Hydro-acoustic characterization of trawl marks in the German Exclusive Economic Zone (North Sea) as a function of sediment properties

Abstract

Marine habitats are increasingly impacted by human activities. Bottom trawling, in particular, contributes to this impact by causing sediment displacement and disrupting the habitat of benthic species. An indicator of sediment reworking caused by bottom trawling on the seafloor is the presence of trawl marks. These elongated furrows are left behind by trawl nets in the sediment and can be observed in hydroacoustic data such as side-scan sonar images. Such trawl marks are known from various marine habitats, and their significance for sedimentation - for example, as an indication of increased erosion - has been the subject of extensive research. However, detailed investigations of such marks have been scarce for the North Sea region within the German Exclusive Economic Zone (EEZ). The present dissertation includes a comprehensive mapping of trawl marks based on approximately 4800 km² of side-scan sonar backscatter data collected in the North Sea part of the German Exclusive Economic Zone (EEZ). Detailed analysis of the trawl marks revealed seasonal variations in their spatial density and allowed for estimates of their persistence. Additionally, sediment strength data provided evidence of the physical impact of trawl nets on the seafloor and reflecting surface sediment reworking. The extensive side-scan sonar dataset also facilitated the development of an approach for automated trawl mark detection.