Influence of bottom trawling on sediment resuspension in the 'Grande Vasiere' area (Bay of Biscay, France)
Sea trials were performed on two zones with different fishing efforts on the continental shelf of the Bay of Biscay 'Grande Vasiere' area of muddy sand) in order to assess particulate matter resuspension and seabed disturbances (i.e. penetration, reworking, grain size changes) induced by different types of trawls. Optical and acoustic measurements made in the water column indicate a significant trawling induced resuspension mainly due to the scraping action of doors. It manifests as a highly dynamic turbid plume confined near the seabed, where suspended sediment concentrations can reach 200 mg l 1. Concentration levels measured behind an "alternative" configuration (trawls with jumper doors instead of classical doors penetrating the sediment) are significantly lower (around 10 20 mg l 1) which indicates a potential limiting impact regarding the seabed. Grain size analyses of the surficial sediment led to highlight a potential reworking influence of bottom trawling. On the intensively trawled zone, this reworking manifests as an upward coarsening trend in the first 5 cm of the cores. A significant decrease in mud content (30 has been also witnessed on this zone between 1967 and 2014, which suggests an influence on the seabed evolution. The geometric analysis of bottom tracks (4 5 cm depth, 20 cm width) observed with a benthic video sledge was used to compute an experimental trawling induced erosion rate of 0.13 kg m 2. This erosion rate was combined with fishing effort data, in order to estimate trawling induced erosion fluxes which were then compared to natural erosion fluxes over the Grande Vasiere at monthly, seasonal and annual scales. Winter storms control the annual resuspended load and trawling contribution to annual resuspension is in the order of 1 However, results show that trawling resuspension can become dominant during the fishing high season (i.e. until several times the natural one in summer) In addition, the contribution of trawling induced resuspension is shown to increase with water depth, because of the rapid decay of wave effects. Finally, the seasonal evolution of the respective contributions for erosion (mainly trawling and waves) could be mapped for the whole study area.
Ocean Dynamics