Methode de cartographie quantitative des ulves "offshore"

Recent studies in the bays of Douarnenez and La Forêt-Fouesnant have proved that drifting stocks of « offshore » ulva exist between -2 and -20 meters depth. These stocks are nearly as important in weight as those located in the tidal zone, the only ones which are generally studied. It is likely that such « offshore » stocks also exist elsewhere in Brittany. The sampling method by discrete stations visited by divers, which has been used till now, is considered heavy and not precise enough. The new method involves a film camera set on a fish towed at a speed of a few knots. The fish is kept above the sea bottom at a chosen altitude controlled by a guiderope. The system is sturdy, autonomous, and can be easily transferred from one boat to another. The percentage of seaground covered by green algae is automatically obtained by processing the recorded signal. It is converted to biomass via a table. The pictures are georeferenced by coupling them with the recorded global positioning data. Hence, it is possible to get biomass maps of « offshore » ulva. The whole system is already operational, but can be further improved.