The social behaviour of the common bottlenose dolphin Tursiops truncatus (Montagu, 1821) in the coastal waters of Lampedusa Island (Strait of Sicily, Italy)

Research on social structure and residency provides important information on the dynamics of a population, improving knowledge of the species' behaviour and ecology. This study investigated the social network, site fidelity, and residency patterns of common bottlenose dolphins (Tursiops truncatus) off Lampedusa Island. Direct visual detection and photo-identification methods were used to collect data during 124 boat-based surveys conducted from 2013 to 2020. A total of 117 groups (mean size 4.5 ± 3.3) were sighted; 73 well-marked animals were identified, of which 59% were resighted over the years. The estimate of social differentiation was 0.72 (SE = 0.07), suggesting differentiated social groups within the community. Data were somewhat representative of the true social system, as the correlation coefficient was close to 0.4. However, the half-weight index averaged 0.07 (±0.03), indicating individuals' low tendency to associate. Five social clusters were identified with strong relationships between individuals within each cluster, in which 94% of maximum association indices were ≥0.4. The data also highlighted preferred and avoided associations and stable relationships. The removal of individuals with the highest values of strength and eigenvector centrality from the network did not affect its connectivity, suggesting inherent robustness. Despite the different levels of site fidelity among individuals, the results indicated rather high levels of yearly sighting rate. Temporary migration into and out of the area shown by this study indicates that Lampedusa coastal waters are an important, regularly used bottlenose dolphin habitat. These results suggest extensive ranging behaviour, which implies the need for networking between institutions and organizations from the various states bordering the Strait of Sicily to address conservation efforts and inform regional management of the Mediterranean bottlenose dolphin population.