fisheries. Most abundance estimates have been less than a few hundred dolphins, but there appear to be at least 1,200 animals (CVs range from 17-119%) in the Pearl River Estuary of southern China, adjacent to and including Hong Kong and Macau (Jefferson 2000, Jefferson 2005). The Pearl River Estuary population is the only one for this geographic form with quantitative data on population trends, and despite the heavy development in the area and numerous threats, the population has shown no evidence of significant decline in the last 11 years (Jefferson 2005).
Other places where abundance has been estimated are Xiamen, with an estimate of 80 (CV=1.08 - Jefferson and Hung 2004), and eastern Taiwan Strait, which is thought to have a population of only about 99 individuals (CV=52% Wang et al. 2007). Declines have been inferred in both of these areas, based on qualitative environmental information. An estimated 237 (95% CI = 189-318) humpback dolphins inhabit waters around the Leizhou Peninsula, southern China (Zhou et al. 2007). Data on the status of humpback dolphins in Australia are scarce, but by analogy with sympatric (and better-studied) dugongs (Dugong dugon), Corkeron et al. (1997) suggested that they were in decline there. The only statistically defensible estimates for Australian waters are of 34-54 (CVs=13-27%) in Cleveland Bay, Queensland (Parra et al. 2006a), and 119-163 (95% CIs = 81-251) in Moreton Bay, Queensland (Corkeron et al. 1997).
Plumbea-type
As in the case of the chinensis-type, there is no overall estimate of total population size for plumbea-type dolphins. All available subpopulation estimates are in low tens to low hundreds: ~ 450 dolphins (95% CIs = 447-485) in the Algoa Bay region, Eastern Cape coast of South Africa (Karczmarski et al. 1999a), 170-244 in the Richard’s Bay region on the KwaZulu-Natal coast, South Africa (Atkins and Atkins 2002), 105 (95% CIs = 30-151) in Maputo Bay, Mozambique (Guissamulo and Cockcroft 2004), ~ 60 dolphins in Bazaruto Archipelago, Mozambique (Guissamulo and Cockcroft 1997) and 58-65 (95% CIs = 56-102) off Zanzibar (Stensland et al. 2006). Quantitative trend data are not available anywhere in the plumbea-type’s range, but there are indications that some subpopulations have declined in numbers in recent years. For instance, the numbers in the Bazaruto Archipelago decreased from ~ 60 in 1992 (Guissamulo 1993) to probably fewer than 30 in 2003, along with considerable deterioration of the shallow-water habitat across the archipelago (Guissamulo and Karczmarski pers. comm.). Mortality in anti-shark nets off the KwaZulu-Natal coast in the late 1980s was estimated to likely exceed the dolphins’ replacement rate (Cockcroft 1990), but there is no more recent information from that area. Mixing among neighbouring populations is uncertain, although in South Africa none was documented between groups inhabiting locations 800 km apart. Quantitative data are limited, but there are indications that the distribution is discontinuous elsewhere in the plumbea-type’s range, with fragmented and likely discrete populations (e.g.. Karczmarski 2000, Baldwin et al. 2004, A.T. Guissamulo pers. comm., V.G. Cockcroft pers. comm.).
Current Population Trend: Decreasing
Additional data:
? Population severely fragmented: Yes
Habitat and Ecology [top]
Habitat and Ecology: Humpback dolphins occur in tropical to warm temperate coastal waters, including open coasts and bays, coastal lagoons, rocky and/or coral reefs, mangrove swamps and estuarine areas (Ross et al. 1994, Jefferson and Karczmarski 2001, Ross 2002). They are rarely encountered more than a few kilometres from shore. They sometimes enter rivers, but rarely move more than a few kilometres upstream and usually remain within the range of tidal influence.
Indo-Pacific humpback dolphins appear to be opportunistic feeders, consuming a wide variety of nearshore, estuarine, and reef fishes. They also eat cephalopods in some areas, but crustaceans are rare in their diet (Jefferson and Karczmarski 2001, Ross 2002).
Chinensis-type
Chinensis-type dolphins often enter rivers, estuaries, and mangroves, preferring coasts with mangrove swamps, lagoons, and estuaries, as well as areas with reefs, sandbanks, and mudbanks (Jefferson and Karczmarski 2001). In at least China and southern Asia, they are rarely found far from estuaries and mangrove habitats (Jefferson and Karczmarski 2001; Wang et al. 2007), and they show a strong preference for river mouths in northern Australia (Parra 2006; Parra et al. 2006b). Aerial surveys of the Great Barrier Reef region demonstrate that humpback dolphins occur mostly close to the coast but also in offshore waters that are relatively sheltered, and near reefs or islands (Corkeron et al. 1997). Fine-scaled resource partitioning between humpback and Australian snubfin dolphins (Orcaella heinsohni) has been documented off Queensland, where the two species favour river mouths and modified habitats but the humpback dolphins occur in slightly deeper (2-5 m deep) waters (Parra 2006).
Plumbea-type
Plumbea-type dolphins are usually seen within a narrow strip of shallow near-shore waters and in estuarine areas (Ross et al. 1994, Jefferson and Karczmarski 2001), seldom in water deeper than 20-30 m (Karczmarski et al. 2000). Seasonality of occurrence, movements, group sizes, and reproduction has been reported in several studies (e.g. Durham 1994, Karczmarski et al. 1999a, 1999b, Guissamulo 2007). The dolphins appear to be selective in their habitat choice (e.g. Karczmarski et al. 2000; Stensland et al. 2006). Dependence on shallow-water habitats as feeding grounds is often evident throughout the year (Karczmarski and Cockcroft 1999; Karczmarski et al. 2000), although the details of the preferred key habitats may differ between groups and locations (Jefferson and Karczmarski 2001; Atkins et al. 2004; Stensland et al. 2006).
