An experimental dredging operation was carried out in a small sandy bay in Scotland, with the aim of quantitatively assessing the effects of scallop dredging on the benthic fauna and the physical environment. An area within the 10-m depth contour was selected; a 1.2-m modified scallop dredge was operated at frequencies of 2, 4, 12 and 25 dredges, carried out over a period of nine days. The effects on the bottom topography, the physical characteristics of the sediment and the fauna were investigated by grab and core sampling, and direct observations were carried out by a diving team. Observed changes in bottom topography were not translated into changes in the disposition of the sediments, their grade distribution and the organic carbon and chlorophyll content, all of which showed no effects. The infaunal community, which consisted of bivalve molluscs and peracarid crustaceans, both taxa adapted morphologically and behaviourally to a dynamic environment, did not show any significant changes in abundance or biomass. Sessile forms such as polychaetes showed a noticeable decrease, and the burrowing spatangid Echinocardium was substantially reduced from the dredged area. Corresponding changes in the biomass of the different taxa were also evident but not significant. However, the most important effect of this experiment was on the epifaunal and large infaunal organisms recorded by the divers. Large numbers of molluscs (Ensis), echinoderms (Asterias) and crustaceans (Cancer) were killed or damaged by the dredging operations. Very large concentrations of the burrowing sand eel Ammodytes were also destroyed. The overall conclusion to be drawn from this experimental dredging operation is that its effect was limited to the selective elimination of a fraction of the fragile and sedentary components of the infauna, and the destruction of the large epifaunal and infaunal organisms. |
