Broadbandwidth total target strength measurements of Antarctic krill (Euphausia superba) from reverberation in a cavity

Total scattering cross-sections (st) of Antarctic krill, Euphausia superba, were acoustically measured over a broadbandwidth (36 to 202 kHz) using a new technique [J. De Rosny and P. Roux, 2001, J. Acoust. Soc. Am. 109(6): 2587:2597]. From 18 February to 9 March 2002, mean total target strengths ( ??10log(??4?) t TTS ), were measured from groups of 57 to 1169 krill (average standard length = 31.6 mm; standard deviation = 6.6 mm), at the Cape Shirreff field station, Livingston Island, Antarctica, and aboard R/V Yuzhmorgeologiya. Chirp pulses were sequentially transmitted with an omnidirectional emitter into one of three glass carboys containing groups of krill swimming in 9.3, 19.3 or 45.9 l of seawater ( 0.6 ??temperature??3.6 °C). Between each pulse, the krill moved within the fixed-boundary-tank and the modulated reverberations were sensed bi-statically with three omnidirectional receivers. At each center frequency (fc), the coherent energy in 200-pulse ensembles identified sound scattered from the tank. The incoherent energy described total sound scattering from the krill. Thus, the TTS at each fc was extracted from a correlation analysis of energy reverberated in the tank. Measurement accuracy was determined using standard metal spheres for references [Demer et al., in-press, J. Acoust. Soc. Am.], and the precision was estimated from the variability in krill TTS measurements. Moreover, empirical estimates of mean s t were statistically compared to a recently proposed krill scattering model based on the distorted wave Born approximation (DWBA) [McGehee et. al, 1998, Deep Sea Res. Pt2, 45(7): 1273-1294], which has been enhanced to account for the stochastic nature of sound scattering (SDWBA) [Demer and Conti, submitted, ICES J. Mar. Sci.], and integrated over all scattering angles (SDWBATTS). This study improves upon methods for acoustical identification and target strength estimation for Antarctic krill, thus reducing the uncertainty in biomass estimation using multi-frequency echo sounder data and echo integration methods.