Knowledge of the acoustic response of single fish (or Target Strength: TS) is of prime importance for acoustic target classification and abundance estimation. This report investigates the effect of TS~length equation selection on the anchovy and sardine biomass estimates derived from PELGAS acoustic and fishing survey data. We aim at assessing the respective effects of b20 and depth correction terms on anchovy biomass acoustic estimates derived from the PELGAS survey, in order to evaluate the magnitude of a potential non linear effect of fish depth on acoustic biomass estimates. The PELGAS 2012, 2014, 2015 and 2016 survey data are re-analysed using several TS equations with different b20 values and/or depth correction terms to assess the respective effects of each single term on PELGAS anchovy and sardine biomass estimates. Our results confirm that the main TS equation parameter influencing the spring Biscay anchovy and sardine biomass estimations is the b20. In comparison, the Ona (2003)'s depth correction term yields marginal and more or less constant changes in the biomass estimates. The hypothesis of a strong and non linear effect of a depth-correction term in the TS equation used to derive acoustic biomass estimates of anchovy or sardine in the Bay of Biscay is invalidated by these results. The annual depth distributions of anchovies, as estimated based on PELGAS catch and ESDU data, show moderate variability, which does not seem to be related to biomass fluctuations.