Systems: Freshwater; Marine
Use and Trade [top]
Use and Trade: There are small-scale fisheries that focus on this species in some parts of its range.
Threats [top]
Major Threat(s): Most humpback dolphins inhabit coastal or estuarine waters of developing nations, i.e. countries with limited resources and means for environmental protection. Range-wide incidental mortality in fishing gear and habitat degradation and loss represent the greatest threats to this species throughout its range (Ross et al. 1994, Jefferson and Karczmarski 2001).
Chinensis-type
Chinensis-type dolphins are not known to be hunted directly in significant numbers anywhere in their range. However, they are often caught in fishing nets, such as gillnets and trawls, and in anti-shark nets set to protect bathing beaches from large sharks along the coasts of Queensland and New South Wales, Australia (Heinsohn 1979, Ross et al. 1994, Parra et al. 2004). Accurate catch data for humpback dolphins in the Australian nets are unavailable, but kills in anti-shark nets off Queensland are high relative to estimated abundance (Paterson 1990, Corkeron et al. 1997). The greatest direct sources of human-caused dolphin mortality in Hong Kong appear to be incidental catches in fishing gear (most likely pair trawls) and vessel collisions (Jefferson 2000, Parsons and Jefferson 2000). Between 1993 and 1998, at least 3 humpback dolphins were killed by boat strikes and another death was suspected of being caused by a boat strike. This represented 14% of all documented humpback dolphin strandings in Hong Kong during that period (Parsons and Jefferson 2000).
Concentrations of organochlorines in cetaceans from Hong Kong coastal waters are significantly higher than those found in cetaceans in other parts of the world (Parsons and Chan 1998, Minh et al. 1999) and it has been suggested that the reproductive success of Hong Kong’s humpback dolphins (including neonatal survival) is being affected (Parsons 2004; Jefferson et al. 2006). In Hong Kong, high volumes of sewage discharge and the close proximity of contaminated mud pits means that there is considerable potential for trace metal contamination of local dolphins (Parsons 1997). Indeed, mercury concentrations in the tissues of Hong Kong humpback dolphins were found to be an order of magnitude higher than in prey items and in some cases, were high enough (max: 906 µg kg-1 dry weight) to be considered potentially health-threatening (Parsons 2004). Hong Kong discharges over 2 billion litres of sewage into the surrounding waters daily. Parsons (1997) estimated that a humpback dolphin’s minimum daily intake of sewage bacteria through ingestion of contaminated seawater could be up to 70,500 faecal coliforms. To put this in context, a one-off ingestion rate of 200-300 coliforms is considered unacceptable for humans (Parsons 2004).
The disposal of contaminated mud from Hong Kongs dredging and reclamation projects poses an indirect risk to humpback dolphins via their consumption of contaminated prey (Clarke et al. 2000). Humpback dolphins inhabit the waters of several coastal ports in Asia that host large volumes of ship traffic, such as Shanghai, Singapore and Hong Kong. Therefore, it is likely that they are highly contaminated with butyltin (BT) (see Tanabe et al. 1998, Tanabe 1999; Parsons 2004).
Underwater industrial activity, such as pile-driving during pier and bridge construction, are likely to cause acoustic disturbance (e.g. the development of Hong Kong’s new airport). Boat traffic also might interfere with the dolphin’s acoustic communication (Van Parijs et al. 2001).
Plumbea-type
Their near-shore distribution and preference for shallow-water habitats make these dolphins particularly susceptible to the effects of human activities in the coastal zone – all similar to what is described above for chinensis-type animals. Habitat loss (through alteration or destruction of inshore environments) and incidental mortality in fishing gear are among the greatest threats (e.g. Baldwin et al. 1999; Cockcroft and Krohn 1994; Guissamulo 1993; 2007; Karczmarski 2000; 2002; Keith et al. 2002; Rozafindrakoto et al. 2004; Stensland et al. 2006). Deliberate killing for human consumption is known to occur in Africa and Madagascar, and at least in some areas human-caused mortality (deliberate and incidental) is likely to be close to, or even exceed, the dolphins’ reproductive rate. Other threats include coastal and offshore development, oil and gas exploration, pollution, and boat traffic (e.g. Karczmarski 2000, Baldwin et al. 2004). Oil-related deaths of dolphins have been reported from the Arabian (Persian) Gulf (Baldwin et al. 1999). Where investigated, tissue concentrations of organochlorines and other pollutants have been high (among the highest of all marine mammal species in the region), causing concern that the reproductive potential of adults and survival of neonates might be impaired (Cockcroft 1999). Behavioural responsiveness to boat harassment has been recorded in several locations (Karczmarski et al. 1998, Karczmarski 2002, Stensland et al. 2006).
Conservation Actions [top]
Conservation Actions: Sousa spp. Are listed in Appendix I of CITES.
For both forms, but especially the plumbea-type, conservation actions are currently either meagre or non-existent. Research is needed to help design effective conservation programmes.
Citation: Reeves, R.R., Dalebout, M.L., Jefferson, T.A., Karczmarski, L., Laidre, K., O’Corry-Crowe, G., Rojas-Bracho, L., Secchi, E.R., Slooten, E., Smith, B.D., Wang, J.Y. & Zhou, K. 2008. Sousa chinensis. The IUCN Red List of Threatened Species 2008: e.T20424A9197694. http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T20424A9197694.en. Downloaded on 16 January 2017.
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