10 11 Microplastic pollution is a growing cause of concern for the marine environment, particularly in the 12 Mediterranean Sea, which is considered to be one of the most polluted seas worldwide. In this study, 13 the gastrointestinal tracts of 102 bogues (Boops boops), sampled from three areas off the Catalan 14 coast (Spain) subject to different degrees of industrialization, were analysed to assess microplastic 15 ingestion and thus estimate local levels of microplastic pollution. Microplastics were detected in 46% 16 of samples analysed. As expected, the abundance and frequency of occurrence of ingested 17 microplastics were higher off the most anthropized area of Barcelona. The majority of ingested 18 microplastics were blue fragments ranging 0.1 -0.5 mm, and the most common polymer type was 19 polypropylene. The results of this study indicate the area off Barcelona as a possible area of 20 concentration for microplastics, further supporting the use of B. boops as a bioindicator to assess 21 microplastic pollution. 22 23 Keywords: bogue; indicator species; marine litter; Mediterranean Sea; fish 24 25 Capsule 26 The results of this study indicate the area off Barcelona as a possible area of concentration of 27 microplastics and support the use of Boops boops as a suitable bioindicator for monitoring 28 microplastic pollution in the Mediterranean Sea. 29 30 Declarations of interest: none 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 Highlights 10 manufacturing larger plastic items. Once in the sea, microplastics are driven by oceanic currents, 11 travel long distances due to their buoyancy and durability (Eriksen et al., 2014), and they represent 12 a considerable portion of the litter found in marine waters (de Haan et al., 2019). Recent studies 13 estimated that 5 trillion microplastics are currently floating in the world's oceans and that the 14 concentration of plastic particles floating in the surface waters of the Mediterranean Sea is 890,000 15 particles km -2 (Eriksen et al., 2014). 16 17 Microplastics may pose a threat to the marine environment (Rezania et al., 2018). Marine species at 18 all levels of the trophic chain, including zooplankton (e.g., Cole et al., 2014), worms (Wright et al., 19 2013), shellfish (e.g., Digka et al., 2018), fish (e.g., Bellas et al., 2016), seabirds (Codina-García et 20 al., 2013), sharks (Fossi et al., 2014) and cetaceans (Fossi et al., 2016) have been reported to ingest 21 microplastics. Despite evidence of the translocation of microplastics from the gastrointestinal tract 22 to other tissues, i.e., the presence of microplastics in the hepatic tissue of the mullet (Mugil cephalus) 23 under laboratory conditions (Avio et al., 2015) and in eviscerated flesh of four commonly consumed 24 dried fish species (Karami et al., 2017), related adverse effects in wild organisms are still lacking 25 (Avio et al., 2015). Furthermore, although microplastics are chemically inert, the organic 26 compounds used as plasticizers to improve the properties of plastics might ...