West Hebrides (2004)
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.
Further reading
- Mitchell, A. 2008. Broadscale subtidal biotope mapping to the
west of the Outer Habrides, Scotland, UK. JNCC Report
424.
Project partners
If you have any further questions about JNCC's offshore survey
work then please contact us.