Bioacoustics of Sousa chinensis

Movements, Home range and Social organization. The Indo-Pacific Humpback Dolphin is commonly found alone or in small groups of ten or fewer individuals. In some cases in Chinese waters, as many as 30-40 individuals have been seen together, and in Arabian and Malaysian waters, groups of more than 100 individuals have been reported. Seasonal variations in group size have been detected in populations from Algoa and Plettenberg Bays in South Africa, where group size increases in summer and winter. This variation does not seem to occur among Indo-Pacific Humpback Dolphin in waters off Australia, Hong Kong, or Mozambique. All age classes can be found in larger groups, with adults generally making up one-half to two-thirds of the group. The Indo-Pacific Humpback Dolphin has been observed in mixed groups with the Indo-Pacific Bottlenose Dolphin ( Tursiops aduncus ), the Common Bottlenose Dolphin (7. truncatus ), the Southern Right Whale (Eubalaena australis ), the Snubfin Dolphin ( Orcaella heinsohni ), and the Afro-Australian Fur Seal (Arctocephalus pusillus). Social vocalizations of Indo-Pacific Humpback Dolphins are very important in the murky waters they inhabit. Frequencies of vocalizations from individuals in Australian waters are 0-6-200 kHz. Photo-identification and genetic studies have indicated that the social structure of some populations of Indo-Pacific Humpback Dolphins is quite fluid; associations are shortterm and it is uncommon for individuals other than mother—offspring pairs to have strong bonds. This social structure tends to occur in waters off South African. In Maputo Bay, Mozambique, more long-term associations are observed among individuals other than mother—offspring pairs. In Pearl River Estuary off Hong Kong, there are two communities of Indo-Pacific Humpback Dolphins, one with more long-term associations than the other. Allomaternal care of offspring has been suggested in populations of Indo-Pacific Humpback Dolphins off South Africa and Taiwan. The Indo-Pacific Humpback Dolphin does not undertake any large-scale migrations, but seasonal changes in distribution have been detected in South Africa, Mozambique, and Hong Kong. During the rainy season, Indo-Pacific Humpback Dolphins in Pearl River Estuary range more widely, with increased freshwater input influencing the delta.

Status and Conservation. CITES Appendix I. Classified as Near Threatened on The IUCN Red List. The eastern Taiwan Strait subpopulation is listed as Critically Endangered. Presently, there is no reliable global abundance estimate for the species. However, some estimates of Indo-Pacific Humpback Dolphins in specific areas include ¢.1200 individuals in the Pearl River Estuary of southern China; ¢.80 individuals in Xiamen, China; c.h4=74 individuals in the eastern Taiwan Strait; ¢.237 individuals in the waters around Leizhou Peninsula, southern China; ¢.34-54 individuals in Cleveland Bay, Australia; c.119-163 individuals in Moreton Bay, Australia; c.450 individuals in Algoa Bay, South Africa; ¢.170-244 individuals in Richard’s Bay on the KwaZulu-Natal coast, South Africa; c.105 individuals in Maputo Bay, Mozambique; ¢.60 individuals in Bazaruto Archipelago, Mozambique; and ¢.58-65 individuals in the waters off Zanzibar. For the distinct and isolated eastern Taiwan Strait population, the best estimate is fewer than 75 individuals. The species as a whole is likely in decline. The most serious threats are fisheries catch (incidental and direct) and habitat loss and degradation. The Indo-Pacific Humpback Dolphin is often caught in gillnets, trawls, and anti-shark nets. Vessel collisions have had a large impact off Hong Kong, where 14% of documented strandings in 1993-1998 were attributed to boatstrikes. Environmental contaminants are also a major threat. In Hong Kong, concentrations of organochlorines in cetaceans are significantly higher than in other parts of the world. High levels of neonatal mortality (53% ofstrandings) in Hong Kong may be related to high organochlorine levels. Indo-Pacific Humpback Dolphins dying from oil-related causes in the Persian Gulf have high tissue concentrations of organochlorines and other contaminants. Mercury levels in Indo-Pacific Humpback Dolphins off Hong Kong were an order of magnitude higher than in their prey and, in some cases, were high enough to be considered detrimental to dolphins’ health. These contaminants are known to cause pathological problems, such as immunosuppression and reproductive failure, in other marine mammals. Underwater noise from industrial activities, including pile-driving and dredging, threaten to cause acoustic disturbances. In some areas of Africa and Madagascar, Indo-Pacific Humpback Dolphins are intentionally killed for human consumption. Other threats include noise disturbance from military sonar and seismic surveying, overfishing of prey species, reduction offreshwater flow from upstream damming, and effects of climate change on habitat and prey availability.