Group definition for underwater observation of wild Indo-Pacific bottlenose dolphins

The group definition for an animal population is important for building associations among individuals (i.e., social network). Indo-Pacific bottlenose dolphins ( Tursiops aduncus ) have a society with high fission–fusion dynamics that changes their group memberships over time, making it difficult to define groups clearly. In addition, current group definitions of small delphinid species are mainly related to boat-based or land-based surveys, and few definitions exist for underwater surveys. We propose a group definition for underwater observations after comparing 234 group transitions from 44 surveys of simultaneous boat-based and underwater observations targeting the resident Indo-Pacific bottlenose dolphins around Mikura Island, Japan. Our results highly matched group transitions between boat-based and underwater observations when all video-recorded dolphins during one encounter (from entering to exiting the water) were defined as a group, and groups with at least one shared individual during one survey were treated as the same group. This definition successfully produced highly similar association indices to boat observation. This is useful for underwater surveys of highly maneuverable delphinids, especially for commercial-tour-based studies in which researchers do not have the full decision-making authority for the observations.