Modelling the target strength of Trachurus symmetricus murphyi based on high-resolution swimbladder morphometry using an MRI scanner

The acoustic target strength (TS) of Chilean jack mackerel (Trachurus symmetricus murphyi) was computed from three-dimensional reconstructions of the swimbladder form. The primary morphometric data were derived from high-resolution images made with a clinical whole-body 3-tesla magnetic resonance imaging (MRI) scanner. The fish were obtained from a commercial purse-seiner operating off Chile during 2003. Transverse MRI sequences of 18 fish were performed with a volumetric pixel 0.5 x 0.5 mm in cross section and 0.7 mm in the axial direction. From each transverse section, the swimbladder contour was traced manually using a digital pen tablet and smoothed using an automatic interpolation routine. The digital contours were used to build a triangular mesh spanning the swimbladder surface. The high-resolution swimbladder models revealed details such as undulations in the dorsal surface of the swimbladder caused by the backbone, and holes where the haemal spines penetrate the posterior end of the swimbladder. The Kirchhoff approximation was used to compute TS as a function of fish orientation at 38 kHz. The mean dorsal-aspect TS was computed for several orientation patterns and regressed on the logarithm of total fish length to derive operationally simple formulae.