Researchers study the survival of rays caught by bottom trawling

Palma, Thursday, 16 September 2025. The Instituto Español de Oceanografía (IEO) and the Instituto de Ciencias Marinas de Andalucía (ICMAN), both from the Consejo Superior de Investigaciones Científicas (CSIC), and the NGOs Sociedad para el Desarrollo de las Comunidades Costeras (SOLDECOCOS) and the World Wildlife Fund for Nature (WWF Spain), have evaluated the post catch survival rate of rays after being caught by bottom trawl gear. The survival experiment was carried out during the MEDITS survey conducted last June in waters off Mallorca and Menorca on board the R/V Miguel Oliver from the General Secretariat for Fisheries of the Ministry of Agriculture, Fisheries and Food. For two days after capture by bottom trawl, the behaviour, respiratory rate, and various physiological parameters measured in blood and dermal mucus were monitored in a total of 121 individuals from five species of rays. The methodologies used are innovative and include an on board aquarium system with temperature, oxygen, and water quality control, where the animals were kept after capture.

The analysis of the collected data will not only allow the evaluation of survival rates, but also the physiological recovery of the individuals, and ultimately, the establishment of a relationship between their recovery capacity and behaviour after capture. The experiment supports the activities of the LIFE-PROMETHEUS project in the Balearic Islands, where the assessment of post-catch survival will allow to estimate the actual number of successfully marked individuals in the mark and recapture experiments included in this project.

Preliminary results indicate very high survival rates, above 90%, for individuals from more coastal species, and more moderate rates, 30–50%, for deeper species. These should be considered conservative estimates, as under commercial conditions the individuals would be returned directly to their natural environment. Precisely, estimating survival under commercial conditions is the goal of the actions launched, in coordination with these experiments, by the NGOs SOLDECOCOS/WWF and the fishing sector, with the participation of the Motril Producers Organisation (OPP85). On board commercial trawlers, and under strictly commercial conditions, the behaviour of discarded individuals is assessed just before their release back to the sea, and dermal mucus samples are also collected.

Xisco Ordinas, Ignacio Ruiz-Jarabo and Jorge Sáez, researchers at IEO, ICMAN and SOLDECOCOS respectively, agree in stating that: “The results generated will help improve handling protocols for these species on commercial vessels and reduce fishing mortality of discarded individuals from commercial, vulnerable and protected species — a key aspect to advancing the sustainability of the trawl fishery in the Mediterranean.”

The MEDITS surveys are carried out under the coordination between IEO-CSIC and the Ministry of Agriculture, Fisheries and Food, which provides the vessels and their equipment to the research staff. They are co-financed by the European Union through the European Maritime, Fisheries and Aquaculture Fund (EMFAF) within the National Fisheries Data Collection Programme, whose results are submitted to the European Commission in accordance with Regulation (EU) 2017/1004 of the European Parliament and of the Council.