TARGET STRENGTH OF SOUTHERN BLUE WHITING (MICROMESISTIUS-AUSTRALIS) USING SWIMBLADDER MODELING, SPLIT BEAM AND DECONVOLUTION

The tilt-averaged target strength, [TS], of southern blue whiting (Mic romesistius australis) at 38 kHz was measured using swimbladder modell ing and two in situ methods. Split beam estimates of [TS] For southern blue whiting were carefully screened for multiple echoes, but a few u seful values of [TS] were obtained from the periphery of shoals even a t 150-300 m depths. We compare [TS] of southern blue whiting derived f rom swimbladder modelling to split beam and deconvolution estimates of [TS]. The [TS]-length regression for M I. australis has similar slope but lower intercept than the published regression for blue whiting, M icromesistius poutasou. Predicted [TS] for M. australis are 2.9 to 4.3 dB lower than for ill. poutassou of the same size. In contrast to the swimbladder results, split beam and deconvolution estimates of [TS] f or M australis were in line with the [TS]-length relationship for M. p outassou. The magnitude of the difference between modelling and br sit u results could arise From the assumed tilt distribution of fish used in the modelling calculations and the actual. but unknown lilt distrib ution of the fish bl situ. Acquiring information on the tilt distribut ions of southern blur whiting is essential to resolve the measured dif ference between Br situ and modelled estimates of target strength. (C) 1998 International Council for the Exploration of thr Sea.