Seabed Stories:tracks from bottom-towed fishing gears

ottom-towed fishing gear, including trawls and seines, is among the most widespread anthropogenic activities impacting seabed habitats. Depending on the gear type, towing intensity, sediment characteristics, and habitat type, these gears impact seabed morphology, resuspend sediments, and disrupt biogeochemical processes. They also cause significant ecological impacts, including mortality of benthic organisms, biodiversity loss, and shifts in species composition. This study uses and combines AIS data and side-scan sonar track observations to examine fishing gear impacts on seabed features in the North Sea. AIS data identified 20 active fishing vessels of beam trawlers, otter trawlers, and Danish seiners, totalling 808 fishing activities in the study period. Sonar surveys covered ~110 km across 18 trials, detecting fishing gear tracks in 14 trials. Beam trawl tracks were the most frequent, characterised by paired depressions, while otter trawl tracks featured parallel furrows and clump marks in a few cases. Danish seine tracks were not confidently observed. Overall, 38% of beam trawl AIS fishing activities were observable tracks using the sonar and only 5% of otter trawl fishing activities.<br/>Tracks appeared at different depths, with beam trawl deeper than otter trawl tracks, but both depths correspond to the average depth of their fishing activities. Longevity analysis revealed that beam trawl tracks persist for up to 30 days while otter trawl tracks for up to 7 days. Combining AIS and sonar data highlighted that beam trawls sweep the largest subsurface area, while Danish seine covers the largest surface area. Hourly swept area rates align with these findings, with beam trawl having the largest subsurface impact and Danish seine dominating surface impacts. This integrative approach provides insights into gear-specific seabed impacts and temporal footprints. Applying only sonar data would minimise the effects of Danish seines, which sweep a large area of the surface sediment, potentially causing sediment resuspension and impacting the marine fauna. Otter trawl and beam trawl effects would be less at the surface level, but based on the sonar, the subsurface impact would appear with implications for fauna recovery. The longevity of the beam trawl also indicates that the tail of fishing activity disturbance exceeds the natural disturbance, which can cause ecological changes due to new conditions.