Typology and behaviour of tuna aggregations around fish aggregating devices from acoustic surveys in French Polynesia

Quatre-vingt-sept prospections acoustiques, d'une duree de deux heures, ont ete effectuees entre decembre 1995 et fevrier 1997 en Polynesie francaise autour de 17 dispositifs de concentration de poissons (DCP). Les densites en thons, entre la surface et 500 m de profondeur et dans un rayon de 0.8 mille nautique autour des DCP, ont ete estimees par echo-comptage en presence de poissons disperses, ou par echointegration en presence de poissons agreges en bancs. Lors de 27 prospections, (representant respectivement 81 % et 15 % des prospections nocturnes et diurnes effectuees), aucune detection de thon n'a ete observee. Les 60 autres prospections ont montre 3 differents types d'agregation : (1) le type « poissons disperses profonds » observe 45 fois, (2) le type « poissons disperses intermediaires » observe 16 fois el (3) le type « poissons en bancs superficiels » observe 16 fois. Differents types d'agregation ont parfois ete observes sous un meme DCP. La taille des poissons (determinee a partir des valeurs d'indice de reflexion individuel ou TS), la distance entre les individus et la profondeur des detections diminuent, alors que les densites augmentent, entre les types « poissons disperses profonds » (7.3 poissons par km 3 entre 100 et 300 m de profondeur), « poissons disperses intermediaires » (26 poissons par km 3 entre 50 et 150 m) et « poissons en bancs superficiels » (801 poissons par km 3 entre la surface et 50 m). Les densites les plus fortes ont ete observees de jour, alors que de nuit, differentes situations, depuis l'absence totale de detection jusqu'a la presence de densites elevees ont ete rencontrees.

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