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| Subspecies: | Unknown |
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| Est. World Population: | |
| CITES Status: | NOT LISTED |
| IUCN Status: | Vulnerable |
| U.S. ESA Status: | NOT LISTED |
| Body Length: | |
| Tail Length: | |
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| Jumping Ability: | (Horizontal) |
| Life Span: | in the Wild |
| Life Span: | in Captivity |
| Sexual Maturity: | (Females) |
| Sexual Maturity: | (Males) |
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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, Parra and Ross 2009). In Hong Kong waters, they are known to feed on at least 24 species of fishes and one cephalopod. The most common prey species in Hong Kong are the croaker Johnius sp., the lionhead Collichthys lucida, and anchovies Thryssa spp. (Barros et al. 2004). Dolphins often follow trawlers in the Pearl River Estuary, and appear to feed on species that evade or are stirred up by these nets (Jefferson 2000, Hung 2008). In the Bay of Bengal, Bangladesh, they are frequently observed preying on fish that fall out of set-bag nets and gillnets when they are being pulled to the surface (Smith et al. 2015). In Matang, western Peninsular Malaysia, humpback dolphins are often observed preying on marine catfishes, particularly the Sagor catfish (Hexanematichthys sagor), often consuming larger catfish individuals by eating only the body and leaving behind the catfish head with sharp dorsal and pectoral spines (Kuit et al. 2014). Predation on Indo-Pacific Humpback Dolphins is almost unknown, and the estuarine waters where the species most often occurs generally have lower densities of potential predators, including large sharks and killer whales Orcinus orca.
Reproductive biology has only been studied in detail in HK/PRE (Jefferson et al. 2012), and to a lesser extent in Xiamen (Wang 1965, 1995). Typically a single calf is born after a gestation period of slightly less than a year (Parra and Ross 2009). In Hong Kong, calving occurs throughout the year, but there is a significant peak in births from March to June, near the start of the wet season (Jefferson et al. 2012). The average calving interval among a sample of 60 females studied through long-term photo-identification was five years, which is quite long for dolphins (Jefferson et al. 2012). Newborns calves are on average 101 cm in length, and sexual maturity occurs at ages of about 9-10 years for females, and a few years later for males (Jefferson et al. 2012). Physical maturity is reached at ages of about 14-17 years, and the oldest known individual was 38 years of age, although it is suspected that some dolphins may live into their forties (Jefferson et al. 2012). Apparent survival for adults and juveniles was estimated to be 0.85 (95% CI 0.725-0.919) in coastal waters of Bangladesh (Smith et al., 2015). Age-specific mortality rates have been estimated for the HK/PRE subpopulation, and for adults from 10-25 years of age ranges from about 0.03-0.15 (Huang et al. 2012).
Indo-Pacific Humpback Dolphins throughout most of their range occur most commonly in groups of 2-6 individuals (Parra and Ross 2009). However, aggregations of several dozen have been observed, especially when they are following fishing vessels, in Hong Kong (Hung and Jefferson 2004). Group sizes in the Bay of Bengal, Bangladesh, are considerably larger than those recorded elsewhere, with a median estimated size of 19 individuals (n=55) and groups of at least 81 and 205 documented (Smith et al. 2015). In the Langkawi Archipelago, western Peninsular Malaysia, humpback group sizes range from 1-152 individuals, although group sizes are usually observed between 4-20 animals (Ponnampalam and Jamal Hisne 2011, Ponnampalam et al. 2014). Group sizes are also large in Kuching—up to 45 with mean of 18 (Minton et al. 2016). Social organization, in the few places where humpback dolphins have been studied, is largely characterized by a fission/fusion society of mostly short-term associations—e.g., Hong Kong/PRE (Jefferson 2000, Dungan et al. 2012), Zhanjiang/Leizhou area (Xu et al. 2012) and the Beibu Gulf area (Chen 2013). However, there is some evidence that humpback dolphins in Taiwanese waters have stronger social bonds and more stable association patterns (Dungan et al. 2012). This may be related to the very small subpopulation size and restricted habitat there.
Indo-Pacific Humpback Dolphins are not known to undergo large-scale migrations, although seasonal shifts in abundance have been identified in the HK/PRE area (Jefferson 2000, Hung 2008). Ranging patterns of individuals have been studied in only a few locations, using photo-identification data. In Hong Kong, individual movements tend to occur over relatively small areas of 39-339 km², with an average of about 135 km², much smaller than the subpopulation’s overall range of several thousand km² (Hung and Jefferson 2004, Hung 2008). Similar patterns were found for Xiamen, with an average range of 84 km² (Chen et al. 2011) and the Zhanjiang/Leizhou area, with an average estimated range size of 169 km² (Xu et al. 2015).
