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Some Introduction to the topic:

The social structure of animal communities is complex in many mammals, playing an important role in population dynamics and behaviour patterns(Genov et al., 2019). Group living is considered an ecological strategy to enhance individual fitness, mainly by increasing foraging success. It can provide various demographic and ecological benefits for animals, especially for highly social species, while disruption of the social structure might lead to long-term negative impacts by decreasing survival of certain individuals, in some cases by decreasing foraging success and resilience of populations to environmental changes (Busson et al., 2019).

Cetaceans display a great diversity of social systems among species from typically matrilineal groups in Killer Whales, where groups are established and from which generally neither males or females leave their natal pod (Bigg et al., 1990), to more fluid systems where individuals part of a wide group form subgroups that frequently change in size and composition(Shane et al., 1986). Beyond that, even within each specie, depending on the region, changes in social systems are found depending on the surrounding environment(Genov et al., 2019). Understanding these systems may play a key aspect in making effective conservation and management plans.

Most of the times social structure of common dolphin (Delphinus delphis) is described as a fission-fusion system with some segregation by sex and age as already demonstrated by genetic analysis (Virisel et al., 2008). It can occasionally form aggregations of hundreds or even thousands of individuals, that result of subgroups’ pods junction. In the Mediterranean Sea the subgroups typically are small ranging from 50-70 individuals to much smaller pods of less than 15 individuals depending on the region (Bearzi et al., 2003) Information regarding social structure of this species in Mediterranean Sea is mainly scarce result of few studies being conducted in the area targeting specifically this topic.

References:

Busson, M., Authier, M., Barbraud, C., Tixier, P., Reisinger, R. R., Janc, A., & Guinet, C. (2019). Role of sociality in the response of killer whales to an additive mortality event. Proceedings of the National Academy of Sciences of the United States of America. https://doi.org/10.1073/pnas.1817174116

Bigg, M. A., Olesiuk, P. F., Ellis, G. M., Ford, J. K. B., & Balcomb, K. C. (1990). Social organization and genealogy of resident killer whales (Orcinus orca) in the coastal waters of British Columbia and Washington State. Report - International Whaling Commission, Special Issue, 12.

Genov, T., Centrih, T., Kotnjek, P., & Hace, A. (2019). Behavioural and temporal partitioning of dolphin social groups in the northern Adriatic Sea. Marine Biology. https://doi.org/10.1007/s00227-018-3450-8

Shane, S. H., Wells, R. S., & Würsig, B. (1986). ECOLOGY, BEHAVIOR AND SOCIAL ORGANIZATION OF THE BOTTLENOSE DOLPHIN: A REVIEW. Marine Mammal Science, 2(1). https://doi.org/10.1111/j.1748-7692.1986.tb00026.x

Bearzi, G., Reeves, R. R., Notarbartolo-Di-Sciara, G., Politi, E., Canadas, A., Frantzis, A., & Mussi, B. (2003). Ecology, status and conservation of short-beaked common dolphins Delphinus delphis in the Mediterranean Sea. Mammal Review, 33(3–4). https://doi.org/10.1046/j.1365-2907.2003.00032.x

Viricel, A., Strand, A. E., Rosel, P. E., Ridoux, V., & Garcia, P. (2008). Insights on common dolphin (Delphinus delphis) social organization from genetic analysis of a mass-stranded pod. Behavioral Ecology and Sociobiology, 63(2). https://doi.org/10.1007/s00265-008-0648-7