Biogenic detritic beds with accumulation of free-living calcareous algae, known as “maërl” or “rhodoliths”, are one of the most biodiversity-rich habitats in the Mediterranean Sea. Rhodoliths form a perennial, heterogeneous habitat that houses a complex structural and functional community, helping to create an ecosystem with high species diversity, as well as a nursery area, food and shelter for many species of commercial interest. The importance of this habitat has determined its designation as habitat of Community importance of the Barcelona Convention and the inclusion of some of its constituent species in Annex V of the Habitats Directive (92/43) of the European Union. Maërl/rhodolith beds are threatened by human impacts; among them, bottom trawl fishing is a major cause of disturbance. However other fisheries may also produce impacts. The trammel-net, one of the most traditional fishing gears in the Mediterranean, is widely used by artisanal fleets to catch various species of high commercial value, including the European spiny lobster Palinurus elephas (Fabricius, 1787) and the red scorpionfish Scorpaena scrofa Linnaeus, 1758, and its impacts on maërl habitats are poorly known. The ubiquity of fishing activities over maërl beds has hampered research on their impacts for lack of reference values of a pristine state. Maërl / rhodoliths beds are characteristic of the Marine Reserve Columbretes Islands (NW Mediterranean). The reserve, 30 miles away from the coast and with an extension of 55 km , is protected from extractive activities since its creation in 1990, while both bottom trawl and artisanal fisheries continue to operate in its surroundings. This study examines maërl species composition and structure within the marine reserve and in open fishing grounds adjacent to it to assess the cumulative impacts of fishing. The results of the first sampling survey are presented

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