West Hebrides (2004)

 

Blue sponge (Hymedesmia paupertas), cup corals and red fish (Helicolenus sp.) on circalittoral cobble © JNCC/SNH/QUB/AFBIColonial ascidian (Diazona violacea) and Axinella cup corals on circalittoral boulder © JNCC/SNH/QUB/AFBICommon sunstar (Crossaster papposus) and erect bryozoans © JNCC/SNH/QUB/AFBI

 

The aim of this survey was to investigate and characterise the habitats and biological communities of five areas of potential Annex I reef off the western Outer Hebrides, west and south-west of the island of Barra. This survey was run in collaboration with Scottish Natural Heritage, the Agri-Food and Biosciences Institute (AFBI) and Queen's University Belfast.

 

Multibeam echosounders were used to generate complete-coverage bathymetric maps and stratify the video surveys for bedrock habitats. Bathymetric data collected on this survey 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.

 

Video tows were completed at each site using either a towed sledge or drop-frame camera system. Where coverage was adequate, habitat maps were produced surrounding the ground-truthed area within each survey site. Video images were semi-quantitatively analysed to give SACFOR epifaunal species abundances and detailed sediment descriptions made to permit biotope or biotope complex classification. This data was spatially linked to the acoustic datasets, guiding the interpretation of areas surrounding the video tows into habitats. Grab samples were used to describe the sedimentary basins surrounding the bedrock outcrops.

 

The bedrock reefs at each survey site showed habitats typical of deep, high energy, exposed environments, with encrusting and erect sponges dominating the epifauna. Encrusting and erect bryozoans, cup corals and keel worms were also common members of the reef epifauna. A total of seven habitats were identified from the reef sites: CR.HCR.DpSp, CR.HCR.DpSp.PhaAxi, CR.HCR.XFa, CR.HCR.XFa.ByErSp, CR.MCR.EcCr.CarSp.Bri, CR.MCR.EcCr.CarSp.PenPor and CR.MCR.EcCr.CarSwi. The bedrock was generally highly fissured. In all but one site, deep gullies separated the bedrock topographic highs, which were often infilled by coarse sands, gravels, cobbles and boulders. The sloping edges of the reefs typicaly showed the highest species richness, particularly further from the surrounding sediment where sand scour may have constrained species richness. The surrounding sedimentary areas were composed of fine to coarse sands, often with a gravel component, which were frequently rippled or megarippled. Boulder fields and extensive areas of coarse sands with cobbles fringed the base of each reef, characterised by the habitat SS.SCS.CCS.PomB and in the south-west Barra site by the habitat CR.MCR.EcCr.CarSwi, with the seafan Swiftia pallida common.

 

Click to download a pdf of the map

 

Further reading

  • Mitchell, A. 2008. Broadscale subtidal biotope mapping to the west of the Outer Hebrides, Scotland, UK. JNCC Report 424.

 

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