The density and sound-speed contrast values of sakura shrimp (Lucensosergia lucens)

Abstract

Sakura shrimp (Lucensosergia lucens) are an important fishery resource in Suruga Bay, Japan. We investigated the density contrast $g$ and sound-speed contrast $h$ of the shrimp. Acoustic survey methods using scientific echosounders are expected to be applied in the future for the estimation of shrimp abundance, but the $g$- and $h$-values, which are essential parameters for predicting the target strength (TS) by theoretical acoustic scattering models, have not been clarified. In this study, the $g$-values were determined based on body-density measurements conducted six times with the density-bottle method. The mean $g$-values for each measurement ranged from 1.0461 to 1.0590, and the overall mean and the standard deviation (SD) for a total of 118 individuals were 1.0508 and 0.0050, respectively. The $h$-values were inferred by best-fitting the predicted TS spectra by the distorted wave Born approximation-based deformed cylinder model to the measured TS spectra by a broadband pulse-echo system using a linear frequency modulated signal with a sweep frequency range at 20–220 kHz. The $h$-values of 12 individuals ranged from 0.990 to 1.029, and the mean and SD were 1.006 and 0.012, respectively. These results will contribute to establishing the acoustic survey method. According to a simple examination, variations of ± 1 SD in the g- and h-values resulted in changes in TS ranging from −3.1 to 2.2 dB. To clarify the variations in $g$ and $h$ associated with the shrimp growth and seasons, further investigations need to be conducted regularly.