Population biology of the Indo-Pacific hump-backed dolphin in Hong Kong waters

The Indo-Pacific hump-backed dolphin (Sousa chinensis Osbeck, 1765) is foun d throughout the western Pacific and Indian oceans, from southern China and northern Australia in the east to South Africa in the west. Throughout mos t of its range it has not been well studied, and in southern China very lit tle is known of its biology. The goal of the present study was to provide s cientific information needed for the long-term conservation and management of the population that occurs in Hong-Kong waters. From September 1997 to N ovember 1998, 38,105 km of systematic line transect surveys were conducted throughout marine waters of the Hong Kong Special Administrative Region (SA R) of the People's Republic of China (PRC) and adjacent waters to provide d ata on distribution and abundance. Photo-identification of individual dolph ins allowed for examination of movement patterns, home ranges, and social o rganization. Collection of stranded dolphin carcasses and detailed neeropsi es provided information on causes of death as well as samples for life hist ory studies, such as feeding habits, growth and reproduction, ecotoxicology , and stock structure. The dolphin population appears to be centered around the Pearl River Estuar y, and Hong Kong waters represent the eastern portion of the range, which e xtends far into mainland Chinese waters (Lingding Bay) and covers at least 1,800 km(2). Within Hong Kong, dolphins only occur in western waters around Lantau Island. The area north of Lantau Island is heavily used throughout the year and represents by far the most important habitat in Hong Kong. Lin e transect analyses indicate that between 88 (spring) and 145 (summer) dolp hins occur in Hong Kong. Based on 27,6000 photographs taken, 213 individual dolphins were identified. The total size of the Pearl River breeding popul ation is unknown, but is estimated to consist of at least 1,028 dolphins, b ased on line transect analysis. An apparent decline in the number of dolphi ns in the North Lantau area over the period from 1996-1998 was not statisti cally significant. Individual dolphins have overlapping home ranges of abou t 30-400 km(2) in different sections of the population's overall range. Gro ups of dolphins in Hong Kong number up to 23 animals, with an average group size of 3.8 +/- s.d. 3.63 animals. There is no significant seasonal variat ion in group size, but groups feeding behind pair trawl fishing vessels (me an = 8.3 +/- s.d. 7.84) are significantly larger than those in Hong Kong. G roups are very fluid and change composition frequently, with association in dices ranging from 0-0.333. Behavioral patterns are similar to those of oth er coastal dolphins, but Hong Kong hump-backed dolphins only rarely ride bo w waves. Following pair trawlers represents an important feeding strategy f or some individuals. Construction work on an airport fuel facility in the d olphins' main habitat appears to have caused some disturbance (indicated by increased swimming speeds) and possibly temporary evacuation of the surrou nding area. There is a great deal of developmental variation in the color pattern of so uthern Chinese hump-backed dolphins, with a general lightening from newborn to adult stages. Males appear to retain more spots in adulthood than femal es. Length at birth appears to be about 100 cm, and postnatal development i s characterized by rapid growth in the first year and a levelling-off of th e growth curve after reaching adulthood. Asymptotic length is reached at ar ound 243 cm. Length and weight are related exponentially, with the maximum weight about 250 kg. Calving occurs throughout the year, but most young are born from January through August, with a peak in spring/summer. Scant evid ence suggests that sexual maturity in females is reached at about 9-10 year s of age. Dolphins feed mainly on several demersal and pelagic fish species that are often associated with estuaries. There is a lack of evidence for long-distance movements (on the order of hundreds of linear kilometers), an d this presumably results in isolation of groups around major Chinese river mouths. Thus it appears that there may be at least 8 separate populations of hump-backed dolphins along the coast of southern China. The Pearl River Estuary, including Hong Kong, is apparently inhabited by one of these popul ations, although preliminary genetic work has shown only equivocal evidence of population separation from dolphins in the Xiamen area. Human-related c auses of mortality include entanglement in fishing nets and vessel collisio ns. Some environmental contaminants (especially the heavy metal mercury and the pesticide DDT) were found in high levels in some dolphins, and prelimi nary evidence suggests that these may be affecting the health of the animal s. A series of recommendations for management and for further research have be en made to aid in the conservation of these animals. Principles for the con servation of wild living resources should be followed, and information from both the natural and social sciences are needed for proper management. Res earch and long-term monitoring of the population must continue for manageme nt strategies to be evaluated and refined. Overall, the population of hump- backed dolphins that occurs in Hong Kong waters appears to be viable and sh ould be able to survive with appropriate conservation efforts.