The purpose of this broad-scale mapping survey in the Sound of Barra and in the Sound of Eriskay in the vicinity of the causeway was to: extend the coverage of subtidal biotope mapping to encompass the central and western parts of the Sound; to conduct intertidal biotope mapping over the whole of the Sound including those areas surveyed subtidally in 2001 and to produce comprehensive biotope maps; and to assess biological changes occurring in the Sounf of Eriskay. This study utilised satellite imagery, underwater acoustic sensing techniques and biological ground surveys to develop a biotope distribution map of the area and relate this distribution to prevailing environmental conditions. The work was undertaken for SNH by the Universities of Heriot-Watt (HWU) and Edinburgh (EDU) between the 9th July and 12th August 2006 and acoustic data was supplied under subcontract by Agri-Food and Biosciences Institute (AFBI). To collect this detailed biological information on the seabed species and habitat features present, records were acquired from a total of 204 subtidal stations. Of these, 137 were surveyed by drop-down video and 67 by diver. A total of 46 shore locations were visited and records acquired from 258 shore zones. Sediment infauna samples were collected from 56 locations (40 subtidal and 16 intertidal). 5 sites, previously surveyed in 2001 around the Eriskay causeway, were revisited and resurveyed by divers recording details of substrate, environmental conditions and biological communities. The images, analysed under SNH contract by Heriot-Watt University, University of Edinburgh and Agri-Food and Biosciences Institute, Belfast, were used to describe the nature of the seabed in terms of the physical structure and the species assemblages and assess anthropogenic induced change.