The footprint of bottom trawling in European waters: distribution, intensity, and seabed integrity

Abstract

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.

Global analysis of depletion and recovery of seabed biota after bottom trawling disturbance

“…Our finding that mean community r increases with F conforms with previous observations of shifts toward species with faster life histories in disturbed communities (36). This effect is apparent across a range of plausible trawling frequencies from >0 to 10 y −1 but would be small for the great proportion of most fishing grounds, where swept area ratio is less than 1 y −1 (7). Although this shift means that previously trawled communities may be more resilient to additional trawling, it does not mean that they will recover any faster to the original pretrawling state.…”

“…None of the studies that met the criteria examined the effect of trawling on biogenic habitats, but there were sufficient studies in other habitats. Many gear-habitat combinations were not represented in the studies reviewed, because many fishing gears are only suitable for fishing on particular seabed types or species associated with those habitats (SI Appendix, Table S1) and because some habitats are less widespread than others (7).…”

“…S1 and Table S6). The increase in r, which results from changes in community composition to favor biota with faster life histories, is, therefore, relatively slight across ranges of trawling frequencies that dominate those on real fishing grounds (e.g., 0-1 y −1 ) (7,31,32). The r estimate of 0.82 y −1 enables estimates of median time to recovery (T) to 0.95K for a range of levels of depletion (Fig.…”

“…These assessments are used to assess the sustainability of bottom trawl fisheries, formulate priorities for habitat protection, and ultimately, achieve a balance between fisheries production and environmental protection. The distribution of bottom trawling is increasingly well-characterized by vessel tracking and other monitoring systems (7), but impacts depend on the magnitude of trawlinginduced mortality and recovery rates of biota, for which the current evidence base is incomplete, dispersed, and often contested (4,8).…”

Deep-Sea Mega-Epibenthic Assemblages from the SW Portuguese Margin (NE Atlantic) Subjected to Bottom-Trawling Fisheries

“…1,000 m water depths and is associated with a large footprint per unit of landing with ca. 13-17 km −2 t −1 depending on the depth range considered (Eigaard et al, 2016;BuenoPardo et al, 2017). Moreover, few studies have attempted to understand the trawling impacts on the benthic assemblages and are limited by the absence of adequate control areas (Morais et al, 2007;Fonseca et al, 2014).…”