Small-scale Mapping of Sea-bed Assemblages in the Eastern English Channel Using Sidescan Sonar and Remote Sampling Techniques

A survey was conducted in the eastern English Channel to investigate the use of sidescan sonar, used in conjunction with traditional biological sampling methods, to map the variety and distribution of benthic biotopes (i.e. sea bed habitats and their associated biological communities). An area of sea-bed, approximately 28km×12km in size, offshore from Shoreham, U.K., was surveyed using a digital sidescan sonar system and a mosaic of the output was produced covering 100% of the survey area. This was used to divide the area into acoustically distinct regions, around which subsequent benthic ground-truth surveys were designed. Benthic communities and sediment types within each of the these regions were sampled using a Hamon grab fitted with a video camera, and using a heavy duty 2-m beam trawl. Further information concerning the sea-bed was obtained through the application of additional video and photographic techniques. Substrates within each acoustic region were generally homogeneous in distribution, and sediment types ranged across the survey area from cobbles and coarse gravel through to muddy sands. Analysis of the faunal data revealed the presence of statistically distinct biological assemblages within most of the acoustic regions, although species similarity between samples collected from within each acoustic area was often low. Using a combination of all the data sets, five discrete biotopes could be identified and mapped across the area. The application of acoustic techniques, used in conjunction with biological sampling techniques, to map the distribution of sea-bed habitats and associated benthic communities is discussed.