Impact of Trawling on the Benthos Around Oil and Gas Pipelines

Scottish Marine and Freshwater Science Vol 9 No 13
Where fisheries actively target specific areas there may be a disproportionately higher impact on the seabed than in less targeted areas. Previous analysis of VMS data has demonstrated a high level of fishing around oil and gas pipelines in the North Sea. This is thought to be due to a reef effect which attracts fish to the pipeline. We present side scan and photographic imaging which clearly reveals evidence of bottom trawling within an area of 500 m either side of the pipelines. Investigation of individual photographs on transects running over the pipeline, point towards evidence for a reduction in benthic fauna on seabed where there are trawl marks compared to seabed where there are not. This likely effect is also evident on sea pens which were commonly found on the muddier ground in the survey areas. Two of the most frequent biotopes, “burrowed mud” and “sea pens and burrowing megafauna in circalittoral fine mud”, are the focus of conservation efforts through OSPAR and as ‘Priority Marine Features’ in Scottish waters. As the North Sea is a mature basin for exploitation of oil and gas, many pipelines are being considered for decommissioning.The report considers the implication of this benthic impact of fisheries on decisions for pipeline decommissioning. The accompanying dataset was collected in order to inform users of the habitats and biotopes in areas surrounding oil and gas pipelines in the northern North Sea. A drop-frame TV camera system was towed behind the MRV Scotia at a speed of ~1 knot. A digital stills camera (Canon) was mounted on the drop-frame together with a high definition and standard definition video (Kongsberg Simrad). The drop-frame was suspended 1 m above the seabed, guided by a steel weight attached by a line to the drop-frame. Maintaining the steel weight (63.5 mm diameter) on or just above the seabed ensured the correct height for accurate focussing of the video and digital camera. Video was recorded continuously together with digital photographs taken at one minute intervals for the duration of the transect including the pipeline feature itself. Two laser pointers set 68 mm apart provided a scale for identifying features.