Laboratory target strength measurements of free swimming Antarctic krill (Euphausia superba)

Target strength measurements of free swimming krill at 120kHz were made using a single beam monostatic system in a 10 m3 laboratory tank. Krill (grouped according to length classes) swam freely in the tank triggering the data acquisition system when generating a back scattered signal larger than a threshold, determined by the system noise level. Dorsal and ventral target strength estimates were calculated indirectly by deconvolution of the cumulative probability function of echo ensembles of single animal insonifications. For mean length classes in the range [29.6, 36.2] mm the median single-animal target strengths are in the range [-76.7,-71.8] dB. Monte Carlo computer simulations were used to evaluate the effects of varying the ratio of largest to smallest echo amplitudes for a given ensemble, thus enabling the estimation of threshold induced bias in the target strength estimates. The threshold induced bias was then determined for each ensemble of experimental data and used to determine corrections which were in the range of [-0.84, -0.33] dB. An error analysis of the target strength estimates detailing the components due to measurement accuracy and precision, and the indirect signal processing techniques used is also presented.