Preliminary target strength measurement of Sprattus sprattus and its influence on biomass estimates in the Adriatic Sea (Mediterranean Sea)

Abstract

Post-processing data analysis has the ability to affect target strength (TS) measurement, which in turn influence the biomass estimates of acoustic surveys. In situ experiments are considered as the best method to measure species-specific acoustic reflectivity since fish are measured in their natural environment without constrains. Nevertheless, this approach poses some problems concerning the methods to be applied to analyze acoustic data, especially when only a small number of monospecific hauls are available. In this study we tested several alternative ways to compute the b20 of sprat (Sprattus sprattus) in a poor data state in the Mediterranean Sea where the TS of the species is seldom studied. The method proposed by MacLennan and Menz (1996) and a pilot approach were used in parallel with the implementation of three single target detection algorithms to analyze two monospecific sprat hauls performed in the Adriatic Sea in 2014 and 2020. Next, we applied a Monte Carlo simulation to investigate the influence of TS variability on biomass estimates. We obtained six b20 values (range, −68.8 dB to −65.5 dB), all higher than the current known value of − 71.7 dB. The resulting coefficient of variation in the overall uncertainty on absolute biomass estimates overcame the 17%. The different models provided different total biomass estimates, however, the method proposed herein applied on high-density filter single target detection data suggest a value of b20 between − 67.5 dB and − 68.8 dB for sprat in the Mediterranean Sea, which significantly reduces absolute sprat biomass values between 2014 and 2021.