Improved estimates of bottom contact and recovery (RESONANCE)

Seafloor impacts from fishing are a management concern and may constrain UK’s bottom-tendered gear fleets’ ability to access fishing grounds. Empirical evidence on the nature of this interaction is limited and requires further development in order to refine estimates of contact and outline a framework for defining and managing impacts. Technological developments in seafloor mapping, increased accessibility of hardware and software solutions are improving the methods that can be used to help improve these estimates. This study uses a novel approach to determining bottom contact with accelerometer-based tilt sensors, coupled with a before after controlled impact (BACI) design with multibeam echosounder, analysis of sediments to explore chemistry (Carbon and Nitrogen) and particle size. Results show geomorphological changes in surface difference in all areas across all time steps (from <24 hours (min) to 37 days (max), across a gradient of fishing intensities and control areas. Analysis of the sediment showed that the spatial heterogeneity at the site accounted for the differences observed, and that there was limited difference in sediment horizon to depths of 4 cm. In terms of chemistry, there was no evidence of any differences in either inorganic or total carbon that could be attributable to fishing level three weeks after the experimental hauls. Dynamics at the site were greater than expected or predicted by previous studies. This may have been influenced by the weather conditions during the experiment. Textural analysis of backscatter data showed evidence of interaction at timescales <24 hours, beyond this no evidence was apparent. Validation using video was complicated by poor visibility due to heavy rainfall, turbidity and self-shading from algae. Acquisition artifacts caused by sea state and relative orientation of survey lines to sea state compounded the comparison where the impact of these issues was of a comparable magnitude to the effects we were studying. This suggests that the background natural disturbance regime in the site was greater than the detectable impacts using the experimental design in the conditions observed. Those impacts that were detectable were short-lived as far as they could be detected using the technologies deployed. The study was a cooperative effort between academia and a >10 m (12.78 m) fishing vessel MFV Stelimar, using a single rig otter trawl with grass rope ground gear. As such, the applicability beyond this area and gear are limited, although this approach does show a roadmap for coproduction of research between the fleet and science, an aspiration of the Fisheries Industry Science Partnership scheme.