Acoustic characterization of Mysis relicta at multiple frequencies

We measured acoustic backscattering from Mysis relicta, a common invertebrate in northern lakes, using five frequencies (38, 120, 200, 430, and 710 kHz). Acoustic backscattering from mysids was highest at 430 kHz and lowest at 38 kHz ( 19 dB lower). Maximum difference between the four other frequencies was 5.2 dB. Mysid target strength (TS) ranged from -80.1 dB at 430 kHz to -99.4 dB at 38 kHz ( 12 mm average length, range 5-21 mm). A theoretical scattering model (Stanton's fluid-like, bent-cylinder model) predicted TS within 0.3-1.9 dB of observed TS for the different frequencies. The detection range was lowest at 38 and 710 kHz and greatest at 120 and 200 kHz. Fish were common above the mysid layer and produced higher acoustic backscattering at 38 kHz than at the other frequencies. A combination of 38 kHz and 120 or 200 kHz provides a strong contrast between mysid and fish acoustic backscattering that would help separate these groups using acoustic data.