WEIGHT CONVERSION OF THE INES MOVIES ACOUSTIC DENSITIES AND THE THRESHOLD EFFECT ON BIOMASS EVALUATION

The detection and the reverberation measurement of the fish by Acoustics allow to analyze their behaviour, distribution and structures. All these investigations are mainly based on a relative evaluation of densities. The method of echo-integration also allows to quantify abundance. It then necessitates to know accurately the acoustic performances of the apparatus and the definition of a weight conversion coefficient to apply on the relative data. The Ines-Movies system, used during the acoustic surveys of the PELFISH Project, consists of a calculation procedure with conversion coefficient. After determination of the Decapterus russelli reverberation index (or TS), in cage this coefficient has been calculated. The accuracy of evaluations resulting from such an index depends particularly on the TS one. But another parameter has to be taken into account; this is the adjustment of threshold during the prospecting. Even with a controlled adjustment of this latter, the conversion of relative integrated values of the Java Sea shows that the evaluations obtained in that way do not correspond to the richness of the area, according to fishing techniques, the level of commercial captures, and the results of experimental fishing as well. The playback of the rough data, available in the Ines-Movies system, allows to better adjust the threshold for evaluation of the targets, taking into account the reverberating surrounding conditions