Report 424
Broadscale Subtidal Biotope Mapping to the West of the Outer Hebrides, Scotland, UK
(2009)
Mitchell, A
The surveys addressed in this report have focused on a number of sites to the west of the Outer Hebrides, Scotland, on the UK continental shelf (Figure 1). The sites were selected based upon their distance offshore (around 12nm, which represents the extent of national territorial waters and the official boundary between ‘inshore’ waters and ‘offshore’ waters) and existing broadscale bathymetric and sediment information from the British Geological Survey (Graham et al, 2001), which indicated areas of bedrock reef. Bedrock reef habitat is specified under the EC Habitats Directive (Council Directive 92/43/EEC) and although many examples of such habitat have been examined within UK inshore waters, there are far fewer studies of deeper regions offshore. This project aims to map the broadscale biotopes and habitats of the rocky reef sites west of the Outer Hebrides through integrating multibeam echosounder data, acoustic ground discrimination system (AGDS) data, underwater video/camera surveys and grab data, facilitated through the use of GIS. Where possible, biotopes have been resolved and classified using the National Marine Habitat Classification for Britain and Ireland (Connor et al, 2004), and suggestions are made as to their conservation value at a regional, UK and European level. Recommendations have been made for further survey work where appropriate.

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Acoustic remote sensing, video ground-truthing and grab sampling were integrated within a Geographical Information System (GIS) to derive benthic habitat maps for five survey sites off the western Outer Hebrides (west and southwest of the island of Barra). The surveys targeted potential bedrock reef areas, which are a habitat listed under the EC Habitats Directive, at the inshore-offshore waters boundary (12nm from the coast). Background data was limited for these sites due to their distance offshore, and therefore acoustic remotely sensed data were gathered in order to generate bathymetries for each site for use in stratifying the video surveys and to ensure bedrock reef was targeted. Multibeam echosounders were used to generate complete-coverage bathymetric maps (minimum resolution of 5m) and stratify the video surveys for bedrock habitats. Bathymetric data was post-processed to derive hillshaded images, slope angles, rugosity levels and contour lines, which revealed complex topographies at each survey site, with depth ranges of up to 115m. The acoustic backscatter data collected by the multibeam echosounders was analysed to produce backscatter mosaics, which indicate differing acoustic ground-types over each survey area. In addition and where possible, single-beam acoustic ground discrimination system data was gathered to complement the multibeam echosounder data, which enhanced ground-type discrimination.
 
 
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ISSN 0963 8901
 
Please cite as: Mitchell, A, (2009), Broadscale Subtidal Biotope Mapping to the West of the Outer Hebrides, Scotland, UK, JNCC Report 424, ISSN 0963 8901