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The Indo-Pacific Hump-backed dolphin (Sousa chinensis), or locally known as Chinese White Dolphin, is distributed throughout shallow, coastal waters of the Indian and western Pacific oceans, from South Africa in the west to northern Australia and southern China in the east (Ross et al. 1994). One dolphin population appears to be centered around the Pearl River Estuary, and Hong Kong waters represent the eastern portion of the range, which extends far into Mainland waters (Jefferson 2000; Ng and Leung 2003; Jefferson and Hung 2004). The Chinese White Dolphin is a species protected under the Conservation on International Trade in Endangered Species of Wild Fauna and Flora (CITES) (Klinowska 1991), and is classified as an 'insufficiently known' species by The World Conservation Union (IUCN) (Reeves and Leatherwood 1994). In china, this species was listed as a Grade 1 National Key Protected Animal under the Wild Animal Protected Law issued in 1988.
The coastal distribution characteristic of Chinese White Dolphin renders them particularly susceptible to various extreme anthropogenic pressures. These pressures have posed some serious impacts on Chinese White Dolphin. For examples, at least 11 dead dolphins were recorded from 1994 to 1999 in Xiamen province (Huang and Liu 2000). The number of death was increasing to six in 2002 and ten in 2004. Some of these dead individuals were identified to die from underwater blasting in coastal construction projects (Wang et al. 2003). Other human activities, such as agricultural and industrial pollution, vessel traffic, and fishing, etc., also have some adverse impacts on Chinese White Dolphin. For example, destructive fishing, especially trawl netting, is likely to result in habitat degradation and resource depletion of dolphin, and extensive mariculture in the nearshore waters might result in habitat loss for local dolphins (Jefferson and Hung 2004).
The primary aim of this review is to study the ecology and population biology of Chinese White Dolphin and the problems the species faces along the coast of China. The ultimate goal is to provide sufficient data to enable the development of an effective conservation and management plan for this coastal dolphin species.
2. Species Overview
The Indo-Pacific hump-backed dolphin (Sousa chinensis), locally known as Chinese White Dolphin, is distributed throughout shallow, coastal waters of the Indian and Western Pacific oceans, from South Africa in the west to northern Australia and southern China in the east (Fig. 1) (Ross et al. 1994). There are probably about 5 populations of Chinese White Dolphins along the southern China coast, mainly centered around the mouths of large rivers (Zhang and Tang 2008). One of the best-known populations in southern China is centered around the mouth of the Pearl River (Jefferson 2000; Ng and Leung 2003; Jefferson and Hung 2004).
Figure 1. (a).Distribution of Sousa chinensis: shallow coastal waters of the Indo-Pacific Ocean. (Reeves et al. 2008). (b). Reliable records of occurrence of Sousa chinensis in Chinese waters (Jefferson 2000).
There is only a little information about distribution in the coastal waters of China, where 5-7 important populations of S. chinensis can be identified, but populations of this species appear to be decline (Zhang and Tang 2008)
Chinese White Dolphin occurred in all of waters of Hong Kong area, including Lantau Island but was most common in the North Lantau area. Estimates in 1995-1997 ranged from 88 to 155 dolphins varied within a year, with an average of 109 in a year-round. The abundance suggested that between 208 and 246 dolphins used the Hong Kong area by using mark-recapture approach (Jefferson and leatherwood 1997). Jefferson and karczmarski (2001) concluded that more than 1,028 dolphins occurred in Hong Kong waters and the adjacent Pearl River Estuary recently.
In 2005, researchers estimated that the Leizhou population of Chinese White Dolphin was about 237 individuals. Researchers concluded that Leizhou Bay is likely to serve as a "humpback dolphin sanctuary" in Chinese waters (Zhou et al. 2007).
Liu and Hang (2000) recorded 392 individuals in Xiamen waters, with a decline population trend.
Usually, the Chinese White Dolphin is quite difficult to approach and tends to avoid boats by diving or reappearing some distance away in a different direction (Carwardine 1995; Ross et al. 1994). Dolphins associate with a number of types of fishing vessels, such as trawlers. Dolphins often gather in large groups behind active pair trawlers and feed on prey stirred-up by the nets (AFCD 2000). Individual dolphins show different tendencies to feed behind pair trawlers (AFCD 2000).
