Target strength pattern measurement of krill (Euphausia pacifica) and measuring limits in a small tank

Abstract

Euphausia pacifica is an extremely important species for Japanfs northeast coastal fisheries and a key species for ecosystems in the northwest Pacific Ocean. Target strength (TS) of krill information have been required for acoustic surveys; they constitute a scale factor that is used to convert acoustic estimates of abundance. A precise TS pattern measurement system was constructed in a small tank. Target strength patterns of a target are measurable in the angle range of 60 deg at 1‐deg steps. First, TS values of the thinnest fishing line with 53‐mm diameter were measured as it was planned for use in tethering krill. TS measurement limits were also confirmed by measuring echo signals and noise signals. Maximum TS of the fishing line was −95 dB and the measurement limit was −98 dB at the frequency of 200 kHz with 10 dB of signal‐to‐noise ratio (SNR). Second, we confirmed the validity of the target suspension method. The thinnest fishing line was penetrated into the krill body to tether it between two vertical fishing lines. Finally, TS patterns of 12 live krill were measured and compared with theoretical calculations using distorted wave Born approximation. Both agreed well.