K:07 Relating acoustic seabed classification to marine benthic habitats. Examples from the western Portuguese shelf using single-beam echo sounders Rosa Freitas 1 , Ana Maria Rodrigues 2 and Victor Quintino 3 Departamento de Biologia, Universidade de Aveiro, 3810-193 Aveiro, Portugal 1 rfreitas@bio.ua.pt ; 2 anarod@bio.ua.pt ; 3 vquintino@bio.ua.pt Introduction Traditional mapping of seabed habitats is usually based on limited sampling sites due to the time and costs involved. Therefore, because only minor parts of the study areas are sampled, detailed maps are difficult to produce. Advances in acoustic technologies brought new opportunities to avoid these limitations and to explore and describe marine environments. Having the ability to combine non-intrusive characterisation of seabed properties with bathymetric information, the acoustic classification of seabed habitats is becoming widely used for the remote characterisation of the seafloor, including in the marine biological sciences. New instruments, such as QTC VIEW TM seabed classification system used in the present study, map differences in seafloor characteristics by processing the acoustic signals from echo sounders. It has been shown that physical properties of the seabed influence the acoustic response (Collins, 1996; Hamilton et al., 1999; Preston et al., 1999; Kloser et al., 2001; Preston, 2001; Anderson et al., 2002; Ellingsen et al., in press; Freitas et al., in press). The purpose of this study was to analyse the reliability of the acoustic approach to the mapping of marine soft bottom habitats, having in mind its future application on a large-scale perspective. The study was conducted in two areas of the Portuguese coastal shelf: off the Tagus estuary, a mid-shelf area was selected, with depth ranging from 30 to 90 meters; off “Ria de Aveiro”, an area closer to shore was investigated, with depth ranging from 5 to 35 meters. Both areas are characterised by the absence of bedrock and algae mats. Sediment grain size off the Tagus estuary ranges from clean fine sand to mud with silt and clay content above 75% of total