SUBMON has successfully photo identified 700 bottlenose dolphins in the Montgrí and Cap de Creus area, an extraordinary figure that highlights the remarkable richness of the marine ecosystems in this region. This number is the result of almost 10 years of continuous work, including regular field surveys, systematic data collection, and a sustained effort to better understand how these cetaceans live and move. Today, the COSTtERA-2 project continues these efforts to study bottlenose dolphin populations in Cap de Creus and their relationship with fishing activity, especially trawling vessels. From 2017 to the present day, 196 bottlenose dolphin sightings have been recorded, and 700 different individuals have been identified in the Cap de Creus study area. Currently, 63% of the identified individuals are animals we had already observed in previous years in this area, while 37% are individuals recorded for the first time in 2025.
Dorsal fin of individual number 700. Credits SUBMON.
The bottlenose dolphins observed in Cap de Creus move throughout Catalonia and even beyond the Gulf of Lion. Among the identified individuals, some have been sighted every year of the study, such as Tramuntana.
Dorsal fin of Tramuntana, one of the most frequently sighted. Credits SUBMON.
Who’s who? Photo identification
This work is possible thanks to photo identification, a non-invasive research technique that allows us to recognise dolphins based on the marks they bear, mainly on their dorsal fins, helping us understand their distribution and interactions over time. Photo identification uses the unique marks on each individual’s dorsal fin as an identifier. These marks may include tooth rakes, cuts, scars, spots, or variations in fin shape, among others. Each combination is unique and allows individuals to be recognised over the years. Although tools have emerged to help automate the process, visual review by experts remains essential, as markings can change subtly or more noticeably over time. For example, Pythagoras was sighted in 2018, 2020, 2022 and 2025, and changes in its dorsal fin can be observed over these seven years.
Sightings of Pythagoras over the years. Credits: SUBMON.
Thanks to photo identification, we can track individuals and study not only their movements but also their social organisation. This knowledge is especially relevant given that bottlenose dolphins display a fission-fusion social structure, a system in which groups are not stable and individuals join or leave groups depending on their needs, interests, and interactions. In these societies, group composition changes constantly, and the same individual may belong to different groups within a single day. Group size and structure depend on factors such as prey availability, reproduction, the presence of predators, or social preferences. This dynamic makes studying them particularly complex but also highly rewarding, as each observation provides new insights into their social interactions and behaviours.
A study still in progress
Thanks to the COSTtERA-2 project, we continue to expand our knowledge of bottlenose dolphins in Cap de Creus. The data collected, which are still being analysed, not only help us understand the behaviour and movements of these populations, but also provide key information about their interactions with fishing activity, particularly trawling. To achieve this, both standardised and innovative methodologies are used, and collaboration with the fishing sector is essential.
The COSTtERA-2 project is carried out with the support of the Fundación Biodiversidad of the Ministry for Ecological Transition and the Demographic Challenge, through the Pleamar Programme, and is co-funded by the European Union through the EMFAF (European Maritime, Fisheries and Aquaculture Fund). In addition, the project contributes to the objectives of LIFE IP INTEMARES.
