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The COSTtERA Project aims to study the behaviour of bottlenose dolphins (Tursiops truncatus) during their interaction with the fishing nets of trawlers and to determine the degree and type of fishing interaction that occurs in the northern part of Catalonia. The interaction between bottlenose dolphins and trawlers occurs in areas where there is a spatial overlap between their distribution areas, since in many cases the target species of the fisheries are the prey species of cetaceans.

This type of interaction has been described in multiple areas of the Mediterranean, and is regularly observed in waters off the Catalan coast as they are an important habitat for the bottlenose dolphin. However, the impact these interactions have on both fishing operations and the dolphin population in this area is still unknown.

During the development of the project, the first underwater images of bottlenose dolphins interacting and feeding on fish caught in trawl nets in northern Catalonia have been obtained. The involvement and participation of the fishing sector has made it possible to carry out the objectives of the project. Specifically, we have worked with the owners, skippers and crew of the Galandu, Roca Forne and Nova Armonia boats, who have made their experience and knowledge available to the project to find the best methodology and to be able to record these images.

Underwater cameras were installed on trawls throughout 19 shipments. In total, 310 hours and eight minutes of video were recorded, in which 329 dolphin behavior events were detected, 315 while the net was dragging, in this case the most frequent being feeding (52.4%) and travel (46.3%). The interactions were mostly carried out individually (83%), with no cases of cooperation between individuals observed even if the sighting was a group of dolphins. Even so, images of up to four dolphins were also obtained, including a mother with her calf.

One of the main questions to be solved during the realization of this project was to know whether or not the dolphins entered the net completely to catch the fish. However, no image was obtained where any dolphin with this behavior is observed. Dolphins, on the other hand, were detected by scanning the outside of the net and feeding on fish that either stood out or got caught in the net.

In parallel to the shipments, the research team carried out 29 trips to study the presence, distribution and surface behavior during their interaction with trawling. A total of 3,117 km were traveled during which 42 sightings were made where the average group size was 8.5 individuals. Of all the sightings made, in 85.7% of the cases the dolphins were interacting with trawlers. In this way, when comparing these results with those existing for the same study area from previous years, an increase in the association of the bottlenose dolphin population with trawling was detected since 2018, with a higher percentage being obtained in 2022. The opposite occurs with group size, which showed a decrease compared to 2018 and 2019, indicating that the groups are increasingly dispersed and distributed throughout the area.

Throughout these outings, dolphins were also photographed with the aim of identifying them. In total , 228 individuals were photo-identified after analyzing almost 30,000 photographs. Up to 43 individuals were seen on more than one occasion throughout the project and 40 of these had already been identified in previous years. As a result of the surface behaviour data obtained, it was possible to detect a frequent behavioural pattern in relation to the activity carried out by the trawler and the arrangement of the group of dolphins. In those sightings where the fishing boat was dragging, the dolphins were scattered behind the fishing boat (90%) while in those cases where the trawler was picking up the net, the dolphins were grouped together (94%).

The COSTtERA project has contributed to the description of the underwater behaviour of bottlenose dolphins when they interact in trawl fishing nets in the Mediterranean and the Iberian Peninsula. The methodology for carrying out this type of study has been established, which can be extrapolated to other areas and to other types of fishing. In addition, this project has made it possible to improve the knowledge of the bottlenose dolphin that lives in the waters of northern Catalonia, as well as to obtain a clearer vision of the level of interaction and the risks that this poses to dolphins and fishermen.