Skin Marks in Critically Endangered Taiwanese Humpback Dolphins (Sousa chinensis taiwanensis)

SIMPLE SUMMARY: The population of Indo-Pacific humpback dolphin (Sousa chinensis) endemic to the inshore and estuarine waters of western Taiwan was identified as a subspecies (Sousa chinensis ssp. taiwanensis) and designated as “Critically Endangered” by the International Union for Conservation of Nature (IUCN). This population is at a high risk of extirpation, faces being impacted by some major anthropogenic threats, and numbers fewer than 65 individuals. Through a combination of long-term observations and photographic monitoring of identifiable individuals and stranding responses, accurate identification and classification of human-induced injuries and skin lesions is an effective approach and is particularly important to the management of the declining population. The results showed that the prevalence of skin marks is higher or comparable to the previous studies on the coastal cetacean populations across the globe. As the total population comprises dozens of individuals, radical measures to conserve the population and reduce environmental hazards are considered indispensable. ABSTRACT: As long-lived apex predators, Indo-Pacific humpback dolphins (Sousa chinensis) are key indicators of marine coastal environmental health. Skin marks can be observed on dorsal body surfaces that are visible during mark–recapture studies that rely on photo-identification (photo-ID) methods. Skin mark prevalence may be an indicator of environmental or anthropogenic stressors in the ecosystem, which may lead to individual and/or population-level health concerns. The prevalence of marks of anthropogenic origin was assessed in the Critically Endangered S. chinensis taiwanensis population along the coasts of central Taiwan. Fifty, twenty-eight, and thirty-four individuals were identified in 2018, 2019, and 2021, respectively. At least one category of injuries was observed in 47 of 57 distinctive individuals (82%), and adults showed a higher prevalence of deep injuries than the other coloration stages. At least one category of skin lesion was observed in 33 of 57 distinctive individuals (58%), and high prevalence of skin lesions was found in mature individuals. Given the difficulty in taking direct observations, skin mark prevalence is proposed as a proxy for estimating habitat health and anthropogenic stressors upon S. chinensis taiwanensis. The moderate-to-high prevalence of skin marks in this study was designated as a warning of risks. This study provides important updated information for the assessment of the health and survival of this population. More effective management measures are urgently needed to reverse the observed population decline.