Target strength of southern blue whiting (Micromesistius australis) using swimbladder modelling, split beam and deconvolution

The tilt-averaged target strength, , of southern blue whiting (Micromesistius australis) at 38 kHz was measured using swimbladder modelling and two in situ methods. Split beam estimates of for southern blue whiting were carefully screened for multiple echoes, but a few useful values of were obtained from the periphery of shoals even at 150–300 m depths. We compare of southern blue whiting derived from swimbladder modelling to split beam and deconvolution estimates of . The -length regression for M. australis has similar slope but lower intercept than the published regression for blue whiting Micromesistius poutassou. Predicted for M. australis are 2.9 to 4.3 dB lower than for M. poutassou of the same size. In contrast to the swimbladder results, split beam and deconvolution estimates of for M. australis were in line with the -length relationship for M. poutassou. The magnitude of the difference between modelling in situ results could arise from the assumed tilt distribution of fish used in the modelling calculations and the actual, but unknown tilt distribution of the fish in situ. Acquiring information on the tilt distributions of southern blue whiting is essential to resolve the measured difference between in situ and modelled estimates of target strength.