Associations between fish and jellyfish in the NW Mediterranean

Fish–jellyfish associations were studied close to Barcelona (NW Mediterranean) during the summer period from 2008 to 2014. Jellyfish and their associate juvenile fish were collected, identified, counted, and measured. Fish behaviour was described by visual field observations and laboratory experiments were performed to determine the survival of the associated fish after being in contact with the jellyfish. In addition, the possible contribution of jellyfish to the dietary composition of the fish was assessed using a combination of biomarkers. Trachurus mediterraneus , Trachurus trachurus , and Caranx rhonchus were associated with the jellyfish Rhizostoma pulmo and Cotylorhiza tuberculata . Trachurus mediterraneus was the most frequent species and their size during the association ranged between 8.4 and 66 mm standard length. The size and number of T. mediterraneus were slightly correlated with the size of R. pulmo , but not with that of C. tuberculata , although more numerous fish were found swimming with C. tuberculata . Behaviour studies showed that juvenile fish swam around jellyfish and into their oral arms seeking shelter without suffering any pain. This survival capability was corroborated by experimental work in which all the specimens of T. mediterraneus survived after being in contact with both jellyfish species. Stable isotopes and fatty acids also revealed an important contribution of R. pulmo and C. tuberculata to T. mediterraneus diet. Defining better the associations between jellyfish and juvenile fish will help to understand the effects of the association on the survival and recruitment of fish species potentially ecologically and economically relevant.