A model approach to estimate the hydrodynamic drag and sediment mobilisation applied to tickler chain beam trawls and pulse beam trawls used in the North Sea fishery for sole

Bottom trawls impact the seafloor and benthic ecosystem. To estimate the trawling impact, information is required about the dimensions of the gear that determine their footprint, sediment penetration depth and hydrodynamic drag that determines the amount of sediment mobilised in the wake of the trawl. Here we present the dimension of gear components of the different beam trawls used in the North Sea flatfish fishery including the traditional tickler chain beam trawl, chain-mat trawl and pulse trawls. The hydrodynamic drag of the gears is estimated by summing the drag of different gear components using empirical equations that describe the hydrodynamic drag of different shaped objects, including cylinders, blocks and fish nets. Netting contributes most to the hydrodynamic drag, followed by the ground rope, tickler chains and chain mat. The hydrodynamic drag of bottom components, which determines sediment mobilisation, is estimated at 6.2 and 6.3 kN.m-1 for a traditional tickler chain and chain-mat trawl, and 3.8 kN.m-1 for a pulse trawler. Drag of pulse trawls depends on their rigging and ranges between 2.8 – 3.2 kN.m-1 for the rectangular ground rope types and 4.0 – 4.1 kN.m-1 for the sole rope type. The amount of sediment mobilised in a seabed habitat with 20% silt content is 24 kg.m-2 for a large tickler chain and chain-mat trawler and between 12 – 16 kg.m-2 for a large pulse trawler.