Shiang-Lin Huang

Population Size and Habitat Characteristics of the Indo-Pacific Humpback Dolphin (Sousa chinensis) Off Donsak, Surat Thani, Thailand

Investigations on the distribution, population size, and habitat characteristics of animal populations provide the baselines for sound conservation management. Southeast Asia is considered an important habitat for the Indo-Pacific humpback dolphin (Sousa chinensis), but information regarding their population size and habitat characteristics is limited. The present study investigated the distribution and habitat characteristics of humpback dolphins off Donsak, Thailand, and estimated the population size based on photo-identification records. Using the POPAN model, the minimum population size of the humpback dolphins off Donsak was estimated to be 193 (167 to 249; 95% confidence interval), with 36 calves, 58 juveniles, 40 subadults, and 59 adults. The progressively ascending cumulative sighting curve implied that the actual humpback dolphin population size in the investigated area is likely higher than this estimate. Principal components analysis of the environmental characteristics indicated that the adult dolphins tend to occur in deeper and clearer waters relative to the younger age classes. Alterations of the coast environment and coastal anthropogenic activities may be particularly deleterious for younger dolphins. Findings from this study contribute significantly to our understanding of the humpback dolphins in Thailand and provide valuable insight for future conservation management.

A case of epimeletic behavior and associated acoustic records of Indo-Pacific humpback dolphins (Sousa chinensis)

Several cetacean species have demonstrated epimeletic behavior that relies on the tight social bonds between conspecifics. These behaviors and the corresponding vocalizations were recorded during a rare encounter with a group of Indo-Pacific humpback dolphins (Sousa chinensis) that included a presumed mother and deceased calf, in Sanniang Bay, China. The observed dolphins were divided into 2 apparent groups: 1) a central group with the presumed mother and her deceased calf, along with 1 to 6 other individuals swimming in synchrony with the presumed mother; and 2) a following group of several individuals, dispersed over varying distances from approximately 20 to > 300 m, that swam in the same direction as the central group. The mother was seen mostly supporting the calfs body using her back, anterior to the dorsal fin and posterior to the melon, while the other members of the central group exhibited standing-by behaviors (i.e., remaining close to the deceased calf but not providing aid). Whistles in this context were of a longer duration and a higher complexity in the frequency modulation compared to social contexts. Several whistle types were also repeated frequently. This paper provides a detailed description of epimeletic behavior and the whistles possibly associated with that behavior in an endangered population of Indo-Pacific humpback dolphins.