Trophic interactions of the jellyfish Pelagia noctiluca in the NW Mediterranean: evidence from stable isotope signatures and fatty acid composition

Jellyfish have the potential to dominate the pelagic biomass of marine ecosystems, thereby negatively affecting pelagic fish. We investigated the trophic interactions of Pelagia nocti - luca (medusae and ephyrae), one of the most abundant and conspicuous jellyfish on the Catalan coast in the NW Mediterranean. A combination of stable isotope and fatty acid analyses was used to obtain a broad picture of the feeding habits of this jellyfish in order to understand its potential interactions with the most abundant fish species (larvae and adults) during the summer in the area. The results suggested that in addition to predation on fish larvae by P. noctiluca, this jellyfish had similar feeding requirements to those of most fish larvae, suggesting potential competition. The trophic niche of medusae and ephyrae overlapped highly with that of larval Engraulis encrasi colus, Trachurus mediterraneus and Sardinella aurita and to a lesser extent with that of Serranus hepatus, Sparus pagrus and Mullus barbatus. No overlap was observed with Arnoglossus sp. larvae and adult E. encrasicolus, Sardina pilchardus, T. mediterraneus and S. aurita. Our findings demonstrated that P. noctiluca could be an important predator and competitor for fish larvae, but not for adult fish. Moreover, salps were found to be a significant food source for P. nocti - luca. This study provides information that should be considered in near-future ecosystem-based fishery management in regions where P. noctiluca thrives.