Mediterranean fouling communities assimilate the organic matter derived from coastal fish farms as a new trophic resource

•We examine macroinvertebrate species from an active fish farm and a control place.•Fatty acids are used as trophic markers of fish feed and faeces consumption.•Oleic and linoleic acids show high percentages in samples from the active farm.•Almost macroinvertebrate species take advantage of uneaten feed pellets and faeces.•Fatty acid analyses are a reliable tool for assessing impact of aquaculture sea-cages. Currently, the lipid content of fish feeds includes high amounts of terrestrial vegetable oils, rich in n-6 fatty acids and poor in n-3 fatty acids. Sinking organic matter in the shape of fragmented pellets and fish faeces could be ingested by the surrounding fauna attracted to the submerged structures of aquaculture facilities or living in natural benthic habitats. Fatty acids contained in feed pellets were used as trophic markers to shed light on the assimilation and incorporation of aquaculture wastes by the invertebrate fauna associated to sea-cages. Eighteen macroinvertebrate species, and zooplankton, seaweeds and sediments were collected from two fish farms, one of which (control) had not been used as such for two years. This study demonstrates that macroinvertebrate fauna present in fouling can take up sinking organic matter from farms. Further research should be directed at assessing the potential implications of aquaculture production for the surrounding ecosystem.