The mobilisation of sediment and benthic infauna by scallop dredges

We present the results of experiments to assess the immediate impact of scallop dredging on the seabed sediment and on the inhabiting infauna. The passage of the scallop dredge is shown to homogenise the seabed, flattening sand ripples. The turbulent wake entrains up to the equivalent of a 1 mm layer of sediment per unit of swept width, although an analysis of the finer particles material implies that the suspended silt material must originate from depths of at least 10 mm. The species most abundant in the sediment plume either swim actively in the water column or are found in, or on, the upper layers of the substrate, whereas those most abundant in core samples taken from the sediment, but not present in the net samples, are almost all tube-building or deep burrowing. The vertical stratification of sediment concentration and of animal numbers in the water column suggests that even if some of these species respond actively to the presence of the dredge, once entrained, they are transported more or less passively in the same way as the larger sediment particles. There was no difference between the core samples taken before or after towing suggesting that animals mobilised by the dredge resettle in the tow path. Our analysis does not provide any information regarding the fate of these animals.