Abundance estimates of micronekton organisms in tropical Pacific Ocean from trawl sampling, acoustic survey and backscatter models [résumé]
Résumé
Micronekton, ubiquitous to all oceans plays a pivotal role in the trophic organisation and consti-tutes the food of most top predators. Due to the paucity of sampling, estimates of these organ-isms abundance and specie distribution is largely unknown. Such sampling comes either from trawls or active acoustic. One key question remains on how these two means of observations compare and complement each other. Our study focuses on the analysis of active acoustic data from 8 oceanographic surveys in South Pacific. Two active acoustic methods were used simulta-neously: hull-mounted echo sounders and a wideband profiler. These data are examined to-gether with biological samples obtained by trawling which brings a ground truth to the acoustic measurements. By comparing the in situ acoustic response, the modelled response from biologi-cal sampling and the density of organisms calculated from wideband profiles, we obtain an order of abundance estimates of micronekton in depth layers. This comparison enables us to estimate the observed differences of organisms abundance between the three methods and helps under-standing. Over the entire cruises, the average ratio between abundances derived from acoustic sounders and those obtained from trawled samples is on the order of 10 but varies strongly with depth.