Target strength of krill in situ at the frequency of 200kHz

The acoustic survey for krill (Euphausia superba) was carried out in the Indian Sector of the Southern Ocean by the R. V. KAIYO MARU participating in SIBEX I 1983-84. In order to estimate the biomass from the mean volume back-scattering strength (S^^^-_v) measured by the 200kHz echo sounder Furuno FQ-50,the mean target strength (^^^-) of krill in situ at the same frequency was estimated in the area surveyed. The ^^^- was obtained from both density and S^^^-_v for the krill aggregation in the natural conditions. The density was estimated by means of the echo sounder Furuno FE-101 which possesses a resolution at 455kHz high enough to detect the individual krills; the volume of water insonified by the beam was estimated from the recorded duration of the echo traces, and the number of krill in the volume was obtained by counting the echo traces. Two experiments to estimate the ^^^- were made about 1 hour before sunset, on board the vessel driftingoover the aggregation spreading near the surface. The mean densities estimated from the experiments were 50.0 nd 61.6 individ./m^3. The corresponding S^^^-_v were -51.0 and -50.4dB, respectively. The mean wet weight of the krill samples from the aggregation was 0.62g. An estimate of the ^^^- of 0.62g krill was -68.1dB. The ^^^- per kg was calculated as -36.0dB