Skinny dolphins: Can poor body condition explain population decline in Indo-Pacific humpback dolphins (Sousa chinensis)?

Indo-Pacific humpback dolphins (IPHDs) who form resident populations along the Chinese coastline are facing a wide range of anthropogenic disturbances including intense fishing and some populations have been shown to experience a severe decline. Body condition is thought to be a good indicator of health since it is linked to survival and reproductive success. In order to better understand population trends, we investigated whether the body condition of IPHDs is poorer in populations whose status is alarming than in other populations. UAV flights were conducted from 2022 to 2023 in four locations (i.e., Sanniang Bay, Leizhou Bay, Jiangmen, and Lingding Bay) in the northern South China Sea. Body ratios were calculated using the body length and widths of IPHDs and were used to analyze differences among seasons, locations, and demographic parameters. A PCA was then used to obtain a detailed picture of the body condition composition of dolphins at each location. Results showed that dolphins from Leizhou Bay and Jiangmen were in better body condition than those from Sanniang Bay and Lingding Bay. Since populations inhabiting Sanniang Bay and Lingding Bay have been shown to experience a sharp decline, it can be hypothesized that poor body condition may have played a role in such a trend. Further investigations of the factors impacting IPHDs' body condition are needed, including monitoring of prey density, contaminant concentration, stress levels, and impacts of human activities on dolphins' behavior. In addition, the creation of a robust scoring method would allow for regular monitoring of IPHDs' body condition to inform conservation measures.

Ranging pattern development of a declining delphinid population: A potential cascade effect of vessel activities.

It is generally accepted that vessel activity causes various behavioral responses of cetaceans and undermines individual fitness. Whether or how it can lead to a demographic response of populations remains rarely examined. In the northern Beibu Gulf, China, vessel activities have sharply increased in the past two decades, while abnormal demographic dynamics was recently noted for the resident Indo-Pacific humpback dolphins. The present study first examined the humpback dolphins' utilization distribution (UD) from 2003 to 2019. Habitat suitability was then modeled with the sighting data collected before the most recent population reduction. Finally, we tried to disentangle the anthropogenic driver of dolphin demography by cross-referring the spatiotemporal development of dolphins' UD, vessel activities, and habitat suitability. Our results showed that the dolphins' UD shrank substantially during the port expansion in the early 2010s, and we suggest that the consequential increase in vessel activities might impose extra marine stressors on the resident humpback dolphins. To reduce the boat interaction, the dolphins steadily shifted their core area to a less suitable area in the east during 2015-2017, when unnaturally low survivals were recorded. Afterward, the dolphin core area partially shifted back to the more suitable area in the west, which corresponded to the improving dolphin survival in 2018. Our finding suggested that the vessel activity may be responsible for the dolphin displacement, while staying in the less suitable area may further lead to a more severe and acute demographic consequence on the population. The underlying and indirect impact of vessel activities as disclosed by the present study is particularly important for port management, marine planning, and conservation practice regarding coastal cetaceans, especially for those resident and endangered populations inhabiting the urbanized coastal areas.