THE EFFECTS OF BEAM-TRAWL DISTURBANCE ON INFAUNAL COMMUNITIES IN DIFFERENT HABITATS

1. Beam-trawling is a source of physical disturbance to marine sedimen tary communities in areas less than 50 m deep, on the western European continental shelf. Chains attached between the beam-trawl shoes are d esigned to penetrate the upper few cm of the sediment, which leads to the damage or removal of some infaunal and epifaunal species. In some areas, beam-trawling may be frequent and intense, leading to speculati on that it may generate long-term changes in the local benthic fauna. 2. As part of a larger MAFF study examining the ecological effects of beam-trawling, we investigated its local impact on an infaunal communi ty in the north-eastern Irish Sea. Studies of this type are complicate d by the heterogeneity of the environment, hence we adopted a replicat ed, paired control and treatment design to maximize the chances of det ecting any effects due to trawling. 3. A side-scan sonar survey reveal ed that the experimental area was characterized by mobile megaripples in the south-eastern sector of the experimental area and stable sedime nts with uniform topography in the north-western sector. Multivariate analysis of the species abundances from the control areas separated th e fauna into two distinct communities which corresponded to the differ ent substratum characteristics. Data from the two regions were therefo re treated separately when testing for the effects of trawling. 4. In the north-western sector, trawling led to 58% decrease in the mean abu ndance of some taxa and a 50% reduction in the mean number of species per sample. Multivariate analysis revealed that differences between co ntrol and fished sites were largely due to the reduction or removal of less common species. These effects were less apparent in the mobile s ediments of the south-eastern sector, which had a naturally impoverish ed fauna and high level of heterogeneity. 5. Univariate variables, suc h as abundance and the total number of species per sample, indicated t hat the variation between replicate samples increased as a result of t rawling disturbance. However, examination of the community data using an index of multivariate dispersion revealed no difference between fis hed and unfished areas. This suggests that the effects of fishing dist urbance are consistent between replicate samples. 6. Fishing with deme rsal gears modifies communities in relatively stable sediments. Freque nt and repeated physical disturbance by fishing gears may lead to long -term changes in the benthic community structure of these habitats.