Pascal Laffargue

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

Mapping trawling pressure on the benthic habitats is needed as background to support an ecosystem approach to fisheries management (EAFM). The extent and intensity of bottom trawling on the European continental shelf (0–1000 m) was analyzed from logbook statistics and VMS data for 2010, 2011 and 2012 at a resolution of 1×1 minutes 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 (proportion of the seabed trawled 1 or more times every ten years) ranged between 40–90% across EUNIS habitats with largest footprints observed in sandy (A5.2) and muddy (A5.3) habitats. The footprint of the management areas ranged between 52-99% and 5-94% for the depth zone from 0–200 m (Shallow) and from 201–1000 m (Deep), respectively. The footprint was estimated as the total area of all grid cells that were trawled fully or partially. Excluding these untrawled proportions reduced the footprint estimates to 28-85% and 2-77%. Mean trawling intensity ranged between 0.5 and almost 8.5 times per year, but was less in the Deep zone with a maximum intensity of 6.4 times per year. Highest intensities were recorded in the Skagerrak–Kattegat and Adriatic Sea. Largest footprints per unit landings were observed in the Mediterranean Sea. Bottom trawling was highly aggregated. The seabed area where 90% of the effort occurred comprised between 11% and 65% (median 44%) of the total area trawled. 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.

Tagging juvenile seabass and sole with telemetry transmitters: medium-term effects on growth

The effects of tagging with acoustic transmitters on the growth of juvenile seabass, Dicentrarchus labrax (L.) (initial mean mass±SD, 173 g±23.4) in a 47 d tank experiment, and sole, Solea solea (L.) (103.2 g±14.8) in a 72 d tank experiment and (104.0 g±18.4) in a 58 d salt marsh mesocosm experiment were examined. Twenty externally tagged seabass grew more slowly than the 20 with surgically implanted tags, which reached the same mass as nine control fish. Movements of the external transmitters harness caused abrasions of the skin and loss of the tag in 60% of the cases. We thus recommend implanting transmitters for telemetry studies of juvenile seabass weighing between 120 and 214 g and carrying a tag that represents 2.2–2.5% of body mass. Both tank and mesocosm experiments conducted on juvenile sole concluded that the externally attached tag retention rate was good, but at the expense of the fish growth rate.