Abstract
The anthropogenic impact in the German Exclusive Economic Zone (EEZ) is high due to the presence of manifold industries (e.g., wind farms, shipping, and fishery). Therefore, it is of great importance to evaluate the different impacts of such industries, in order to enable reasonable and sustainable decisions on environmental issues (e.g., nature conservation). Bottom trawling has a significant impact on benthic habitats worldwide. Fishing gear penetrates the seabed and the resulting furrows temporarily remain in the sediment known as trawl marks (TM), which can be recognized in the acoustic signal of side-scan sonars (SSS) and multibeam echo sounders (MBES). However, extensive mapping and precise descriptions of TM from commercial fisheries at far offshore fishing grounds in the German EEZ are not available. To get an insight into the spatial patterns and characteristics of TM, approximately 4800 km2 of high-resolution (1 m) SSS data from three different study sites in the German EEZ were analyzed for changes in TM density as well as for the geometry of individual TM. TM were manually digitalized and their density per square kilometer was calculated. In general, TM density was highest in August and October. Moreover, different gear types could be identified from investigating individual TM in SSS data. Beam trawl marks were observed to have widths of up to 22 m whereas otter board marks showed widths up to 6 m. The persistence of TM was estimated to 2–7 days minimum for all three sites based on the SSS data from 2015–2019. A maximum persistence could be defined at one site (Dogger Bank) and it was five months for the investigation period 2016–2017. Besides the main factors driving TM degradation (wave-base impact, sediment-type), different methods for TM detection (SSS, MBES, under-water video) are discussed. The study provides valuable information on the physical impact of bottom trawling on the seabed and can support existing monitoring strategies.
Highlights
The German Exclusive Economic Zone (EEZ) in the North Sea, is extensively used for infrastructure and resource extraction, such as wind farms, offshore cables, pipelines, shipping routes as well as fishery [1,2]
The Wind Farm (WF) site is dominated by sandy sediments and the proportion of mud is slightly lower in the northwestern part (5–10%) than in the rest of the area
Together with the detailed description of trawl marks (TM) patterns related to different trawl gears, the mapping presented in this study provides an insight into the physical disturbance on the seabed caused by bottom contacting trawling, which can help to evaluate the impact of fishing activities on sediments, seafloor morphology, and macrobenthos
Summary
The German Exclusive Economic Zone (EEZ) in the North Sea, is extensively used for infrastructure and resource extraction, such as wind farms, offshore cables, pipelines, shipping routes as well as fishery [1,2]. The impact on the seabed due to different kinds of bottom contacting trawling gears has been globally documented by several authors, e.g., [3,4], and it has multiple effects on the benthic habitats. The mortality rates of non-target species can increase due to the damage caused by the bypassing trawl gear [5,6] and the flattening of small bedforms can influence juvenile fish as they use them as a shelter [7]. Since 1995, large trawlers (>221 kW engine power, >24 m length over all, LOA), which use larger trawl gears and have higher catch rates compared to small trawlers (
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