The analysis of 255 bottom trawl samples obtained in annual experimental surveys (2007–2010) along the western Mediterranean shows the existence of five well-defined demersal assemblages that follow a depth distribution: (a) upper shelf assemblages, including two assemblages differentiated by the type of substrate (sand-muddy and terrigenous muddy bottoms); (b) a middle shelf assemblage; (c) an upper slope assemblage; (d) a middle slope assemblage. Faunally, they are dominated by fish (37% of 452 total species), followed by crustaceans (22%), molluscs (17%), echinoderms (9%), and other invertebrates (15%). The assemblages identified showed major alterations on the shelf and shelf edge and less pronounced ones on the upper and middle slope. The average diversity values were more or less high, evidencing the high species richness in the western Mediterranean. The identified assemblages may facilitate future multispecies fisheries management based on an ecosystem approach. 1. Introduction An assemblage is a group of species whose distribution in space and time overlaps in a certain area, sharing a same habitat, bathymetric range, and so forth. A species assemblage describes the collection of species making up any cooccurring community of organisms in a given habitat or fishing ground. Often, these assemblages are also called communities. In biological terms, a community is a group of interacting organisms sharing a populated environment. In the case of the demersal species, species that dwell at or near the bottom interacting among them according to their behaviour, the identification of these assemblages according to species composition, spatial distribution, and stability will allow to determine the degree of incidence of the impacts of anthropic origin, specially trawl fishing, on them. The impacts of fishing on ecosystems are diverse in nature [1–3]. Trawling cannot only cause damage to physical components of the habitats, and therefore implying a reduction in biodiversity, but also changes the community structure, species abundance, and their size structure. In summary, it can be said that in addition to the physical alteration of habitats, fishing causes a reduction in population size of both target and nontarget species, alters their demographic structure, and substantially modifies the structure and composition of communities [4–8]. Communities less impacted by trawling sustain more biogenically habitat-structured communities (e.g., more abundance of sessile suspension feeders at the less disturbed sites versus higher dominance of small
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