The spatial extent of trawl marks in the German Baltic Sea basins and their relation to the composition of the subsurface.

Bottom trawling represents the largest anthropogenic source of physical disturbance to seafloor morphology, sediment texture and composition, and benthic habitats. Past studies have shown that the morphological traces left by bottom trawling in the Baltic Sea remain stable for a year to more than a decade depending on area. The persistence of trawling-induced morphology is particularly relevant with the currently declining fishing pressure. The steeply declining trawling intensity provides the opportunity to establish baseline maps of trawling impacts and investigate how a trawled seafloor re-naturalizes after trawling has stopped. Here, we train a convolutional neural network to map trawl marks in bathymetric grids of 1 m resolution largely provided by the German Federal Maritime Agency for Kiel Bay, Fehmarn Belt, Mecklenburg Bay and Arkona Basin in the Western Baltic Sea. The model operates directly on bathymetric grids and is robust to artifacts, allowing monitoring of trawl marks with low effort. The calculated trawl mark density is a measure of the cumulative morphological impact of trawling in the different areas. For the Fehmarn Belt marine protected area, where bottom trawling was excluded in 2025, differential bathymetric data show no substantial seafloor recovery after one year, and new trawl marks are observed. Small areas of low trawling activity around seafloor obstacles such as pockmarks, boulders and wrecks allow the direct comparison of a pristine (Holocene-like) seafloor with an adjacent heavily trawled seafloor. Here, seafloor roughness decreases with increasing trawling intensity, potentially related to sediment resuspension and flattening by ground ropes that are not directly image by acoustic surveys. Untrawled seafloor locally elevates slightly above the surrounding trawled seafloor, potentially caused by long-term erosive effects of sediment reworking by bottom trawling. Initial results suggest a relationship of near-subseafloor free methane fronts to areas of intense trawling, suggesting that trawling can also effect the flux of climate relevant trace-gases into the water column. We further analyze vertical profiles of benthic microbial communities at stations with different trawling intensity.