Social structure of the bottlenose dolphin (Tursiops truncatus) in the Thermaikos Gulf

This study represents the first attempt to investigate the social structure of the common bottlenose dolphin (Tursiops truncatus) inhabiting the Thermaikos Gulf, Greece. This area is characterized by high socio-economic and ecological value, but it is subject to intense anthropogenic pressure. Studying the ecology, social organization, and behaviour of the local bottlenose dolphin population is therefore crucial to assess the condition of the species and to build effective management and conservation strategies. Between 2021 and 2025, 85 boat-based surveys were conducted, resulting in 45 sightings and the photo-identification of 133 individuals. Association analyses using the Half-Weight Index (HWI) were performed in SOCPROG 2.3 on 26 well-marked and frequently re-sighted individuals. The overall mean HWI (0.21) was consistent with other values reported for other Mediterranean coastal populations, indicating a fluid fission–fusion society. Although permutation tests did not reveal statistically significant preferred associations, results were close to the threshold of significance and suggested a tendency to non-random relationships and the presence of a probable social pattern. Cluster and network analyses identified three loosely connected subgroups, showing variable degrees of sociability within the population. This study provides a valuable basis for future research on the species in the region and highlights the importance of consistent, long-term, and standardized monitoring to support effective conservation planning in one of the most anthropogenically impacted areas of the eastern Mediterranean.