Application of an analytical approach to characterize the target strength of ancillary pelagic fish species

The lack of data on the species-specific Target Strength (TS) on ancillary species limits the applicationof acoustic surveys in assessing their abundance and distribution worldwide. The TS values of Scombercolias and Trachurus mediterraneus in use in the Mediterranean Sea rely on studies conducted onother species in the Atlantic and Pacific oceans. Nevertheless, the application of backscatteringmodels offers the possibility to overcome the absence of empirical data handling the parametersthat most affect the TS. X-ray scans were performed on 82 specimens to get digital representationsof the swimbladder and the fish body which were used as input for the application of the KirchhoffRay Mode model to measure the TS as a function of frequency and tilt angle. The morphometricdifferences between the two species produced divergent relative frequency responses and broadbandTS patterns. Moreover, comparing the results with one ex-situ experiment, we found a goodagreement considering a mean tilt angle of - 10°, standard deviation = 12°. Our results provide the firsttheoretical insights into the use of backscattering models as a tool to distinguish between species inthe Mediterranean Sea by acoustic method, increasing the knowledge of the acoustic reflectivity ofancillary species.