Résumé
Tuna have a high metabolic rate, yet they often live in regions characterized by low primary productivity rates. Improving knowledge on tuna-habitat trophic relationship is of main importance for understanding tuna distribution and catchability. Distribution of large tuna food webs, i.e., micronekton, was described in French Polynesia at depths up to 500 m using acoustics and pelagic trawls. At large scale, acoustics profile response, morphological characterization of acoustic structures, and quantitative descriptors of sound scattering variability were used. Due to water masses advection and oxygen limitation main micronektonic production was localized 8 degrees south of the equatorial upwelling. At a smaller scale, micronekton was classified into seven scattering type: gas-filled swimbladder fish, absent swimbladder fish, gas-bearing invertebrates, cephalopods, fluidlike crustacean, fluidlike jelly-fish, and elastic-shelled invertebrates. Qualitative and quantitative aspects of the scattering structures were also described. Then scattering structure composition and the tuna trophic interest of aggregations was determined.