|Scientific Name:||Sousa chinensis (Osbeck, 1765)|
|Infra-specific Taxa Assessed:|
Sousa borneensis (Lydekker, 1901)
|Taxonomic Notes:||Currently, all Indo-Pacific Humpback Dolphins are considered to be part of a single widespread and highly variable species, Sousa chinensis. Some biologists consider Humpback Dolphins in the Indo-Pacific to consist of two species: S. plumbea in the western Indian Ocean, from South Africa to at least the east coast of India, and S. chinensis, from the east coast of India to China and Australia. These two geographic forms differ markedly in their external morphology, and are moderately distinct in their skull morphometrics (see Jefferson and Van Waerebeek 2004 for a partial summary). However, the external morphological characters that have traditionally been used to differentiate these forms may be plesiomorphic (ancestral) or the result of convergent evolution, and as such, are not particularly informative phylogenetically. This was pointed out by Jefferson and Van Waerebeek (2004) when comparing the Atlantic S. teuszii and West Indian plumbea form, which are both uniformly gray in color and have dorsal humps, but may in fact not be that closely related. Similarly, a light gray to white adult coloration and lack of a prominent dorsal hump has traditionally been used to define the chinensis form.
Recent mitochondrial (mt) DNA analyses indicate that Humpback Dolphins from Australia (chinensis form) are highly distinct from other Indo-Pacific populations (Frere et al. in press). Phylogenetic analyses comparing chinensis dolphins from Hong Kong and Australia, with plumbea dolphins from South Africa, as well as with Atlantic S. teuszii, strongly suggest that Hong Kong chinensis, South African plumbea, and Atlantic S. teuszii are more closely related to one another than to the ‘chinensis’ dolphins in Australia. Previous mtDNA work by Cockcroft et al. (1997) indicated a similar pattern of relationships among these three Indo-Pacific regions.
It is clear that the taxonomy of the genus Sousa will not be resolved until comprehensive sampling and both genetic and morphological analyses have been conducted throughout the range. There is growing evidence that at least two putative species, and possibly more, are valid (see Rice 1998, Jefferson and Karczmarski 2001, Frere et al. in press). In the meantime, considering that there is a broad area of sympatry and no evidence of intermediate forms, the two morphological forms (plumbea-type and chinensis-type) are assessed separately in the account below. However, the reader should remain aware that the Australian chinensis-type is apparently not closely related to other chinensis-type dolphins.
|Red List Category & Criteria:||Near Threatened ver 3.1|
|Assessor(s):||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.|
|Reviewer(s):||Brownell Jr., R.L. & Cooke, J. (Cetacean Red List Authority)|
When considering the status of the entire species Sousa chinensis (including both the chinensis-type and the plumbea-type together), the total population size probably consists of more than 10,000 mature individuals and therefore criterion C for Vulnerable would not apply (although as indicated below, it might apply to each of the two forms when assessed separately). Population reductions can be inferred over much of the species’ range due to heavy fishing pressure (incidental mortality) and habitat loss in coastal and estuarine areas. This inference is supported in some areas by either direct or indirect evidence, including observations of bycatch, interviews with fishermen, and the high and expanding levels of fishing effort with gillnets and other harmful fishing gear. 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 (about 60 years; see Taylor et al. 2007) including the past and future. Therefore, the species as a whole comes close to qualifying for Vulnerable based on criterion A4cd, where subcriterion d (actual or potential levels of exploitation) is interpreted to include bycatch. Although the species may not meet any of the criteria for Vulnerable at this time, it is likely to do so in the near future (especially considering the implications of the taxonomic uncertainty described above). Therefore the species is assessed as Near Threatened, and it should be reassessed following taxonomic clarification of the two forms.
The available abundance estimates for the chinensis-type humpback dolphin range from a few dozen to over 1,200 for the few small areas of the geographic form’s range that have been studied so far (less than 10%). Although it is possible that the total population numbers in the low tens of thousands, there is no evidence to suggest there are more than that and some reason to suspect the relatively large subpopulation in the Pearl River Estuary, estimated at about 1,200-1,300 individuals, is exceptional. That subpopulation would likely have no more than about 650 mature individuals (estimated % mature = 50% - see Taylor et al. 2007; but also note that there is a direct estimate of 60% mature for that subpopulation – Jefferson 2000, implying as many as 780 mature individuals). Considering the apparently fragmented distribution, the inference of declines in most areas (due to threats as described above and that fact that conservation actions currently are either meager or non-existent in most of the range), and that there could well be fewer than 10,000 mature individuals, the chinensis-type geographic form would qualify as Vulnerable (C2a(i) and possibly also A4cd) if it were assessed separately.
All available abundance estimates for plumbea-type humpback dolphins are low (fewer than 500 individuals), and the total number across their range is unlikely to exceed 10,000 individuals. The distribution is discontinuous across most of the range, with probably discrete local subpopulations. Ongoing environmental degradation and loss of key habitats is likely further fragmenting the aggregate population. Exposure to serious environmental stressors throughout their range makes plumbea-type dolphins highly vulnerable, and there are indications of considerable declines in at least some locations. Conservation actions currently are either meager or non-existent throughout the range. It is possible that the decline of plumbea-type animals has been large and pervasive enough throughout their range to cause a net reduction of at least 30% over a period of 3 generations (about 60 years; see Taylor et al. 2007) including the past and future. The plumbea-type geographic form would qualify for Vulnerable (C2a(i) and possibly also A4cd) if it were assessed separately.
|Previously published Red List assessments:|
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).
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.
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:|
Indian Ocean – western; Indian Ocean – eastern; Pacific – northwest; Pacific – southwest; Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
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).
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|
|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 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 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).
|Use and Trade:||There are small-scale fisheries that focus on this species in some parts of its range.|
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 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 Kong's 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).
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).
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.
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|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.Downloaded on 18 October 2017.|