Some studies have mentioned that Chinese White Dolphin's calves may be born throughout the year, but spring or summer is the valving peaks. Usually, Gestation of this species lasts 10-12 months, and age at sexual maturity is 10 years in females and 12-13 years in males (Jefferson and karczmarski 2001). In addition, the evidence suggests a minimum of a 3-year calving interval and maternal care lasts at least 3-4 years (Karczmarski 1999).
According to Ross (2002), dolphins' food consists mainly of fish and cephalopods. Chinese White Dolphin in China feed on several species of demersal and estuarine fishes. There was a little evidence of predation on cephalopods or crustaceans (Jefferson and Hung 2004).
In China, the rivers influence the hydrography of the region, notably with regard to turbidity, salinity, pH, tides, currents and temperature of the Chinese waters. As well know, most Chinese White Dolphins occur around the mouth of the rivers. The dramatic increase in its freshwater output and the changes of fish distribution leads to influence the abundance distribution of Chinese White Dolphin (Parsons 2002a). For example, dolphins in Hong Kong and the Pearl River Estuary have individual ranges averaging 99.5 kmÂ², which is only a small portion of the population's range (Jefferson and Hung, 2004). Seasonal changes in their abundance are also significantly correlated with water temperature and salinity (Parsons 1998b).
3.1 Habitat degradation and disturbance
Increased use of sensitive habitats poses a threat to Chinese White Dolphins. Reclamation involves converting an area of coastal waters into a piece of land. If that area were used by the Chinese White Dolphins as part of their home range, then the reclamation would result in a permanent loss of their habitat (AFCD 2000). One part of sea areas now occupied by the Chek Lap Kok airport platform in North Lantau used to be a habitat for the Chinese White Dolphins was confirmed (AFCD 2000).
The contaminated mud disposed from Hong Kong's dredging and reclamation projects causes a risk to the Chinese White Dolphin via reduced abundance of prey species (Clarke et al. 2000). Underwater industrial activity, such as pile-driving during land-reclamation as in the construction of Hong Kong Kai Tak airport, causes acoustic disturbance to Chinese White Dolphins too. Although, recently, AFCD (2000) has reported on the successful development of reducing underwater noise of percussive piling by an air bubble curtain, the Chinese White Dolphins are still in threatening.
As well known, Hong Kong is one of the busiest ports in the world with approximately half a million oceanic and river-going vessels per year passing through the area of greatest Chinese White Dolphin abundance (Parsons 1997a). This is another factor that influences the abundance of Chinese White Dolphins.
The volume of sewage discharged into Hong Kong's waters is considerable (Parsons 1998c). Although there is little information available on the effects of sewage on wild dolphin populations, the sewage could be a potential source of pathogenic bacteria, fungi and viruses (Parsons 1997b). Immunosuppression and other health impacts caused by accumulation of environmental contaminants, such as heavy metals and organic compounds, can be another potential long-term threat to the local dolphin population though the causal relationship is difficult to prove conclusively (Parsons 1998c).
Wursig and Greene (2002) reported that heavy noise can interfere with echolocation and communication of dolphins. The noise is related to heavy vessel traffic.
Karczmarski et al. (1998) reported that dolphin-watching tours have a potential impact on the dolphins. Although the impact of seems minimal compared to those heavy vessels, it will cause changes of dolphins' behavior (karczmarski et al. 1998).
3.3 Depletion of food resources
Since feeding habit studies found that there was an overlap in prey of dolphins and targets of fishing vessels, competition exists between fishermen and dolphins for the same fisheries resources (AFCD 2000). It also mentioned that the fisheries resources in Xiamen have declined, which would affect not only the fishermen, but also the dolphins. Both of them will surely suffer if there is a continual decline in fisheries resources of the area (AFCD 2000).
The inshore distribution of these dolphins makes them very susceptible to many human activities in the coastal zone, particularly those relating to fishing. However, unlike in other parts of Asia (e.g. Japan and Indonesia), there appears to be no direct fishery for dolphins in southern China (Chen et al. 2005). Some stranded specimen showed strong evidence of capture in fishing nets. They are however very likely the result of incidental catches. Deliberate capture of dolphins does not appear to be a significant threat (Wang and Han 2007). Dolphins in Hong Kong and Pearl River Estuary however associate with pair trawlers (Chen et al. 2005).
