Correction for shadowing within sardine schools

The acoustically estimated biomass of high-density sardine schools in Moroccan waters is potentially underestimated due to acoustic shadowing. To correct this bias, the attenuation of echo energy, extinction, due to shadowing, must be estimated. In this study, extinction is estimated for high-density schools of sardine (Sardina pilchardus) surveyed during a 1996 survey along the Moroccan coastline. The seafloor echo was used as a reference target (Foote et al. 1992), and the relative reduction of the seafloor echo under the schools was used to estimate the extinction cross section (σ_e). The extinction ratio, the ratio between σ_e and the back scattering cross section (σ_bs) of sardine schools with mean fish length = 23 cm, was estimated to be 0.627, with a 95% confidence interval from 0.486 to 0.768. There was a negative correlation between the extinction ratio and fish density. The mean correction factor for extinction in 19 analysed schools ranged 5-34 %.