The exploitation of offshore mussel farms is becoming important throughout the world, but monitoring this activity remains a difficult task. Here, we propose a specific method for this purpose. A total of 140 long-lines were monitored on a mussel culture ground in the French Mediterranean Sea during four experimental surveys deploying multibeam sonar devices mounted on poles (Reson Seabat 6012, 455 kHz) on small boats. This allowed geo-referenced observations to be made of the submerged mussel long-lines, as well as three-dimensional (3D) drawings of the long-line structures and the sea bed shapes, using long-line longitudinal sonar sampling. Three sonar data-analysis methods were applied: (i) direct two-dimensional (2D) visual interpretation of raw sonar video images; (ii) indirect 2D long-line drawings; and (iii) 3D digital long-line reconstructions. The development of these acoustic methods in shallow water provides scientists, managers and local authorities with a tool for observing the 3D position (geographical position and depth) of mussel cultures, for counting each structure by the 'long-line echo-counting' method, for monitoring their shape in situ, and for classifying the mussel rope segments into three growth categories ('in growth', 'full' and 'empty'). The use of acoustic tools for monitoring underwater mussel culture grounds, for management purposes and for scientific studies, could be extended to other artificial structures in shallow water environments. (c) 2005 Elsevier B.V. All rights reserved.