Benthic impact of fisheries in European waters: the distribution and intensity of bottom trawling

Mapping and monitoring of pressure from fishery on the marine benthic environment is necessary to support an ecosystem approach to fisheries management (EAFM). In many cases this need is not reflected in official fisheries statistics and logbooks, where focus typically is on catch rather than effort. Consequently, most logbook information is not well suited for quantitative estimation of seafloor impact (swept area and impact severity) of the different gears and trips. We developed a method to overcome this information deficiency of official statistics and produced European wide high-resolution fishing intensity maps (total yearly swept area within grid cells of 1*1 minutes longitude and latitude) for 2010, 2011 and 2012. The annual distribution and intensity of bottom trawling on the European continental shelf was analyzed for different management areas and gear groups, distinguishing between surface and sub-surface effects. Fishing pressure indicators were calculated and compared for each management area; i) proportion of area untrawled, ii) proportion of area with an annual swept area intensity ≥ 1, and iii) proportion of area where 90% of the effort is concentrated). The management area with the largest proportion of surface area being trawled ≥ 1 time a year was the Adriatic Sea (64%) and the management area with the lowest proportion of surface area being trawled ≥ 1 time a year was the Northwestern Shelf (15%). Also the Tyrrhenian Sea (45%), the Channel (41%) and the North Sea (36%) have a substantial part the seabed trawled at intensities above 1, indicating a high level of fishing pressure on the benthic habitats. The results of the analysis also showed that in all European seas, between 33% and 72% of the sea bed down to 200 m was not trawled during the study period. When considering all three fishing pressure indicators jointly (proportion of area untrawled, proportion with an annual intensity ≥ 1, and with 90% of the effort) four management areas draw attention; the North Sea, the Channel, the Tyrrhenian Sea and Adriatic Sea. These four areas all score relatively high on all three pressure indicators, and also have a substantial part (> 50%) of the total area impacted at the sub-surface level. Within the soft sediment habitats that dominate the continental shelf areas of Europe, mud habitats appear to be trawled most intensively while at the same time they likely have a higher sensitivity to bottom trawling as compared to the sandy and coarser sediments. Consequently, current fishing practices in parts of these four management areas could potentially compromise seafloor integrity.