Measurements of swimming angles, density, and sound speed of the krill euphausia pacifica for target strength estimation

Abstract

In recent years, acoustic technology has been used extensively to estimate krill abundance. Acoustic assessment is required to obtain precise estimates of krill target strength (TS). However, predictions of target strength from theoretical scattering models are often influenced by the swimming angle, density, and sound speed contrasts between krill and seawater. Density and sound speed contrasts are known to show annual cycles. In this study, swimming angles and seasonal variations of the specific density and sound speed contrasts of Euphausia pacifica are presented. Biological sampling was carried out during twilight, when the sound-scattering layer migrates up to the surface. Sound speed measurements were performed on a vessel 2 hours after net sampling, using a T-shaped velocimeter with two transducers mounted at the ends of a horizontal tube. The swimming angles and specific densities of E. pacifica were measured at a laboratory within 48 hours of net sampling. The specific densities of the krill were measured using a series of glycerol solutions of variable densities. Moreover, the total lipid content, lipid composition, and fatty acid composition of the krill were analyzed. The swimming angles of the krill varied from 30 to 50 degrees in low illumination condition (0.1 lx). The density and sound speed contrasts changed with the season. These variations depended on the lipid content in the krill body. In particular, the density of the krill changed with the wax ester content. The variations of TS estimated by the distorted-wave Born approximation (DWBA) method will be discussed