Effective frequency for acoustic survey of Antarctic krill

The frequency of 120 kHz is exclusively used for the abundance estimation of Antarctic krill. The considerations on the effective frequency for the acoustic survey of Antarctic krill, however, have not been sufficiently done. We examined the effective frequency for the acoustic survey from the viewpoints of the variation of target strength (TS), the detection range, and the variation of absorption coefficient α. Although the variations of TS and α are the smallest for 38 kHz, this frequency is not effective because the detection range is especially small. The frequency of 200 kHz is not effective, because the variation of TS may be large. Therefore, the effective frequencies from the QES frequencies are 70 or 120 kHz; there are not serious problems for using both frequencies. This conclusion is desirable because the frequency of 120 kHz has been generally used for the acoustic surveys of Antarctic krill for a long time.