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.

Automated summary

The definition of animal populations is crucial for understanding social networks among individuals. However, defining groups of Indo-Pacific bottlenose dolphins is challenging due to their fission-fusion dynamics and lack of underwater survey definitions. Through comparing 234 group transitions from simultaneous boat-based and underwater observations, we propose a group definition for underwater surveys of these dolphins. Our definition, which considers all recorded dolphins in one encounter as a group and treats groups with at least one shared individual as the same group, produces highly similar association indices to boat observations. This is valuable for studying highly maneuverable delphinids, particularly in commercial-tour-based studies where researchers have limited decision-making authority for observations.