Mapping organic carbon vulnerable to mobile bottom fishing in currently unfished areas of the Norwegian continental margin

Organic carbon stored in continental margin sediments might be at risk by widespread mobile bottom fishing, potentially leading to reductions of organic carbon stocks, increased ocean acidification, additional atmospheric carbon dioxide emissions and a reduction of the buffering capacity of the ocean. Spatially explicit studies that have been conducted to inform marine management have so far looked at organic carbon stocks that have already been affected by mobile bottom fishing. Here, we focus instead on areas on the Norwegian continental margin that are currently not fished, based on fishing data covering the years 2009-2020. Using these data and spatial prediction methods, we estimate that the surface sediment layer (0-2 cm) in unfished areas covering 765 600 km2 contains 139.2 Tg of organic carbon. Based on data from a meta-analysis of demersal fishing impacts on organic carbon density and estimated reductions in sediment thickness due to fishing-induced erosion, we estimate that 18.7 Tg (1.9-33.5 Tg) of organic carbon might be lost due to mobile bottom fishing in a scenario where each grid cell is fished evenly over the entire area and down to the full depth of the surface layer. Approximately one third of this vulnerable organic carbon is currently located in existing area-based protection measures. Additional protection could be guided by hotspots of vulnerable organic carbon, which are mainly found in the Barents Sea. We argue that the protection of vulnerable organic carbon that is at high risk of being lost e.g. in areas becoming accessible to fishing due to sea ice retreat such as in the northern Barents Sea should be given a high priority.