Sediment remobilisation governed by hydrodynamic drag and not by weight of the gear

Many people think that the bigger the weight of the fishing gear, the bigger the impact on the seafloor. BENTHIS researcher Barry O'Neill has found that this is not always the case: and when looking at the amount of sediment put in to the water column, it’s the hydrodynamic drag of the gear that is the most important factor, not the weight.

The BENTHIS ‘drag team’ used a specially designed sledge to measure the hydrodynamic and the contact drag on different components of the fishing gear that are in contact with the seabed (Figure 1). These include the ground gear, which is made of rubber discs and chains and protects the net from the seafloor; and the otter boards, which spread the fishing gear and ensure it fishes close to the seabed (Figure 2). The sledge also measured the amount of sediment remobilised in its wake. The research team could adjust the weights of the gear components, so that they would obtain independent measurements for each combination of weight, tow speed (between 1 and 2 m per second) and component.

Figure 1. The towed sledge on deck
Figure 1. The towed sledge on deck

O’Neill:  “We found out that the amount of sediment entrained in the wake of the gear components is related to the hydrodynamic drag of the element rather than the weight or the degree to which the element penetrates the seabed. These results support our earlier observations in 2011 that the remobilisation of sediment by demersal fishing gears is essentially a hydrodynamic phenomenon.  As the hydrodynamic drag of a gear element increases,  there is an increase of turbulent shearing and a greater pressure drop in its wake, which leads to an increase of the sediment remobilised.”

Figure 2. Illustrations of some of the trawl gear components that are in contact with the seabed that have been tested : cylindrical clumps, disc and rockhopper ground gears, and trawl otter boards
Figure 2. Illustrations of some of the trawl gear components that are in contact with the seabed that have been tested : cylindrical clumps, disc and rockhopper ground gears, and trawl otter boards

Although there are other factors involved, these results indicate that by developing more hydrodynamic gears the fishing industry could reduce benthic impacts and also save on fuel costs.