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Estimating footprints: gear type and fishing intensity

Published on
May 12, 2018

Different types of fisheries such as beam trawling, Scottish seine or flyshoot differ greatly in their footprint: the area trawled per hour.

Figure 1. Footprint (km2 fished per hour) of an average vessel of the 14 European bottom trawling metiers The black bars indicate the subsurface level (deeper than 2 cm) and the white bars the surface level (0-2 cm). (Eigaard et al. 2016).
Figure 1. Footprint (km2 fished per hour) of an average vessel of the 14 European bottom trawling metiers The black bars indicate the subsurface level (deeper than 2 cm) and the white bars the surface level (0-2 cm). (Eigaard et al. 2016).
Figure 2. Mean annual trawling intensity at the sea surface at a scale of 1x1 minutes latitude and longitude. Results are based on satellite (VMS) recordings of fishing activities and logbook data for the period 2010-2012 from countries indicated in black
Figure 2. Mean annual trawling intensity at the sea surface at a scale of 1x1 minutes latitude and longitude. Results are based on satellite (VMS) recordings of fishing activities and logbook data for the period 2010-2012 from countries indicated in black

Scottish seining has the largest footprint of 1.6 km2 per hour, but the impact is mainly on the surface level: the impact on the subsurface level (deeper than 2 cm) is 0.08 km2/h. In contrast, beam trawling for flatfish has a lower overall footprint, but ranks substantially higher when comparing only impact at the subsurface level  (0.19 km2/h).

Trawling intensity maps were created by combining the fishing position recorded by satellite (VMS) with the metier information of the vessel from the EU-logbooks and the estimate of the typical footprint for each metier. Mean trawling intensity ranged between 0.5 and 8.5 times per year in waters down to 200 m depth, but was slightly less in the deeper water (200-1000m). Highest intensities were recorded in the Skagerrak-Kattegat, Celtic Sea, Bay of Biscay, Iberian Portuguese area, Tyrrhenian Sea and Adriatic Sea.

90% of the fishing effort takes place in 50% of the area

Between 15% and 72% of the seabed area was untrawled across the analysed management areas. This estimate applies to the 3-year study period. It is unlikely, however, that the untrawled seafloor in the study period will remain untrawled when studied over a longer time period. A more cautious estimate of the untrawled seafloor is given by the proportion of the grid cells without trawling activities, which ranged between 1 and 47 %. Within the area trawled, trawling is highly aggregated with 90 % of the effort occurring in around 50 % of the gridcells. This implies room for a significant reduction of trawling impact at a minimal cost for the fishery. Because of the aggregated nature of trawling, the footprint estimates are scale dependent.

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