Drivers of microplastic accumulation in a densely canyoned continental margin: Insights from blackmouth catsharks (Galeus melastomus)

Microplastic pollution is widespread in marine environments, including deep-sea ecosystems. This study investigates the ingestion of microplastics by the blackmouth catshark Galeus melastomus in the densely canyoned continental margin of the Gulf of Lion in the northwestern Mediterranean Sea. The gastrointestinal tracts of G. melastomus specimens collected as by-catch during bottom trawling hauls were analyzed for microplastic contents. Of the examined sharks, 80.32 % had ingested microplastics. Multi-sourced fibers mostly made of polyethylene terephthalate and cellophane were predominant (93 %). After comparison with other Mediterranean locations, our study highlights the role of canyoned margins in, first, trapping microplastics, and, second, easing ingestion by macrofauna. Such a role is likely due to the geomorphological characteristics of such margins, and to specific atmospheric, riverine and oceanographic factors, such as dense shelf water cascading and storms, occurring in the study area. Microplastic ingestion by G. melastomus in the Gulf of Lion is significantly higher than in other Mediterranean regions with different geomorphic and environmental factors, including absence/presence of submarine canyons, relevance of river discharge or occurrence/lack of dense water formation. Our findings suggest that the well-known funneling effect of sedimentary particles in canyoned continental margins also enhances microplastic transport and concentration in these environments, thus increasing exposure of inhabiting deep-sea species. Our results underscore the need for targeted pollution management strategies to protect vulnerable deep-water ecosystems, and place G. melastomus as a candidate sentinel species regarding microplastic pollution in the deep sea.