A time-based method for the study of associations using photo-identification of Grampus griseus in the Northern Ionian Sea.

Social structure is often a key determinant of population biology and therefore investigating the social organization of cetaceans, as top predators and important sentinels of marine ecosystem health, provides useful information for the development of conservation and management plans. Little information is available on Risso's dolphin compared to other species regularly present in the Mediterranean Sea. Its conservation status has recently been classified as Endangered by the International Union for Conservation of Nature (IUCN) in this sea. Therefore, the need to increase information on its biology and ecology is even more urgent. Therefore, this study aims to explore the association pattern of 94 photo-identified individuals of Grampus griseus sighted at least 23 times in the Gulf of Taranto, from July 2013 to September 2024, using a novel time-based method, considering individuals as a group within a 5-min interval, allowing a deeper understanding of their social organisation. The association analysis, performed using SOCPROG 2.9, highlighted the presence of random and short-time associations within the population, however, the test for the null hypothesis of "random association" was rejected and the temporal analysis performed with SLAR suggested the presence of both fluid and stable associations between individuals, showing a sex- and age-related preference at the two levels of association. In particular, both strong associations between individuals leading to male alliances and numerous but less strong associations of females were observed. This reveals the important role of females, in caring for their calves, forming bonds with several other individuals over time and moving within the research area to ensure the protection of their offspring. The cluster and social network analysis showed two socially segregated units but with the presence of floaters between the units. Furthermore, the presence of small organized subgroups within the two main clusters was revealed. All these results describe a basic feature of the fission-fusion model as noted in odontocetes of similar size. However, most Risso's dolphins were observed to be organized in small groups, and with a highly stratified social organization by age and sex classes, characteristics that are not consistent with a fluid society, but have more similarities with stable and long-lasting groups. These results are in line with the results of social studies on groups in the Azores (Hartman et al., 2008) that support the hypothesis of a more complex and structured society, and support the preliminary results obtained by Santacesaria et al, 2022, but this analysis based on a time interval revealed additional insights into social dynamics that the traditional method may have overlooked, highlighting the importance of temporal factors in capturing the complexity of associations within the population. This study confirms the importance of the Gulf of Taranto as a residence and nesting site for Risso's dolphins and therefore a critical habitat that must be taken into account in protected areas of the Mediterranean Sea. Key words: Associations study, Grampus griseus, Social organisation, Time-based method, Nothern Ionian Sea ​