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

Demersal trawling causes significant impact on marine benthic habitats including sediment biogeochemistry and faunal composition. To examine the effect of shrimp trawling on benthic sedimentary habitats a replicated Before/After (B/A) experiment was performed in the Gullmarfjord, Western Sweden, after 6 years protection against trawling activities. The experimental area was randomly divided into three control transects and three trawling transects each f 1.5 km long. The experimental fishery activity was designed to mimic the extent and the trawling procedure used before the banning with 80 hauls per site and year. All transects were sampled three times in 1996 before the experimental trawling activity was started, and three times during the experimental trawling activity in the autumn of 1997. Along each transect, 10 replicated sediment profile images (SPIs) were taken randomly at each sampling. The status of the sedimentary habitat was assessed by the parameterisation of a benthic habitat quality (BHQ) index combining sediment surface and subsurface variables and the redox conditions observed in the SPIs. A significant interaction between B/A and treatment was observed in mean BHQ-index. Changes of the BHQ-indices were larger in trawled areas than the natural variation in the experimental area. Mean BHQ-indices in trawled transects were lower than in control transects after the experimental trawling activity was started. In about 43% of the SPIs a severe mechanical disturbance was observed, which introduced a significantly increased spatial variance of BHQ-indices in trawled areas compared to control areas. D 2002 Elsevier Science B.V. All rights reserved.