4. Conservation and management plan
The long-term goal of this conservation and management plan is to enable the Chinese White Dolphins to continue to use waters as a portion of their population range and to enhance the continued survival of this dolphin population inhabiting the Chinese waters.
To achieve this goal, it includes management, public education, and research. Management will aim at improving the habitat for the dolphins and at minimizing the effects from human activities that may threaten the short-term and long-term survival if the dolphins in China. Public education will improve the community's understanding of the issue and persuade them to support the conservation programme. Research will increase our knowledge of the species (e.g. biology, behavior, distribution, and abundance etc.).
4.1.1 Improve the general marine environmental conditions
Degradation of habitat of Chinese White Dolphin is a threat to the long-term survival of this species. Major efforts should be made to clean-up local waters and improve water quality (AFCD 2000). Due to the high levels of pathogenic bacteria would cause health problems to the dolphins, the standard of treatment of sewage discharging into the dolphins' range should be upgraded to chemically enhanced primary treatment plus disinfection (Parsons 1997b). Illegal use and discharge of toxic substance, such as DDT, should be investigated and eliminated (Minh et al. 1999).
4.1.2 Minimize impacts of coastal development on dolphins
The environmental impact assessment process is an important way to minimize the potential impacts on Chinese White Dolphins. When a project is proposed adjacent to dolphin areas, the proponent would be required to conduct an ecological assessment with special reference to dolphins (AFCD 2000). The mitigation measures would be specified to ensure that the residual impact to dolphins would be kept within acceptable levels (Wang et al. 2008).
4.1.3 Designate more marine protected areas for Chinese White Dolphins
Protecting the areas within the range of the dolphins is one of the most important means of conserving this marine mammal species. Statutory designation of the protected area would go a long way to help maintain the long-term viability of the species.
The Sha Chau and Lung Kwu Chau Marine Park was established near Lantau Island in 1996; The Xiamen Nature Reserve for Chinese White Dolphin was established in the Xigang and Tongan Bay in 1997; The Pearl River Reserve for Chinese White Dolphin was established in Guangdong province in 1999. The function of these Marine Protected Areas is to manage and designate reserves for Chinese White Dolphins, providing good habitats for this specie (Wang et al. 2008).
4.1.4 Ensure protected areas for Chinese White Dolphins are properly managed
The Marine Parks Ordinance and the Marine Parks and Marine Rserves Regulation make provisions for the management and control of marine parks and marine reserves and to prohibit or control certain activities in a marine park. Proper management of the Marine Park is essential to ensure that the Marine Park would benefit the long-term survival of the dolphins (HKDCS).
4.2 Public education
The Chinese White Dolphins' conservation is unlikely to be successful without the support and participation of the community. Effort is needed from government and non-government organizations to arouse public interest and awareness of dolphin protection (HKDCS). On the other hand, increased public interest in dolphins will benefit more commercial or non-commercial dolphin watching activities, which if improperly conducted and managed, may cause harassment to the dolphins (AFCD 2000). Hence, in promoting public interest, the importance of community education should not be lost sight of.
The purpose of protection is to offer more healthy and sustainable development for this species. Therefore, it must develop the research of the Chinese White Dolphins' biology, ecology, and behavior, including distribution, migration, population stocks, population genetic structure, and habitat. Accurate and up-to-date information on the abundance and trends in abundance is vital to conservation and adaptive management of this species. To assess the effectiveness of various actions of the conservation programme, the change in distribution and abundance of Chinese White Dolphin within Chinese waters is an essential piece of information. Continual monitoring of abundance would provide a useful indicator of the success, of the conservation measures that have been taken for the dolphins.
The conservation programme should be a dynamic and interactive programme. It should be implemented in a manner which is responsive to the changing nature of the biological system. The findings and progress of its implementation should be regularly reviewed so that the various conservation actions can be adjusted in accordance with the most up-to-date situation.
Finally, the government and non-government organization is the suitable advisory body to give advice and monitor progress of the implementation of the conservation programme. Scientists, green group members, and fishermen leaders representing views of different stakeholders of the community are also good advisory body to offer advice for conservation programme of Chinese White Dolphins.