Impact of trawling on the benthos around oil and gas pipelines: Scottish Marine and Freshwater Science Vol 9 No 13 | Impact of Trawling on the Benthos Around Oil and Gas Pipelines

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.The TV tows were conducted along a transect perpendicular to the pipeline extending to 500 m either side of it, for a duration of between 30 to 45 minutes. A minimum of three TV tows were carried out at regular intervals along each stretch of pipeline. The location of the drop-frame and the digital stills were recorded directly into ArcView. The substrate and macrobenthos were recorded every minute of the tow. The photographs were analysed in Adobe Photoshop Elements 14 for substrate type and species presence. The species were identified to as high a level as possible.Biotopes were allocated based on the substrate and species present according to the Marine Habitat Classification (Connor et al., 2004). As the area covered by the photograph was less than 25 m2, biotope presence was verified on the corresponding video.

In May 2015 Marine Scotland Science conducted a survey on board the MRV Scotia at seven stations coincidental with oil and gas pipelines in the northern North Sea. These stations were selected based on a study using VMS data which found an aggregation of fishing around these areas (Rouse et al., 2017). Side scan sonar was carried out over the pipelines and TV tows were conducted perpendicular to the pipeline. The purpose of this survey was firstly to gain evidence of trawling in the vicinity of the pipelines and secondly to assess the impact on the surrounding seabed. This report firstly details the biotopes classified from the videos and photographs and secondly presents evidence for trawl scars from the side scan and the photographs. The effect of trawling on abundance of benthic invertebrates is investigated using two measures: the total count of organisms recorded on a photograph frame and the total number of sea pens recorded on a frame. Being widespread on muddy ground and vulnerable to trawling, sea pens in particular provide an ideal test case for the benthic impact of trawling in the vicinity of oil and gas pipelines.