Particle Size Analysis (PSA) and organic carbon content data from marine sediment sampling of two plots within the Trevose Box off south-west England between 14 - 18 April 2023 before and after fishing. This DEFRA funded FISP (Fisheries Industry Science Partnerships scheme) project aimed to understand the acute effects of scallop dredging and beam trawling on seabed carbon. The project was led by Bangor University in collaboration with Imperial College London, CEFAS, Western Fish Producers Organisation and the South Western Fish Producer Organisation. The study was conducted in two areas offshore south-west England, UK. The areas were within the Trevose box, which is a management area closed to bottom fishing for the months of January, February and March each year. This allowed us to sample areas which had not been exposed to fishing immediately before our sampling which would reduce the chance of seeing any effect from our experimental fishing. One plot was designated an area that would be experimentally fished by a beam trawler, and another by a scallop dredger (Figure 1). The plots were chosen to minimise differences in depth, seabed habitat, sediment type, wave energy and tidal currents within and between them. Both were chosen where their respective types of fishing gear had been used previously, although scallop dredging has been less common in this area in recent years. The sampling from the RV Prince Madog and experimental fishing from the fishing vessels took place between 14 – 18th April 2023. Before any fishing activity took place, grab sampling was conducted in the dredge plot and in the beam trawl plot that would be later fished to varying amounts, which will be referred to as ‘Times fished’ or ‘fishing intensity’. A 0.1 m2 Day grab was used. When the day grab was onboard two cores were used to subsample the grab sample as deep as possible, these were then sliced in 1 cm intervals and frozen onboard for later carbon analysis. A homogenised sample from the day grab was also taken for Particle Size Analysis. After the ‘Before’ grab samples were taken, a beam trawler and scallop dredger towed their gear along the sampled lanes in each respective plot. The 'After’ samples were taken after that fishing activity. Please see report for full details on how the PSA and organic content values were determined. Whitton, T.A., Austin, M., Newbould, A., Kennedy, H., Allender, S., Cavan, E., Parker, R., North, C., Hatchman, J., Hiddink, J.G. (2024) The impact of mobile demersal fishing on blue carbon in seabed sediments. Report to DEFRA. https://research.bangor.ac.uk/portal/en/researchoutputs/the-impact-of-mobile-demersal-fishing-on-blue-carbon-in-seabed-sediments(1fa1845a-6669-4b5b-83e1-234a6eafc75c ).html