4th International Submarine Canyon Symposium (INCISE2018)
5-7 November 2018, Shenzhen, CHINA
This is the detail of Towards the Reduction of Trawl-Induced Sediment Resuspension and Seabed Impact through Self-Regulated Deep-Sea Fishery Management in la Fonera Canyon (NW Mediterranean)
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Pere Puig, Marta Arjona-Camas
Towards the Reduction of Trawl-Induced Sediment Resuspension and Seabed Impact through Self-Regulated Deep-Sea Fishery Management in la Fonera Canyon (NW Mediterranean)
Bottom trawling has been identified as the main human activity causing physical loss of -and damage to- the seafloor. Studies in the La Fonera Canyon have shown that the recurrent reworking of the deep seafloor by trawling gears creates periodic resuspension events, causes sediment removal from fishing grounds and ultimately smooths the seabed morphology over large spatial scales.
Recently, there has been a tendency in the fishing sector to shift towards gears with less impact on the surface sediments. In this regard, the fishermen guild of the Palamós harbor initiated in 2013 a self-regulated project with the main goal to reduce the intensive erosion created by trawling gears on fishing grounds along La Fonera Canyon flanks.
Six different types of otter boards were selected to monitor their impact on the sea floor. The fishing ground was closed to the fleet during the monitoring period and it was only visited by a single trawling vessel using different otter boards on consecutive days. Previously, a mooring line equipped with a downward looking ADCP and 3 turbidimeters was deployed on the northern flank of the submarine canyon at 980 m water depth, slightly deeper than the maximum trawling depth. Data was used to select the otter boards that performed better and resuspended less amount of sediment.
The passage of the heavier otter boards that were in contact with the seafloor generated sharp increases in suspended sediment concentration (up to 900 mg/l at 5 m above seafloor) and the consequent intensification of the current speed closer to the seafloor that denoted the occurrence of sediment gravity flows.
On the contrary, the hauls with the nets equipped with pelagic and light (< 500 kg) semi-pelagic and bottom otter boards did not produce any significant increase of the suspended sediment concentration and current speed, which indicated that even they could be in contact with the sea floor, did not cause any noticeable resuspension. These otter board models were selected to be used in all the trawling vessels working at these fishing grounds, and the complete change of trawling gear in the Palamós fleet was accomplished on September 2017.