O uso da acústica multifrequência na caracterização das comunidades pelágicas em Ilhas Oceânicas do Atlântico Sul, Brasil

Abstract

Underwater acoustics have an unrealized potential for multicomponent biotic and abiotic ecosystem characterization. A variety of methods (e.g., backscattering differences, clustering, Gaussian model) are currently used for multifrequency classification. This study aims at proposing and implementing a method based on the distribution of different groups of organisms echoes on multifrequency spatial planes, to be applied on acoustic data collected around oceanic islands of the Northeast Brazil. Initially, echoes of different groups of organism are defined a priori using in situ sampling. Then we apply an objective separation criteria based in the maximum difference of cumulated energy in each bi-frequency component, to maximize energy allocation within a given group when compared to the nearest one. This method was applied on four frequencies (38, 70, 120 and 200 kHz) data collected in the frame of the ABRAÇOS (Acoustics along the BRAzilian COaSt) project, around Archipelago of Fernando de Noronha and Atoll das Rocas, an area characterized by a low productivity but high biodiversity. For this process, up to five bi-frequency components were used for group discrimination. By applying the method we could discriminated six groups: Fish like (Sv38 ~ Sv70 ~ Sv120), two types of High Resonant at 38 kHz (Sv38 > Sv70 > Sv120 > Sv200) associated to gelatinous, Fluid Like (Sv38 < Sv70 < Sv120) associated to crustacean macrozooplankton, High Resonant at 70 kHz (Sv38 < Sv70 > Sv120 > Sv200) associated to algae, and a group of Unclassified echoes. Results are coherent in terms of distribution pattern of each group. Among other we could reveal the importance of gelatinous that were the dominant group in the vicinity of oceanic islands where it formed a dense layer above the thermocline. These results open new perspectives to improve knowledge on the patterns of distribution and the interaction of a variety of functional groups in this and other aquatic ecosystems.