Measurements of sound-speed and density contrasts of Antarctic krill (Euphausia superba) on board RV Kaiyo Maru

In order to calculate the target strength of Antarctic krill (Euphausia superba) using an acoustical scattering model, information on size, morphology, orientation, sound–speed and mass-density contrasts between the animal and the surrounding water are required. Sound-speed and mass-density of krill were measured during the Antarctic surveys conducted by the Japanese RV Kaiyo Maru in 1999/2000 and 2004/2005. Samples of krill were caught by a RMT(1+8). Mass-density of krill was measured by density bottle method. The mean total length and the mean mass-density contrast were 43.5mm and 1.028 near South Shetland Islands in February 2000. These were 21.7mm and 1.049, and 45.1mm and 1.043 in the Ross Sea in January and February 2005. Sound-speed was measured using the "time of flight" method. The corresponding sound-speed contrast of krill with mean total lengths of 44.2mm was 1.011 in the South Shetland Islands. These contrasts of krill with mean total lengths of 25.1mm and 48.6mm were 1.044 and 1.035 respectively in the Ross Sea. To examine the effect of these parameter differences, the target strength and its directivity of krill were calculated using the stochastic DWBA model.