Indo-Pacific Humpback Dolphins have been considered to occur in shallow, coastal waters from central China (the northernmost records are from near the mouth of the Yangtze River) in the east, southward throughout Southeast Asia, and westward around the coastal rim of the Bay of Bengal to at least the Orissa coast of eastern India (Jefferson and Rosenbaum 2014; see map). However, as mentioned above, humpback dolphins along the coast of Bangladesh (and possibly along the coast of eastern India) were recently found to be phylogenetically distinct. The species regularly occurs in enclosed seas, such as the Gulf of Thailand, and they appear to have their highest densities in and around estuaries (Jefferson and Karczmarski 2001, Parra and Ross 2009). Their distribution is apparently fragmented, with relatively long stretches of coastline between river mouths often having very low or zero densities. The range maps shows potential habitat within the known range < 30 m in depth. It appears the only places where they range reasonably far offshore are limited to waters of the continental shelf <30-40 m deep. Due to the uncertain taxonomic status of humpback dolphins from Bangladesh, eastern India and Sri Lanka, at this time the confirmed range of S. chinensis should only be considered to extend west to the Bangladesh/Myanmar border (see map). Specific range limits within the Indo-Malay archipelago also need to clarified.
There is evidence from several sources suggesting that range contraction has occurred, at least in southern and central China, where fishermen reported seeing humpback dolphins nearly continuously along the mainland coast several decades ago, but where today only about 6-8 areas of regular occurrence are thought to occur (X. Wang et al. 2012, Wu et al. 2014). This means that in some areas, fragmented subpopulations may now occur in what was once occupied by one or more larger, continuous population units
Sousa chinensis is listed in Appendix I of CITES. Throughout most parts of its range in Southeast and South Asia, conservation actions have been either extremely limited or non-existent. There is active management in Hong Kong (by the Agriculture, Fisheries and Conservation Department, and other government entities), where since about 1993 the species has been protected by the Wild Animals Protection Ordinance, Marine Park Ordinance, and the Environmental Impact Assessment Ordinance. The dolphins have been the subject of long-term monitoring and extensive environmental impact assessment for over 20 years (Jefferson et al. 2009, Hung 2014). The establishment of several marine parks form an integral part of the management strategy in Hong Kong, although it is also recognized that other measures are needed for the long-term conservation of the overall subpopulation (Jefferson et al. 2009). Management is less active in Taiwan, but there has been recent progress in monitoring and environmental impact assessment (Ross et al. 2010). In mainland Chinese waters, the species is a Protected Species of the First Order, though in most areas there is little actual management of threats, beyond the establishment of marine protected areas (Chen et al. 2009), which generally appear to be inadequately managed and in fact are little more than ‘paper parks’. Therefore, recommendations for the establishment of national nature reserves as the main conservation measures for these dolphins in China must be viewed with some degree of skepticism, without details on how the borders were established and the protective measures provided and their effectiveness.
In 2014, the Government of Bangladesh signed into law the country’s first Marine Protected Area (MPA) spanning about 1,700 km2 and encompassing more than 50% of the total number of humpback dolphin sightings made in waters offshore of the Sundarbans mangrove forest between 2004 and 2012. Efforts are currently underway to establish conservation management in this new MPA through fishing closures for entangling gears and modifying fishing practices. Efforts are also being made to generate interest in expanding the MPA into a larger bi-national MPA that includes adjacent coastal habitat in India supporting a portion of the ‘superpopulation’ in Bangladesh described above (Smith et al. 2015). One promising approach for reducing humpback dolphin bycatch in these waters is an initiative that requires gillnet fishermen to attend their nets, release entangled dolphins, and collect data and biological samples from mortalities in exchange for measures taken to improve their safety at sea
In 2015, research data on Langkawi Sousa was used to make recommendations in the drafting of the Langkawi Plan (Revision) 2030 (by the Department of Town and Country Planning) for speed limit zones in the main ferry travel routes in and out of Langkawi. However, actual on-the-ground conservation action based on this recommendation has yet to take place.
More conservation-oriented research is needed throughout the range of Indo-Pacific Humpback Dolphins to design effective management programs. There is a vital need to identify and mitigate the main threats facing demographically-isolated subpopulations and reduce their impacts to sustainable levels. Particular attention should be given to addressing bycatch and protecting critical habitat in areas where there are strong competing interests for human development (Parra and Ross 2009, Ross et al. 2011). Where marine protected areas are established for humpback dolphin conservation, it is essential that they be managed using the best available scientific information, and that the major threats to animals in those area be eliminated or at least effectively reduced to sustainable levels.
A key outstanding question is the taxonomic identity of humpback dolphins in Bangladesh and eastern India. Further investigation is needed on the genetic identity of these animals with comparisons made among samples obtained from previously unsampled areas and the analysis of additional genetic markers. Due to their apparent phylogenetic distinctiveness, once the taxonomic identity of this form has been confirmed, it should be assessed separately.