Acoustic extinction in dense herring layers, measured from a bottom-mounted transducer

Abstract

Herring is often found in dense aggregations or schools, and acoustic shadowing is thus a problem in acoustic survey estimates. Knowledge of the attenuated echo intensity in herring aggregations is essential for correcting the measured area backscattering coefficient for extinction. The attenuation is measured through the extinction coefficient (σe/σb), which can be found by analyzing the reduction in echo intensity from a reference target while different densities of fish are located between the transducer and the reference target. In this study, the transducer was bottom-mounted and the sea surface was used as a reference target. The paper will present new data on the extinction coefficient for herring, as well as its variability. This variability has further been used estimate the uncertainty in the correction algorithms for a typical herring survey.