Effects of demersal trawling on marine sedimentary habitats analysed by sediment profile imagery

Demersal trawling causes one of the most widespread physical and biological changes in marine shallow and shelf sedimentary habitats: trawl otter boards may create furrows in the sediment surface, while trawl nets and attached weights scrape the sediment surface. As a consequence, benthic animals are disturbed or killed, and resuspension of particles increase. The impact of trawling on benthic animals has traditionally been analysed by changes in species composition and abundance, whereas frequency and distribution of trawl tracks are frequently analysed by side-scan sonar. We have used sediment profile images (SPIs) (30 x 22 cm) and observed furrows and other physical disturbances on the sediment surface that we attribute to trawling. In a manipulative experimental trawl study in Sweden (BACI design), significant impacts were found in trawled benthic habitats (73-93 m deep) compared with pre-trawling conditions and with reference areas. In particular, furrows from trawl boards had a severe ecological impact. In the Gulf of Lions (northwest Mediterranean), similar patterns were observed in the vast majority of 76 images taken at random at depths between 35 and 88 m in four different areas. Epifauna and polychaete tubes were generally either rare or not observed at all on trawled sediment surfaces. Burrows and feeding voids were, however, frequently present in some trawled areas and seemed to be comparatively less affected. Such biogenic structures in the sediment were generally associated with rather deep (3-4 cm) mean apparent redox profile discontinuities (aRPDs), which were measured digitally as the visible division zone between oxidised (sub-oxic) and reduced sediments. Increased roughness caused by the trawl boards acting on the sediment surface, e.g. depressions and protrusions, could have effects on sediment solute fluxes. (C) 2002 Elsevier Science B.V. All rights reserved.