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The Gulf of Cádiz represents an area of extensive seepage of the south-western Iberian Peninsula that is located between two continents and basins, where a high geomorphologic, sedimentary and environmental complexity occurs. In the present study, epibenthic communities were investigated in a mud volcano field containing four mud volcanoes (Gazul, Anastasya, Pipoca and Tarsis), one diapir/mud volcano complex (Chica) and several diapirs that are all located in the upper and middle slope of the Spanish margin of the Gulf of Cádiz (300–700 m depth). Faunistic samples were collected in 2010 and 2011 using a beam-trawl in different sectors of the mud volcano field, including fluid venting submarine structures and their adjacent seabeds. Environmental variables from the sediment and water column as well as the commercial bottom trawling activity were also measured in the same sampled areas for studying relationships with the fauna. A total of 325 taxa were found in the samples, including sponges, molluscs, cnidarians and decapods the most diverse groups. The epibenthic communities displayed differences between sectors that were related to depth, sediment features, characteristics of the water masses and the trawling activity. Six different assemblages were found including deep-sea sponge aggregations and cold-water coral banks in Gazul, echinoid and solitary coral aggregations in Gazul, sea-pen communities that were sometimes intermixed with sponge and gorgonian aggregations in Tarsis, Chica and Pipoca, and low density sea-pen communities with components of chemosynthesis-based communities in Anastasya. Species richness was higher in Gazul, Pipoca and Chica due to the presence of complex habitats (e.g. cold-water coral banks, sponge aggregations) and low in Anastasya due to the scarce epifauna, high trawling activity and low near-bottom current speed. Typical cold seep chemosymbiotic bivalves of the Gulf of Cádiz were not detected in beam-trawl samples due to their infaun

Bottom trawling activity, commercial species and benthic and demersal assemblages were studied integrating Vessel Monitoring System data (VMS), fishing logbook data and otter trawl samples in a mud volcano (MV) field of the Spanish margin of the Gulf of Cádiz (GoC) in 2011. High trawling activity was detected around Anastasya MV and between the Cádiz and Guadalquivir diapiric ridges, and low trawling activity around Gazul MV and in the deepest areas (Huelva Channel and southwards). A total of 11 938 individuals from 126 benthic and demersal species were collected in the samples. Multivariate analyses mainly grouped samples according to depth and sediment type and 3 different assemblages were detected. The Stratum 1 assemblage (Gazul MV, 441–470 m depth, muddy sand, low trawling activity and high near-bottom current speed) was dominated by Parapenaeus longirostris, Leptometra phalangium and Plesionika antigai, among others, and displayed high species richness and abundance. The Stratum 2 assemblage (Anastasya and Tarsis MVs, Triangle, and the External zone, 528–622 m, mud/sandy mud, medium to high trawling activity and low near-bottom current speed) was dominated by P. longirostris, Plesionika martia, Nezumia aequalis and small chondrichthyans (mainly sharks), among others, and displayed low species richness, abundance and biomass. The Stratum 3 assemblage (Huelva Channel > 700 m, sand, low trawling activity and high near-bottom current speed) was dominated by similar species to Stratum 2, but also included large demersal sharks (Centrophorus granulosus, Galeorhinus galeus, Dalatias licha) that resulted in a high biomass. Depth and near-bottom current speed showed strong relationships with the benthic and demersal assemblages in Principal Component Analyses. The occurrence of medium to intense trawling activity in and around some MVs (Habitat 1180, Council Directive 92/43/ECC-Habitats Directive) should imply an effective regulation that allows a balance between susta