Impact of scallop dredging on benthic megafauna: a comparison of damage levels in captured and non-captured organisms

The impact of scallop dredging on benthic megafauna was assessed by direct observation of damage, both in the bycatch and in organisms encountering dredges but not captured. Damage was assessed using a simple 4point scale adapted for different taxonomic groups. Experimental dredging was undertaken on a scallop fishing ground in the north Irish Sea, off the Isle of Man. Divers were deployed immediately after dredges had passed, to record levels of damage to megafauna left in the dredge tracks. Mean damage levels, and the proportions of the 4 damage scores in the bycatch and on the seabed, were the same in most species. Some common species did show differences. The edible crab Cancer pagurus was more severely damaged when not captured, while the starfish Asterias rubens and whelk Neptunea antiqua received greater damage within the bycatch. Capture efficiency for the megafauna was low, ranging from 2 to 25% among species. The results indicate that the majority of damage to large benthic invertebrates during scallop dredging occurs unobserved on the seabed, rather than in the bycatch.