Mapping trawling pressure on the benthic habitats is needed as background to support an ecosystem approach to fisheries management. The extent and intensity of bottom trawling on the European continental shelf (0–1000 m) was analysed from logbook statistics and vessel monitoring system data for 2010–2012 at a grid cell resolution of 1 × 1 min longitude and latitude. Trawling intensity profiles with seabed impact at the surface and subsurface level are presented for 14 management areas in the North-east Atlantic, Baltic Sea and Mediterranean Sea. The footprint of the management areas ranged between 53–99% and 6–94% for the depth zone from 0 to 200 m (Shallow) and from 201 to 1000 m (Deep), respectively. The footprint was estimated as the total area of all grid cells that were trawled fully or partially. Excluding the untrawled proportions reduced the footprint estimates to 28–85% and 2–77%. Largest footprints per unit landings were observed off Portugal and in the Mediterranean Sea. Mean trawling intensity ranged between 0.5 and 8.5 times per year, but was less in the Deep zone with a maximum intensity of 6.4. Highest intensities were recorded in the Skagerrak-Kattegat, Iberian Portuguese area, Tyrrhenian Sea and Adriatic Sea. Bottom trawling was highly aggregated. For the Shallow zone the seabed area where 90% of the effort occurred comprised between 17% and 63% (median 36%) of the management area. Footprints were high over a broad range of soft sediment habitats. Using the longevity distribution of the untrawled infaunal community, the seabed integrity was estimated as the proportion of the biomass of benthic taxa where the trawling interval at the subsurface level exceeds their life span. Seabed integrity was low (<0.1) in large parts of the European continental shelfs, although smaller pockets of seabed with higher integrity values occur. The methods developed here integrate official fishing effort statistics and industry-based gear information to provide high-resolution pressure maps and indicators, which greatly improve the basis for assessing and managing benthic pressure from bottom trawling. Further they provide quantitative estimates of trawling impact on a continuous scale by which managers can steer.

SignificanceWe conducted a systematic, high-resolution analysis of bottom trawl fishing footprints for 24 regions on continental shelves and slopes of five continents and New Zealand. The proportion of seabed trawled varied >200-fold among regions (from 0.4 to 80.7% of area to a depth of 1,000 m). Within 18 regions, more than two-thirds of seabed area remained untrawled during study periods of 2–6 years. Relationships between metrics of total trawling activity and footprint were strong and positive, providing a method to estimate trawling footprints for regions where high-resolution data are not available. Trawling footprints were generally smaller in regions where fisheries met targets for exploitation rates, implying collateral environmental benefits of effective fisheries management.

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There is an implicit requirement under contemporary policy drivers to understand the characteristics of benthic communities under anthropogenically-unimpacted scenarios. We used a trait-based approach on a large dataset from across the European shelf to determine how functional characteristics of unimpacted benthic assemblages vary between different sedimentary habitats. Assemblages in deep, muddy environments unaffected by anthropogenic disturbance show increased proportions of downward conveyors and surface deposit-feeders, while burrowing, diffusive mixing, scavenging and predation traits assume greater numerical proportions in shallower habitats. Deep, coarser sediments are numerically more dominated by sessile, upward conveyors and suspension feeders. In contrast, unimpacted assemblages of coarse sediments in shallower regions are proportionally dominated by the diffusive mixers, burrowers, scavengers and predators. Finally, assemblages of gravelly sediments exhibit a relatively greater numerical dominance of non-bioturbators and asexual reproducers. These findings may be used to form the basis of ranking habitats along a functional sensitivity gradient.

Bottom-trawl fisheries are expanding into deeper habitats and higher latitudes, but our understanding of their effects in these areas is limited. The ecological importance of habitat-forming megabenthos and their vulnerability to trawling is acknowledged, but studies on effects are few. Our objective was to investigate chronic effects of otter trawl fishery on substratum and megabenthos on the shelf (50–400 m) and slope (400–2000 m) in the southern Barents Sea. The study area represents a wide range in the history of fishing intensity (FI). Physical impact of trawling, density of trawl marks (TMs), was quantified on 250 video transects from shelf and slope, and megabenthos (>2 cm) composition was studied on 149 video transects from the shelf. The number of satellite-recorded vessels within grid cells 5 × 5 km was used as a proxy for FI in the TM analysis and for the megabenthos records within a 2-km radius around the transects. The effects of using different search area sizes were tested. Patterns in the density of TMs and megabenthos composition were compared with FI using linear regression and correspondence analysis. Occurrence of TMs was not directly related to FI but to bottom type, whereas megabenthos density and diversity showed a negative relation. For 79 of the 97 most common taxa, density was negatively correlated with FI. The sponges Craniella zetlandica and Phakellia / Axinella were particularly vulnerable, but also Flabellum macandrewi (Scleractinia), Ditrupa arietina (Polychaeta), Funiculina quadrangularis (Pennatulacea), and Spatangus purpureus (Echinoidea) were negatively correlated with FI, whereas asteroids, lamp shells, and small sponges showed a positive trend. Our results are an important step towards the understanding of chronic effects of bottom trawling and are discussed in relation to the descriptors “Biological diversity” and “Seafloor integrity” in the EU Marine strategic framework directive.