Estimation d'un stock naturel de coquille Saint Jacques par une méthode combinant les dragages et la plongée. Traitement des résultats par une méthode géostatistique

The efficiency of the fishing drags in the Saint Brieuc Bay, Brittany, has been determined through the analysis of the tracks left on the seabed. The results, which focus mostly on the age class n°2 (2.5 years), show that the efficiency and the damages caused by the drags vary heavily according to the nature of the seabed. For soft bottoms, the average efficiency is 67 % and the global damage is 2 %. Concerning hard bottoms, the values are respectively 30 % and 17 %. These efficiency results combined with those of a dragging expedition in a set sector of the Saint-Brieuc Bay enable a direct assessment of the abundance. A result treatment method deriving from the geostatistical theory is used: it takes into account the spatial structure of the king scallop distribution as observed when dragging based on a systematic grid on a 3,300 hectares area. On the dragged area, one of the richest of the bay, a tonnage of 2,550 tonnes has been found (standard deviation of 407), which corresponds to an average density of 0.6 king scallop per square meter.