Underwater Observations on Scallop (Placopecten magellanicus) Behaviour and Drag Efficiency

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Abstract
The efficiency of an 8-ft scallop drag was estimated from population density measurements by scuba divers. Density measurements had to be made with an enclosed quadrat because of scallop swimming activity. Scallops responded to approaching objects by facing away from them and swimming. A steep rise usually preceded level swimming at a mean height of 0.4 m from bottom. Point-to-point swimming distances of up to 4 m were recorded with ground speeds in excess of 67 cm/sec. Few scallops over 100 mm could be induced to swim.Despite level bottom conditions, overall drag efficiency was low (2.1%) but increased progressively over the size range encountered (20–150 mm). Direct observations of drag function showed that swimming activity rather than selection by the drag was responsible for the low drag efficiency for the capture of scallops smaller than 100 mm.An indirect fishing mortality was established for recessed scallops buried by the drag. Dragging resulted in dislodgement of dead shell to the substrate surface, and aggregation of benthic predators in the drag tracks.

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