Résumé
Tuna distribution may be directly linked to food availability, the density of which can be assessed by echo-sounding. A study was performed in the French Polynesian EEZ (Economic Exclusive Zone) between 1995 and 1997 with a 38 kHz echo sounder working down to 500 m depth. With the settings used, acoustic back-scattered energy may be considered as representative of micronektonic fish biomass and provided information on horizontal and vertical micronekton variabilities and the structure of the echograms. Three different zones were defined by multivariate analysis. The classical biomass decreasing gradient from the equator to higher latitudes did not appear. Instead, the maximum micronekton abundance was found between Marquesas Archipelagos and a west-northwest/east-southeast oriented line stretching between 11 and 14° S, i.e. in a weak convergence, favourable to micronekton development due to the concentration of lower trophic levels with no oxygen limitation in the deep layers. Two zones with very different hydrological features, but with comparable micronektonic abundances, surround the richest micronekton zone. To the north, waters are enriched by the equatorial upwelling, but intense organic matter remineralisation limits oxygen availability under the thermocline. To the south, waters are influenced by the great southern gyre and present oligotrophic characteristics less favourable to micronekton development. The present results suggest that prediction of tuna forage distribution should take not only trophic parameters into account, but also environmental ones. They also suggest that echo-sounding data should be used more extensively in the validation of models predicting tuna forage.