Measurements of density and sound speed contrasts for estimating krill target strength using theoretical scattering models

Abstract

In recent years, acoustic technology has been extensively used for estimating krill abundance. This acoustic assessment is required for precise estimate of target strength (TS) of a krill. However, prediction of the target strength from the theoretical scattering models is generally dependent on the swimming angle, the density and sound speed contrasts between krill and seawater. In these parameters, the density contrasts and sound speed contrasts are known to change during the year. In this study, the seasonal variations of the specific density and sound speed contrasts of Euphausia pacifica are presented. The specific densities of E. pacifica were measured in a series of saline or glycerol solutions of different densities. The measurements of the sound speed were performed using T-shaped velocimeter with two transducers mounted at the ends of the horizontal tube. The specific densities of E. pacifica changed about 1% with season. The sound speed contrasts also varied about 3% with season. These seasonal changes yielded difference about 5 dB in target strength of E. pacifica calculated from a straight cylinder model.