Preliminary report of sound-speed contrast and density of krill measured on board RV Kaiyo Maru

The target strength (TS) of Antarctic krill, Euphausia superba, is a scaling factor to convert the acoustic backscattering strength into the population density. Therefore, it is important to improve the accuracy and precision of the TS value. Since in situ and experimental TS measurements of small animals like krill are difficult, the theoretical scattering models are used to predict the characteristics of the TS. For the calculation by the acoustical scattering model, the information of size, morphology, orientation, and material properties of krill are required. There are two important material properties: one is the ratio of the density of the animals to that of the surrounding water (density contrast) and the other is the ratio of the sound speed in animal to that in the surrounding water (sound-speed contrast.). Although these parameters are inevitable for the model calculation, the reports of them are still few. During Antarctic surveys of the Japanese R/V Kaiyo Maru in 2000 and 2004/ 2005, the measurements of sound-speed and density of krill were carried out. We report on those preliminary results. The mean total length and mean density were 43.5mm and 1.0562gcm-3, respectively in 2000 survey, and were 41.7mm and 1.0720gcm-3 in 2005 survey. The corresponding sound-speed contrasts of mean total length 25.1mm and 48.6mm were 1.0442 and 1.0348, respectively.