Recovery of substrates and macro-benthos after ¢shing trials with a new Portuguese clam dredge

The e¡ect of dredging on bottom structure was assessed, to estimate the damage in£icted on the benthic macrofauna left on the dredge path and to evaluate aggregations of scavengers within the track. Sediment suspended during dredging rapidly resettled both on sand and sandy-mud bottoms. Dredge tracks were deeper on sandy-mud sediments and persisted longer than in sandy sediments. The disappearance of tracks in both sediment types depended both on current strength and wave action. Damage and mortality induced by dredging on the macrobenthic animals left on the dredge path was relatively low. Post-¢shing diver observations showed that damaged animals left on the dredge path rapidly attracted scavengers, mainly Ophiura albida. Immediately after the tow ophiuras reached densities 8 times greater in the track region than in the background. However, they dispersed again rapidly because only few damaged fauna were available on the dredge tracks. Undamaged or slightly damaged shell¢sh started to rebury immediately after escaping from the dredge.