Abstract
Bottom trawling and seabed littering are two serious threats to seabed integrity. We present an overview of the distribution of seabed litter and bottom trawling in Norwegian waters (the Norwegian Sea and the southern Barents Sea). Vessel Monitoring System (VMS) records and trawl marks (TM) on the seabed were used as indicators of pressure and impact of bottom trawling, respectively. Estimates of TM density and litter abundance were based on analyses of seabed videos from 1778 locations, surveyed during 23 cruises, part of the Norwegian seabed mapping programme MAREANO. . The abundance and composition of litter and the density of TM varied with depth, and type of sediments and marine landscapes. Lost or discarded fishing gear (especially lines and nets), and plastics (soft and hard plastic and rubber) were the dominant types of litter. The distribution of litter reflected the distribution of fishing intensity (density of VMS records) and density of TM at a regional scale, with highest abundance close to the coast and in areas with high fishing intensity, indicated from the VMS data. However, at a local scale patterns were less clear. An explanation to this could be that litter is transported with currents and accumulates in troughs, canyons, and local depressions, rather than reflecting the fisheries footprints directly. Also, deliberate dumping of discarded fishing gear is likely to occur away from good fishing grounds. Extreme abundance of litter, observed close to the coast is probably caused by such discarded fishing gear, but the contribution from aggregated populations on land is also indicated from the types of litter observed. The density of trawl marks is a good indicator of physical impact in soft sediments where the trawl gear leaves clear traces, whereas on harder substrates the impacts on organisms is probably greater than indicated by the hardly visible marks. The effects of litter on benthic communities is poorly known, but large litter items, such as lost fishing gear may add to the direct negative effects of bottom trawling.
Highlights
The Norwegian exclusive economic zone (EEZ) is large (2.1 Mkm2), and most of it is deep sea (>200 m)
The areas indicated in red had a mean annual fishing intensity (FI) >800 Vessel Monitoring System (VMS) records/km2
The largest continuous areas with high FI were found in the North Sea, outside the range of the MAREANO mapping area
Summary
The Norwegian exclusive economic zone (EEZ) is large (2.1 Mkm2), and most of it is deep sea (>200 m) It includes a long coastline with numerous fjords, a wide shelf with banks, inserted canyons, and troughs, and an abyssal plain with depths down to 3,970 m. The target species in this area are mainly cod and haddock, and deep sea prawns. These commercial stocks have for periods been fished extensively. Physical damage Mortality Removal Dislodgement a ban against trawling on known coral reefs had been implemented. The petroleum industry represents activities that cause physical impact at more restricted scale than bottom trawling, and represents local sources of litter. Resuspended particles from bottom trawling could lead to similar effects on a greater scale, and studies have indicated great impact on sediment distribution and even changes bottom topography (Palanques et al, 2006; Puig et al, 2012).
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