Ecological and Biological Characteristics for the Risso's Dolphins (Grampus griseus) off Taiwan, with Conservation Evaluations on Potential Anthropogenic Threats

Risso's dolphin (Grampus griseus) is one of the cetacean species commonly encountered off the eastern coast of Taiwan. The species appears to occur in Taiwanese waters year-round. Many biological and ecological characteristics, including group size, habitat preference, and ages at weaning and sexual maturity, are by-and-large in accord with earlier reports for populations in other geographic regions. Morphological assessments, dietary analyses, and genetic data altogether suggest the dolphins found in Taiwanese and Japanese waters are from the same population, which exhibits high genetic diversity and a sign of recent demographic expansion. The dolphin species is endorsed a "Rare and Valuable Species (Class II)" status and is protected by the Wildlife Conservation Act in Taiwan. However, it is still vulnerable to human activities, in terms of whale-watching tourism and coastal gill-net fisheries, with a growing concern on the level of heavy metal residues found in dolphin tissues. Current knowledge for the species is mostly derived from relatively limited information. Assessments on key biological and ecological features, e.g., population size, residency, social structure, and group composition, shall be prioritised for conservation initiatives.